CS601 Data Communication Final Term Papers Solved ...

Cs601 Solved Subjective (Mid+ Finals)

CS601 Data Communication Final Term Papers

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FINAL TERM EXAMINATION SPRING 2011

What is the difference between guided and unguided media? Answer: Guided Media are those media that provide a conduit from one device to another. Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path while Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. It passes through a vacuum; it is independent of a physical pathway.

Write Commercial advantage and characteristics of token bus Answer: (Page 232) Other LANs are not suitable for this purpose Token Bus has no commercial application in data communications Token Ring allows each station to send one frame per turn Access method: Token passing

What is the difference between FDM and TDM Answer: 1)FDM-Frequency division multiplexing where as TDM mean Time division Multiplexing. 2)In FDM spectrum is divided into frequency whereas in TDM divided into time slot. 3)FDM is used in 1st generation analog system whereas TDM is used in 2nd generation analog system.

Write the types transmission noise Answer: (43) Thermal Noise: Due to random originally sent by TX Induced Noise: Comes from sources like Motors and Appliances Crosstalk: Effect of one wire on another Impulse Noise: Spike (A signal with high energy in a very short period of timepower lines,

lightening etc.

What is power bandwidth Answer: The power bandwidth of an amplifier is sometimes taken as the frequency range (or, rarely, the upper frequency limit) for which the rated power output of an amplifier can be maintained (without excessive distortion) to at least half of the full rated power. OR

Power Bandwidth Answer: (Page 67) Frequency band in which 99% of the total power resides.

What does the CRC generator append to data unit? [2] Answer: (Page 175) Appending it to the end of the data must make the resulting bit sequence exactly divisible by the divisor

How much bandwidth for modem is required in case of FSK? [2] Answer: BW required for FSK is equal to the Baud rate of the signal plus the frequency shift. Because of the limitations of voice-grade telephone lines, these modems are restricted to a bandwidth of about 3 kHz

What is even parity generator in VRC error detection mechanism? [2] Answer: (Page 172) Even parity generator counts the 1‟s and appends the parity bit (1) to the end.

What is the difference between angle of incident and angle of reflection? [2] Answer: (Page 126) The difference between them is that Angle of refraction passes from less dense to denser medium whereas angle of incidence passes from more dense to less dense medium.

What is daisy chaining in 1Base 5 star Lan? [2] Answer: (29) Slower speed in star lan can be increase by the use of DAISY CHAINING.

What is the responsibility of Application layer? [3] Answer: Page 53

Enables the user either human or software to access the network It provides user interface and support for the services such as Electronic mail, Remote File access and Transfer, Shared Database Management and other services

What is critical angle? Answer: (Page 127) We have a beam of light moving from a more dense to a less dense medium. We gradually increase the angle of incidence measured from vertical axis. As angle of incidence increases, so does the angle of refraction. The angle at which refracted line lies on the horizontal axis is called Critical Angle

Hamming code-Redundancy bit (5 marks Q) Answer: (Page 181) Redundancy Bits (r) r must be able to indicate at least m+r+1 states m+r+1 states must be discoverable by r bits Therefore, 2r ≥ m+r+1

Tree topology advantages (3 or 5 marks) Answer: (1) Because of Secondary Hub, More devices can be attached to a Central Hub and therefore increase the distance a signal can travel Enables Differentiated Services: Allows to prioritize communication, e.g. computers attached to one secondary hub can be given priority over others Therefore, TIME SENSITIVE data will not have to wait for access to the network Rest of the advantages are almost the same as STAR

Q #41: Whether VRC error detection method is used for single bit error or burst error. (2) Answer: (Page 173) VRC can detect all single bit errors Can also detect Burst errors as long as the total number of bits changed is ODD

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Q # 42: Which modem was first developed commercially in 1970? (2) Answer: (Page 114) Bell modems –First commercial modems by Bell Telephone Co. –Developed in early 1970s

Q # 45: Consider a major telecom company using RZ encoding for its signals conversion. What will be the major problem faced by using such type of encoding? (2) Answer: (Page 75) Any time, data contains long strings of 1’s or 0’s, Rx can loose its timing. The only problem with RZ encoding is that it requires two signal changes to encode one bit and therefore occupies more BANDWIDTH

Q # 47: Geosynchronous Satellite? (3) Answer: (Page 139) Line of sight propagation requires the sending and receiving antennas must be locked into each other To ensure continuous communication, satellites must move with the same speed as earth. So that they seem fixes w.r.t earth These satellites are called Geosynchronous Satellites

FINAL TERM EXAMINATION 2011

Question#1 What are the Asynchronous protocols in data communication layer? ………….Marks (10) Answer: (Page 206) Asynchronous protocols Treat each character in a Bit stream independently. Employed mainly in

Modems. Transmission does not require timing coordination; Timing is done by using extra bits

Different Asynchronous Protocols

XMODEM oThe first field is a One Byte start of header (SOH) field oThe second field is a two-byte Header. –The first header byte , the Sequence number carries the Frame number –The second header byte is used to check the validity of the sequence number

o o

The fixed data field holds 128 bytes of data The last field CRC checks for errors in the data field only

YMODEM YMODEM is similar to X-MODEM with only the following major differences: o1024-Byte data unit oTwo CANs to abort Transmission oITU-T CRC-16 for Error Checking oMultiple files can be sent simultaneously

ZMODEM : Newer Protocol Combines features of XMODEM and YMODEM.

BLAST oBlocked Asynchronous Transmission oMore powerful than XMODEM oFull Duplex oSliding Window Flow Control oAllows transfer of Data and Binary Files

KERMIT oDesigned at Columbia University oMost Widely used Asynchronous Protocol oFile Transfer protocol is similar in operation to XMODEM, with sender waiting for an NAK before it starts TX oKermit allows the transmission of control characters as Text

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Question#2 What is Frequency division multiplexing ?.......Marks (5) Answer:(Page 149) Frequency division multiplexing (FDM) oAn analog technique that can be applied when BW of the link is greater than the combined BW of the signals to be TX oSignals generated by each sending device modulate difference carrier frequencies oThese modulated signals are then combined into a single Composite signal that can be transported by the link oCarrier frequencies are separated by enough BW to accommodate the modulated signal oThese BW ranges are the channels through which the various signals travel

Question#4 What is stop and wait ARQ in error control ?....Marks (3) Answer: Page 197 Stop-and-Wait is an extended form of flow control to include retransmission of data in case of Lost or Damaged frames. There are four main features added in it. 1.Sending device keeps a copy of the last frame transmitted until it receives the ACK for that frame. 2.Both data and ACK frames are numbered 0 and 1 alternately. 3.A data 0 frame is acknowledged by a ACK 1 frame indicating that the receiver has received data 0 and is now expecting data 1 . 4. For retransmission to work, 4 features are added to the basic flow control mechanism.

Question#5 What is Interleaving ?........Marks (3) Answer: Page 153 Synchronous TDM is considered as a very fast rotating switch. When this switch opens in front of a device, the device has the opportunity to send a specific amount of data on to the path. The switch moves from device to device at a constant rate and in a fixed order. This process is called INTERLEAVING. Interleaving can be done by BITS, BYTES or by any other DATA UNIT

Question#6 What is DSU in terms of digital services?.........Marks (3) Answer: Page 163 DSU (Digital service unit) changes the rate of digital data created by the subscriber’s device to 56 Kbps

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and encodes it in the format used by service provider. It used in dialing process and is more expensive than MODEM. But it has better speed, better quality and less susceptibility to noise.

Question#7 Which architecture of Ethernet developed by ITU_T and ANSI?........... Marks (2) Answer: 236 FDDI (Fiber Distributed Data Interface)architecture of Ethernet developed by ITU_T and ANSI.

Question#8 What is a spike in noise term?.........Marks (2) Answer:143 Spike is a signal with high energy in a very short period of time that comes from power lines, lightening etc,

Question#9 Answer: Page 172 What is even parity generator in VRC error detection mechanism?...........Marks (2) Even parity generator counts the 1’s and appends the parity bit (1) to the end.

Question#10 Compare line discipline methods ENQ/ACK and Poll/ Select? Answer: Page 188-189 =>ENQ/ACK coordinates which device may start a transmission and whether or not the intended recipient is ready and enabled. => Using ENQ/ACK, a session can be initiated by either station on a link as long as both are of equal rank. => In both half-duplex and full-duplex transmission, the initiating device establishes the session. => In half duplex, the initiator then sends its data while the responder waits. The responder may take over the link when the initiator is finished or has requested a response. => In full duplex, both devices can transmit simultaneously once the session has been established. POLL/SELECT: => The poll/select method of line discipline works with topologies where one device is designated as a primary station and the other devices are secondary stations. => Multipoint systems must coordinate several nodes, not just two. => The primary device controls the link and the secondary device follow sits instruction It is up to the primary to determine which device is allowed to use the channel data given time

Why addressing is required in Poll / Select method and not required in ENQ/ACK method?3


Answer: (Page 190) Addressing is required in Poll / Select method as it is a not point-to-point configuration, For the primary device in a multipoint topology to be able to identify and communicate with a specific secondary device, there must be some addressing, while ENQ/ACK method is a point-to-point method and for point-to-point configuration, there is no need for addressing.

What do you know about ITU-T Modems?3 Answer:(Page 114) ITU-T modem : V-series: Today’s most popular modem standards Bell modem compatible: V.21/22/23/26/27/29

Following abbreviations stands for what?3 Answer: (Page 224) ARP…….…. (Address Resolution Protocol) RARP……... (Reverse Address Resolution Protocol) ICMP ……… (Internet Control Message Protocol)

Write names of Link Access Protocols developed by ITU-T?3 Answer: (Page 211) LAPs: LAPB, LAPD, LAPM, LAPZ etc. all based on HDLC

Write the names of different types of noise in the medium?3 Answer:(Page 144) Thermal Noise Induced Noise Crosstalk Impulse Noise

Write down some disadvantages of star topology.3 Answer: (Page 30) Although Cabling required is far less than Mesh Still each node must be connected to a Hub , so Cabling is still much more

What are the two major classes of synchronous protocols at data link layer?2 Answer: (Page 206) Character – Oriented Protocols Bit – Oriented Protocols

Whether Hamming code is the technique used for error detection or error correction?2 Answer: (Page 181)


Hamming code is the technique used for error correction

Define Multiplexing? What is its advantage?2 Answer: (Page 147) Set of techniques that allows the simultaneous transmission of multiple signals across a single data link It allows multiple users to share total capacity of a Transmission Medium.

What is the purpose of dual ring?2 Answer: (Page 34) Unidirectional traffic movement is overcome by dual ring technology.

Which modem was first developed commercially in 1970?2 Answer: (Page 114) Bell modems

Write any two functions of physical layer?2 Answer: (Page 45)

It defines characteristics of Interface between device and transmission Medium It also defines the type of transmission medium Physical Layer is also concerned with Line Configuration


Which one has more overhead, a repeater or a bridge? Explain your answer. [3] Answer: A bridge has more overhead than a repeater. A bridge processes the packet at two layers; a repeater processes a frame at only one layer. A bridge needs to search a table and find the forwarding port as well as to regenerate the signal; a repeater only regenerates the signal. In other words, a bridge is also a repeater (and more); a repeater is not a bridge

Define high frequency [HF] and super high frequency [SHF], which devices uses these frequencies [3] Answer: Page 135 and 136


High frequency. HF uses ionospheric propagation. These frequencies move into the ionosphere where the density difference reflects them back on earth. It is used for Citizen’s Band Radio, International Broadcasting, Military Communication, Telephone, Telegraph and Fax

Super high frequency. SHF waves are TX using mostly line-of-sight and some Space propagation. It is used for Terrestrial and Satellite Microwave and Radar Communication devices.

Write all steps of checksum method. [3] Answer: (Page 179) o The sender subdivides data units into equal segments of ‘n’ bits(16 bits) o These segments are added together using one’s complement o The total (sum) is then complemented and appended to the end of the original data unit as redundancy bits called CHECKSUM o The extended data unit is transmitted across the network o The receiver subdivides data unit as above and adds all segments together and complement the result o If the intended data unit is intact, total value found by adding the data segments and the checksum field should be zero o If the result is not zero, the packet contains an error & the receiver rejects it

Differentiate internet and the internet? [3] Answer: (Page 240) INTERNET oAn internet is a generic term used to mean an interconnection of individual networks oTo create an internet, we need networking devices called routers and gateways oAn internet is different from the Internet oInternet is the name of a specific worldwide network

What is the differences in between bit oriented and character oriented protocols [5] Answer: (Page 206) Character – Oriented Protocols oAlso called Byte- Oriented Protocol oThese protocols interpret a transmission frame or packet as a succession of characters, each usually composed of one byte oAll control information is in the form of an existing character encoding system

Bit – Oriented Protocols


oCharacter –Oriented Protocols are not as efficient as bit – oriented protocols and are seldom used oThey are easy to comprehend and employ the same logic as bit-oriented protocols oTheir study will provide the basis for studying the other data link layer protocols oIBN’s BSC is the best known character oriented protocol

Question#1


What is the formula to calculate the number of redundancy bits required to correct a bit error in a given number of data bits? [2] Answer: Messages(frames) consist of m data (message) bits, yielding an n=(m+r)-bit codeword.

Question#2 What is R G rating of coaxial cable? Answer:-(Page 126) Different coaxial cable designs are categorized by their Radio government ( RG ) ratings Each cable defined by RG rating is adapted for a specialized function: RG-8 •Used in Thick Ethernet RG-9 •Used in Thick Ethernet RG-11 •Used in Thick Ethernet RG-58 •Used in Thin Ethernet RG-59 •Used for TV

Question#3 What are the advantages of thin Ethernet? Answer:(Page 228) The advantages of thin Ethernet are: 1.reduced cost 2.ease of installation Because the cable is lighter weight and more flexible than that used in Thick net


Question#4 What is the difference between a unicast, multicast, and broadcast address? [3] Answer: Broadcast: transmitting a packet that will be received by every device on the network Unicast: the sending of information packets to a single destination Multicast: delivery of information to a group of destinations. Question#5 T lines are designed for Digital data how they can be used for Analog Transmission? Answer:(Page 166) o DS-1 requires 8 Kbps of overhead o To understand this overhead, let’s examine format of a 24-voice channel frame o Frame used on T-1 line is usually 193 bits divided into 24 slots of 8 bits each + 1 bit for synchronization (24*8+1=193) o 24 segments are interleaved in one frame o If a T-1 carries 8000 frames, the data rate is 1.544 Mbps (193 * 8000=1.544 Mbps) which is capacity of the line

Question#6 What are the three types of Guided Media? Answer:(0) 1.Coaxial cable 2.Twisted-pair cable 3.Fiber optic cable.

Question#7 Why do we need Inverse Multiplexing? [5] Answer:(Page 159) •An organization wants to send data, voice and video each of which requires a different data rate •To send voice it needs 64Kbps, •To send data, it needs 128 Kbps link •To send video it may need 1.544 Mbps link •It can lease a 1.544 Mbps line from a common carrier and only use it fully for sometime •Or it can lease several separate channels of lower data rates •Voice can be sent over any of these channels •Data & Video can be broken into smaller portions using Inverse Multiplexing and TX

Question#8 Describe method of checksum briefly? Answer:(Page 180) o The sender subdivides data units into equal segments of ‘n’ bits(16 bits) o These segments are added together using one’s complement o The total (sum) is then complemented and appended to the end of the original data unit as redundancy bits called CHECKSUM o The extended data unit is transmitted across the network


o The receiver subdivides data unit as above and adds all segments together and complement the result o If the intended data unit is intact, total value found by adding the data segments and the checksum field should be zero o If the result is not zero, the packet contains an error & the receiver rejects it

Question#9 Explain Asynchronous Time Division Multiplexing in detail? Also discuss its advantages over synchronous TDM? Answer: Asynchronous time-division multiplexing (ATDM) is a method of sending information that resembles normal TDM, except that time slots are allocated as needed dynamically rather than pre-assigned to specific transmitters. ATDM is more intelligent and has better bandwidth efficiency than TDM. asynchronous time-division multiplexing comprising receive circuits (CRl/i) supplying cells received via input links, transmit circuits (CTl/j) transmitting retransmitted cells on output links, a buffer memory (MT) storing the received cells and delivering the cells to be retransmitted and a buffer memory addressing device (SMT) including a write address source (SAE) and a read address source (fsl/j).

Advantages asynchronous TDM: In asynchronous TDM, the timeslots are not fixed. They are assigned dynamically as needed. The objective would be to switch from one user to another user whenever the one user is idle, and to asynchronously time multiplex the data. With such an arrangement, each user would be granted access to the channel only when he has a message to transmit. This is known as an Asynchronous Time Division Multiplexing System (ATDM). A segment of a typical ATDM data stream is shown in Figure 2. The crucial attributes of such a multiplexing technique are: 1.An address is required for each transmitted message, and 2.Buffering is required to handle the random message arrivals.


Question No: 31( Marks: 2 ) What is hybrid topology? Answer: Hybrid topology is a kind of topology, In which Several topologies combined in a larger topology

Question No: 32( Marks: 2 ) What is combined station of HDLC? Answer: (Page 211) A combined station is one of a set of connected peer devices programmed to behave either as a


primary or as a secondary depending on the nature and the direction of the transmission.

Question No: 33( Marks: 2 ) What kind of error is undetectable by the checksum? Answer: (Page 180) Error is invisible if a bit inversion is balanced by an opposite bit inversion in the corresponding digit of another segment

Question No: 34( Marks: 2 ) What's the name of the telephone service in which there is no need of dialing? Answer: (Page 161)

In Analog Leased Service there is no need of dialing

Question No: 35( Marks: 3 ) How Bit Rate & Baud rate are related? Answer: (Page 85) Bit rate equals the baud rate times the no. of bits represented by each signal unit. The baud rate equals the bit rate divided by the no. of bits represented by each signal shift. Bit rate is always greater than or equal to Baud rate

Question No: 36( Marks: 3 ) Following abbreviations stands for what? Answer: (Page244) 1.ARP: Address Resolution Protocol 2.RARP: Reverse Address Resolution Protocol 3.ICMP: Internet Control Message Protocol

Question No: 37( Marks: 3 ) Differentiate between Polling and Selecting. Answer: (Page 189) If the primary wants to receive data, it asks the second-arise if they have anything to send, This is called Polling If the primary wants to send data, it tells the target secondary to get ready to receive, This function is called Selecting.

Question No: 38( Marks: 3 ) T lines are designed for Digital data how they can be used for Analog Transmission? Answer: repeat

Question No: 39( Marks: 5 ) What is the difference between character oriented and bit oriented protocols? Answer: repeat

Question No: 40

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Write down the function of Primary-Secondary communication in line discipline. Answer: (Page 189) 1.Poll method works with topologies where one device is designed as a Primary station and the other devices are Secondary stations 2.The primary device controls the link and the secondary device follow sits instruction 3.It is up to the primary to determine which device is allowed to use the channel at a given time. 4.The primary therefore is always the initiator of the a session 5.Whenever a multipoint link consists of a primary device and multiple secondary devices using a single TX line , all exchanges must be made through the primary device even when the ultimate destination is a secondary device


Question No: 31( Marks: 2 ) What kind of error is undetectable by the checksum? [2] Answer: Page 180 (repeated)

Question No: 32 ( Marks: 2 ) What are properties of signals? Answer: (Page 17) Capable of being propagated over TX. Medium ,Interpretable as data at the Receiver

Question No: 33( Marks: 2 ) Whether in Asynchronous or Synchronous TDM, addressing is used? Answer: (8) Addressing is used only in Asynchronous TDM.

Question No: 34( Marks: 2 ) What is the basic purpose of Router? Answer: Basic purpose of Router: A router is a device that extracts the destination of a packet it receives, selects the best path to that destination, and forwards data packets to the next device along this path. They connect networks together; a LAN to a WAN for example, to access the Internet. Question No: 35( Marks: 3 ) Why we need a Null Modem? Answer: (Page 106) A null modem provide DTA –DTE interface w/o DCEs Question No: 36( Marks: 3 ) Count LRC for the following bits? 10011010 10100101


Question No: 37( Marks: 3 ) What are the categories of multiplexing? Answer: Page 148 There are three catogaries of multiplxing FDM TDM Have a two other catagories Synchronous and asyrounce WDM

Question No: 38( Marks: 3 ) What are the three purposes of control frames at data link layer? Answer: (Page 209) Control frames serve 3 purposes:

Establishing Connections Maintaining Flow and Error Control during Data Transmission Terminating Connection

Question No: 39( Marks: 5 ) Compare line discipline methods ENQ/ACK and Poll/ Select? Answer: (Page 188-189) =>ENQ/ACK coordinates which device may start a transmission and whether or not the intended recipient is ready and enabled. => Using ENQ/ACK, a session can be initiated by either station on a link as long as both are of equal rank. => In both half-duplex and full-duplex transmission, the initiating device establishes the session. => In half duplex, the initiator then sends its data while the responder waits. The responder may take over the link when the initiator is finished or has requested a response. => In full duplex, both devices can transmit simultaneously once the session has been established. POLL/SELECT: => The poll/select method of line discipline works with topologies where one device is designated as a primary station and the other devices are secondary stations. => Multipoint systems must coordinate several nodes, not just two. => The primary device controls the link and the secondary device follow sits instruction It is up to the primary to determine which device is allowed to use the channel data given time

Question No: 40( Marks: 5 ) What is the difference between character oriented and bit oriented protocols? Answer: repeat



Question No: 31( Marks: 2 ) What are the advantages of a multipoint connection over a point-to-point connection? Answer: Point-to-point connection is limited to two devices, where else more than two devices share a single link in multipoint connection. Multipoint connection can be used for fail-over and reliability.

Question No: 32( Marks: 2 ) What's the name of the telephone service in which there is no need of dialing. Answer:(Page 164) DSS (digital data service) is the telephone service in which there is no need of dialing.

Question No: 33( Marks: 2 ) Which type of frames is present in BSC frames? Answer: (Page 206)

There are two types of frames that are present in BSC. 1.Control Frames and 2.Data Frames

Question No: 34( Marks: 2 ) What methods of line discipline are used for peer to peer and primary secondary communication? Answer: (Page 187) Line discipline is done in two ways: 1.ENQ/ACK(Enquiry Acknowledgement)

This is used for peer to peer communication. 2.Poll/ Select This method is used for primary secondary communication.

Question No: 35( Marks: 3 ) How does the checksum checker know that the received data unit is undamaged? [3] Answer: (9)


Checksum Checker or generator: The sender subdivides data units into equal segments of ‘n’ bits(16 bits) 1.These segments are added together using one’s complement. 2.The total (sum) is then complemented and appended to the end of the original data unit as redundancy bits called CHECKSUM. 3.The extended data unit is transmitted across the network. 4.The receiver subdivides data unit and adds all segments together and complement the result. 5.If the intended data unit is intact, total value found by adding the data segments and the checksum field should be zero. 6.If the result is not zero, the packet contains an error & the receiver rejects it.

Question No: 36( Marks: 3 ) Which one has more overhead, a repeater or a bridge? Explain your answer. [3] Answer: A bridge has more overhead than a repeater. A bridge processes the packet at two layers; a repeater processes a frame at only one layer. A bridge needs to search a table and find the forwarding port as well as to regenerate the signal; a repeater only regenerates the signal. In other words, a bridge is also a repeater (and more); a repeater is not a bridge.

Question No: 37( Marks: 3 ) Write down disadvantages of Ring Topology. Answer: (Page 33) Disadvantages of Ring Topology

Unidirectional Traffic

Can be solved by using:

Question No: 38( Marks: 3 ) How parity bits are counted in VRC error detection method technique in case of odd parity generator?


Question No: 39( Marks: 5 ) How are lost acknowledgment and a lost frame handled at the sender site? [5] Answer: A lost or damaged frame is handled in the same way by the receiver; when the receiver receives a damaged frame, it discards it, which essentially means the frame is lost. The receiver remains silent about a lost frame and keeps its value of R.

Question No: 40( Marks: 5 ) Explain Protocol Data Unit (PDU)? Answer: (Page 221) Protocol Data Unit (PDU) The data unit in the LLC level is called the Protocol Data unit (PDU) The PDU contains 4 fields familiar from HDLC: –A destination service access point (DSAP) –A source service access point (SSAP) –A control field –An Information field



CS601 Data Communication Lecture Wise Question and

Answers

For final Term Exam Preparation By

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CS601 Data Communication Lecture no.23

what is ITU-T stand for?

ITU stands for International Telecommunication Union (ITU) and it coordinates the standards for telecommunications (T) .That why it is called ITU-T.

Is PCM is in the modem?

PCM is used to convert analog signal to digital signal. As you know, modem is modulator and demodulator for the analog signal. So, modem uses PCM for analog to digital conversion.

What is ISP and what is ISP stands for?

ISP is short for Internet Service Provider. It refers to a company or a body that provides Internet services, including personal and business access to the Internet. For example, PTCL (Pakistan Telecommunication Company Limited) is serving as an ISP in Pakistan.

What type of signal conversion required when we r talking to overseas friend ?

When two people communicate via a telephone line then this type of communication requires Analog to Analog conversion because human voice comes in Analog category of signals and typical telephone lines carry Analog signals. So, Analog to Analog conversion is required.

What are the essential elements of “Shannon’s formula” ?


The essential elements of “Shannon’s formula” are: 1. Proportionality to bandwidth W 2. Signal power S 3. Noise power P 4. A logarithmic function The channel bandwidth sets a limit to how fast symbols can be transmitted over the channel. The signal to noise ratio (P/N) determines how much information each symbol can represent. The signal and noise power levels are, of course, expected to be measured at the receiver end of the channel. Thus, the power level is a function both of transmitted power and the attenuation of the signal over the transmission medium (channel).


Is PCM is in the modem ?

PCM is used to convert analog signal to digital signal. As you know, modem is modulator and demodulator for the analog signal. So, modem uses PCM for analog to digital conversion.

Explain why we are using modem if data is to be sent by digitizing it. Let me clear, from PC digital data is converted into analog using modem, then via local loop data is sent to Switching where it is converted into Digital using PCM. Then why did not PCM used directly from PC ????

The local loop is basically provided by any telephone service provider and obviously it is analog. We use same local loop for computers which are digital. So it is necessary to convert our digital data into analog signal for transmitting the data through the local loop. That’s why we use modem in our system to convert data from digital to analog conversion.

What is FDM?


FDM stands for Frequency division multiplexing .It is the process by which the total bandwidth available to the system is divided. This division is into a series of non-overlapping frequencies bands that are then assigned to each communicating source and user pair.

Define noise source? and give some example of it because it is outside the wire????

In communication, noise is an error or undesired random disturbance in a useful information signal, introduced before or after the detector. The noise is a sum of unwanted or disturbing energy from natural and sometimes man-made sources.

Noise sources:

Fluctuations of the electric current in an electrical conductor

A signal or process with a frequency spectrum that falls off steadily into the higher frequencies etc

What is male connector and what is female connector?

A male connector is a connector that is attached to a wire, cable, or a piece of hardware, having one or more exposed, unshielded terminal and constructed in such a way that it can be inserted tightly into a holder (female connector) to ensure a reliable physical and electrical connection.

The "female" connector is generally a receptacle that receives and holds the "male" connector.

Can we calculate BW by physical measurement of Cables’ metal wire diameter? 2) What is the Role of X, Gamma, Cosmic Rays in Data Communication?

Bandwidth is actually the capacity of the channel. The amount of data travels through cable in a unit time is referred to as bandwidth. The X-rays and Gamma rays have uncertain photon energy.

Gamma radiation is often used to kill living organisms, in a process called irradiation. Applications of this include sterilizing medical equipment (as an alternative to autoclaves or chemical means), removing decay-causing bacteria from many foods or preventing fruit and vegetables from sprouting to maintain freshness and flavor.

The downloading via cable modem is 36Mbps and uploading is 12Mbps and it uses QAM and QPSK technique for uploading and downloading respectively; my question is same network card is used for uploading and downloading; then why not use the same modulation for upload and download, i mean to say on the network card there will be an electronic component which is


responsible to implement QAM for decoding the internet information received from the cable, and there would be another component on the network card which is implementing QPSK for upload information; why not use same technique for both upload and download.??

This term "Cable Modem" refers to a network interface card that communicates with the cable network, sending and receiving data in two slightly different ways. In the downstream direction, the digital data is modulated and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750MHz. Currently, 64 QAM (Quadrature Amplitude Modulation) is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals. In a two-way activated cable network, the upstream (also known as reverse path) is transmitted between 5 and 42 MHz. This tends to be a noisy environment with RF interface and impulse noise. Most cable companies use QPSK (Quadrature Phase Shift Keying) or a similar modulation scheme in the upstream direction, because QPSK is a more robust scheme than higher order modulation techniques in a noisy environment. The drawback is that QPSK is slower than QAM. For downstream and upstream different standars are used I.e Telephone cable and cable Modem that’s why different techniques are used .

DSL technology is using same telephone line which has restricted bandwidth of 4 Khz; then how using DSL modems we can have very good download speed as compared to dial-up internet connection?

DSL, which stands for Digital Subscriber Line, is a broadband communication technology designed for use on regular phone lines. It has the ability to move data over the phone lines at speeds up to 140 times speedier than the fastest analog modems available today.

Traditional phones do not utilize the full amount of the signals that are available and as a result the maximum speeds that you can get using a dial-up connection are only 128 Kbps. A DSL modem takes the analog signals and converts them into digital signals or bits.DSL provides always-on high-speed Internet access over a single dedicated telephone line; cable modems offer always-on high-speed Internet access over a shared cable television line, the bandwidth is shared among all users in a neighborhood, and will therefore vary, perhaps dramatically, as more users in a neighborhood get online at the same time. Cable modem upstream (from the home to the Internet) traffic will in many cases be slower than DSL, either because the particular cable modem is inherently slower, or because too many people in a neighborhood are trying to send or receive data at the same time.

The bandwidth of telephone line is 4 KHz, due to which maximum data rate is restricted. what are those factors which restrict this bandwidth? on the other hand fiber optic has unlimited bandwidth....how?


Bandwidth is determined by the medium used and transmission speed of the networking equipment or Internet .bandwidth is used to view web pages and send data out from your computer (upload) or receive data into your computer (download).Bandwidth is therefore also proportional to the complexity of data for a given level of system performance. so the complexity of data for given system restricts the bandwidth. In digital cable and fiber-optic systems, the demand for ever-increasing data speeds outweighs the need for bandwidth conservation. this is the reason for unlimited bandwidth. In the electromagnetic radiation spectrum, there is only so much available bandwidth to go around, but in hard-wired systems, available bandwidth can be constructed without limit simply by installing more and more cables.

There are different types of TV cable available in the market as RG-7, RG-9, RG-11 and so on, is all these cables have same band width that is 750 MHz or each type has different band width?

RG(Register jack) is the standard connector used in Ethernet standard cables, like twisted pair and coaxial cable. The bandwidth they carry is around 10 Mbps to 100 Mbps, regarldless of the RG numbers, such as RG11, RG7 or RG

A LAN cable connecting a computer with HUB is example of UTP (Unshielded twisted Pair) cable or not ?

Yes commonly in network devices are attached to hub with UTP. Twisted pair cabling comes in two varieties: shielded and unshielded. Unshielded twisted pair (UTP) is the most popular and is generally the best option for school networks. The quality of UTP may vary from telephone- grade wire to extremely high-speed cable. The cable has four pairs of wires inside the jacket. Each pair is twisted with a different number of twists per inch to help eliminate interference from adjacent pairs and other electrical devices. Mostly purchased Category 3 or Category 5. Category 5 cable is highly recommended. A disadvantage of UTP is that it may be liable to radio and electrical frequency interference. Shielded twisted pair (STP) is suitable for environments with electrical interference.

What is Data compression?

Data compression or source coding is the process of encoding information using fewer bits (or other information-bearing units) than an unencoded representation would use, through use of specific encoding schemes.

What is Local loop?

Traditionally, the local loop was wireline in nature from customer to central office, specifically in the form of an electrical circuit (i.e., loop) provisioned as a single twisted pair in support of voice communications. Where the number of local loops was restricted, different customers


could share the same loop, known as a party line. Modern implementations may include a digital loop carrier system segment or fiber optic transmission system known as fiber-in-the-loop.

that who is the owner of internet? who runs it? how does it work? what are the resources for that?

WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management over long distances.

Plz explain the difference between Hub, Modem, Router, and Switches. Plz tell me the uses of it. How does its work? Where do we use it? Plz explain. If there is possible, i want to see them. You can explain with pictures also so that i can see them.???

A hub is used in a wired network to connect Ethernet cables from a number of devices together. The hub allows each device to talk to the others.

A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP?? Network. Routers are located at gateways, the places where two or more networks connect.

A network switch or switching hub is a computer networking device that connects network segments. In networks, a device that filters and forwards packets between LAN segments.

Short for modulator-demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

what are the types of IP. define each.???

IP addresses are basically of two types:

1) Static IP address 2) Shared IP address

An IP Address is a 32-bit number that identifies a computer on the Internet. Every web site on the internet is found not by its domain name but by its IP address.

When someone types the address: www.xyz.com it is translated into an IP address and then the computer is directed to that IP address which is the web site.


Now every single website has an IP address specifically allocated to it. If every site assigned a separate IP address then there could be a problem with running out of IP addresses so a lot of the sites on the server use a single IP address for multiple sites. Thus, using more than one site on an IP address is called a Shared IP address. If a site has its own IP address, and shares with no one else, it is called a Static IP address.

You can always access a site which has a static IP address by using its IP address alone, but you cannot access a site using a shared IP address by typing in the IP address alone because when you type in a shared IP address you arrive at the server but the server does not know which site you are looking for as you have not told it which domain name you want.


What is thick Ethernet and thin Ethernet ? and where is it use ?

Ethernet is the most widely-installed local area network (LAN) technology / architecture. Thick Ethernet also called 10 BASE 5 and Thin Ethernet also called 10 BASE 2 are the two implementations of Ethernet using Coaxial Cables as transmission medium

What is mean by fragility??

Fragility means lacking physical or emotional strength. It also means easily breakable. It is used for the objects that are not made of strong material and the objects which require great care when handled. Fragility is one of the draw backs of Fiber Optics because Fiber Optics is made of glass which is a very fragile material. So, it cannot be used in extreme conditions where hardware portability is required.

What is HUM?

Hum can come from ground loops (cable won't help you if that's the case, but see our isolation transformer below) or, often, from EMI (electro-magnetic interference). High-energy, low- frequency noise, like the 60-cycle hum from nearby power cords, fluorescent lights, and other miscellaneous sources, is the hardest type of interference to shield against, and the best defense here is a dense and highly-conductive braid shield.

What are T connector and Terminators?

T-connector: A T-shaped splitter (one connector to two connectors). For example the BNCT is a T-shaped splitter with one female BNC to two female BNCs (BNCT-1M is one male BNC to two female BNCs)


Terminator: A device attached to the end-points of a bus network or daisy-chain. The purpose of the terminator is to absorb signals so that they do not reflect back down the line. Ethernet networks require a terminator at both ends of the bus, and SCSI chains require a single terminator at the end of the chain.


Why only VLF (3KHz ~ 30Khz) frequency is used for surface propagation; why same frequency cannot be used for e.g. space propagation?

Each frequency range is suited wart to specific layer of atmosphere and is most efficiently transmitted and received by technologies adapted to that layer. Frequency range for surface propagation (3KHz ~ 30Khz) cannot be used for extremely high frequency range of space propagation because radars and satellite communication will not take place with Low frequency band also VLF waves are propagated as surface waves through air so this frequency band is not enough for Extremely High Frequency (EHF)range of space propagation.

The frequency of FM radio is usually 87 ~ 108 MHz, which falls in VHF frequency; and we have studied that in VHF band propagation is either satellite or LOS (line of sight) but FM radio stations are broadcasted; how it is achieved? Sorry sir, i am asking the question of lecture 26 in MDB of 25;

Radio works on VHF (very high frequency) as the channels are operated in MHZ. We receive these channels on the basis of satellite signals with some impact of line of sight in it. It is broadcasted in a sense that all radio devices in the region able to receive it and listen it, as it is simplex mode of communication.

W hat is the meaning of channels in frequency?

Channel represents path of the specific network. Which are allocated different frequencies to represent different ranges of channels?

If i have a high power Radar operating at 4Ghz, and it is facing upward, then does its transmission will pass through ionosphere or not? Will my radar be able to detect satellite in space? Sir one request, first of all i also try to search for answers to my question from Uncle GOOGLE, but once i don't get my precise answer i put it on MDB. I expect specific answers from the instructor in his own words….

As far as your first query is concerned ,for radar operating at a frequency of 4GHZ the transmission is Direct wave line of sight space propagation as this is the super high frequency


band. In your second query you have asked about radar detecting satellite in space ,according to IEEE standards range specified for radar detection is 8.0 to 12.0Ghz if your radar is operating at 4Ghz it will not detect satellite in space. Dear student these MDB's are replied with best possible answers that are comprehensive to get understand by students. it is also advised to you that carefully read replies of your queries and ask one query per mdb. so that you can better understand one topic at a time. if still you are not satisfied then you can ask again and we are here to best facilitate you.

Why VLF cannot pass through the ionosphere layer; whereas SHF (Super High Frequency) can pass through ionosphere layer?

The ionosphere plays a very important role in the propagation of radio waves; the E-layer of the ionosphere reflects the radio waves and enables them to travel beyond the visible horizon. The physiochemical model of the ionosphere is very complicated with a multitude of photochemical reactions going on; ions forming and recombining with the availability or scarcity of energetic photons and particles.

This already complicated model of the ionosphere is further complicated during a Sudden Ionosphere Disturbance (SID) which can be caused by a solar flare or a gamma burst. During a solar flare, the sun releases energy on the order of 1027 to 1032 ergs in the form of electromagnetic waves, energetic particle and mass motions. Although all of this energy obviously does not reach the earth, a fraction of that does reach the earth disrupts the normal balance of ion formation and recombination in the ionosphere.

This disruption disturbs the propagation models of radio waves transmitted through the ionosphere. This disturbance is responsible for distorted waves which can be problematic for the receiver.

During a solar flare, a VLF wave transmitted through the ionosphere shows a sharp rise in intensity. This intensity of the VLF wave rises because the highly ionized D-layer reflects the VLF wave instead of the E-layer. The VLF no longer has to travel through the partially ionized D-layer, while losing its energy, and travel up to the E layer to be reflected.

Hat is the frequency band of mobile phone transmission? Is it a broadcast transmission; if i am in direct LOS(line of Sight) of mobile tower, then i will receive transmission directly; if i am not in LOS then the mobile transmission will be received after reflection from Ionosphere layer? am i correct?

Mobile system which is termed as GSM uses different frequency bands i.e. 900, 1800, 1900. GSM-900 uses 890–915 MHz to send information from the mobile station to the base station (uplink) and 935–960 MHz for the other direction (downlink), providing 124 RF channels


(channel numbers 1 to 124) spaced at 200 kHz. Duplex spacing of 45 MHz is used. Guard bands 100 kHz wide are placed at either end of the range of frequencies.GSM-1800 uses 1710–1785 MHz to send information from the mobile station to the base transceiver station (uplink) and 1805–1880 MHz for the other direction (downlink), providing 374 channels (channel numbers 512 to 885). Duplex spacing is 95 MHz GSM-1800 is also called DCS (Digital Cellular Service) in the United Kingdom, while being called PCS in Hong Kong. Line of sight communication is in the VHF and higher frequencies of the RF spectrum where the wavelength is too short to pass over structures and hills, the transmitter and receiver antennas must be in line. In mobile communication there is only one way which is broadcast when messages regarding services from operators to multiple users are broadcasted.

AM radio band is usually in VLF or LF band (in KHz) does that mean if i install my own high- powered radio station at Islamabad which operates in VLF or LF frequency band; then its transmission can be received by a person sitting in JAPAN; am i correct?

Yes, it is receive anywhere around the globe, if frequency is permitted by that region.

Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF). The frequency allocation is done by ITU.

Can you kindly give us some daily life example of ionosphere propagation?

The ionosphere affects modern technologies such as civilian and military communications, navigation systems and surveillance systems, etc. For many communication and navigation systems, this arises because the systems use signals transmitted to and from satellites, which must therefore pass through the ionosphere. For the most reliable communication and navigation it is necessary to correct the signals for effects imposed by the ionosphere.

The frequency of FM radio is usually 87 ~ 108 MHz, which falls in VHF frequency; and we have studied that in VHF band propagation is either satellite or LOS (line of sight) but FM radio stations are broadcasted; how it is achieved? Sorry sir, i am asking the question of lecture 26 in MDB of 25;

Radio works on VHF (very high frequency) as the channels are operated in MHZ. We receive these channels on the basis of satellite signals with some impact of line of sight in it. It is broadcasted in a sense that all radio devices in the region able to receive it and listen it, as it is simplex mode of communication.


The capability of modems to support 2 wire or 4 wire is it related to telephone network, does it mean that in case of 4-wires i need to hire 02 telephone lines; one to transmission and another for receiving for full duplex mode?

Yes, In case of four-wire (4W) line , a pair of two-wire (2W) lines will be used, one for transmitting and one for receiving, in which the signals in the two directions are to be kept totally separate.

What is MTSO?

MTSO stands for Mobile Telephone Switching Office. The MTSO is the switching office that connects all of the individual cell towers to the Central Office (CO).

The MTSO is responsible for monitoring the relative signal strength of your cellular phone as reported by each of the cell towers, and switching your conversation to the cell tower which will give you the best possible reception.

What is “BAND”?

A band is a small section of the spectrum of radio communication frequencies, in which channels are usually used or set aside for the same purpose.


What is NGN?

Next-generation network (NGN) is a broad term used to describe key architectural evolutions in telecommunication core and access networks.

As it will be more advance network, so it will use the guided and unguided media


What is frequency band?

In telecommunication, a band sometimes called a frequency band is a specific range of frequencies in the radio frequency (RF) spectrum, which is divided among ranges from very low frequencies (elf) to extremely high frequencies (elf). Each band has a defined upper and lower frequency limit.

How can calculate Attenuation in fiber optics?

As we know that Attenuation means any reduction in the strength of a signal. So, imagine a signal travels through a transmission medium and its power is reduced to half. Suppose its original strength or power (P1) is 1 which reduces to its half 0.5 , represented by P2. By putting these values in the formula we can calculate or measure Attenuation.

dB= 10 x Log10 (P2/P1)

dB= 10 x Log10 (0.5 / 1)

dB=10 x Log10 (0.5)

What is FOCUS?

Every line parallel to the line of symmetry (line of sight) reflects off the curve at an angle such that they intersect in a common point called FOCUS.


What are Transmission impairments?

Analog signal consist of varying a voltage with time to represent an information steam. If the transmission media were perfectly, the receiver could receive exactly the same signal that the transmitter sent. But communication lines are usually not perfect, so they receive signal is not the same as the transmitted signal. For digital data this difference can lead to errors. Transmission lines suffers from three major problems

1. Attenuation 2. Delay distortion 3. Noise


What is Los signal?

Line of sight signal (los)los signal is the signal traveling directly following the straight path from transmitting antenna to the receiving antenna without any obstacle in its path. Los signal is also referred to as direct signal or direct wave.

Explain the Concept of line of sight?

Los (line of sight) means something aligned or in the same direction with something. If we talk about microwave's line of sight, it refers to the tower or a repeater which regenerates the signal because microwave needs to have some relay station, in order to avoid signal loss and it must be in line with the one relay station to the other, so to avoid signal loss.

What is Attenuation?

Attenuation is the type of impairment in which signal loses its energy while travelling. It happens when avoiding resistance, the energy of the signal is utilized in breaking the resistance and because of this, attenuation occurs.

What is Distortion?

Distortion is another type of impairment in which signal becomes spitted and lost. It happens in the composite signal, when two or more signals are combined to be sent on the common platform or channel, they combined in such a way that different frequency signals mix up to form distortion.

CS601 Data communication Lecture no 29

What is FDM?

FDM is an analog technique that can be applied when BW of link is greater than the combined bandwidth of the signal to be transmitted.

Difference between MUX and Demur

Multiplexing means combining multiple streams of information for transmission over a shared medium. De-multiplexing performs the reverse function: split a combined stream arriving from a shared medium into the original information streams. As it is clear from definitions they are inverse of each other.


What is Demultiplexing?

Demultiplexing is to separate 2 or more channels that have been multiplexed. Signals are typically multiplexed or combined onto one higher speed channel to efficiently use the bandwidth. When different signals are sent from source to destination, all the signals have to be sent on singal channel from multichannel to save the channel resources, for that process signals have to be multiplexed on receiver side with the help of Multiplexer. Multiplexer combine the signals in one composite signal by modulating every signal on different carrier frequency. On reaching destination the receiver Demultiplex the composite signal for every receiver, this is done with the help of De-Multiplexer. De-multiplexer also demodulate the composite signal on different frequencies for each user.

What is TDM?

TDM is abbreviated as Short for time division multiplexing, It is a digital multiplexing technique that combines data streams by assigning each stream a different time slot in a set. TDM repeatedly transmits a fixed sequence of time slots over a single transmission channel.

What is TX. System?

Those systems which transmit data to destination are called Tx systems, The Term TX is widely used in Satellite Communication, Telecommunication system & data communication system. " TX " stands for Trans and "Rx" stands for Receive.For any Information or data to be communicated over a network there is a node that Generate or transmit that information , The sender node point which is transferring information is "TX" or Transmitter point & the Destination node point that has to receive that information is " Rx" or receiver point.

What is g’(t) and Tax. Stands for

Those systems which transmits data to destination are called Tax systems, The Term TX is widely used in Satellite Communication, Telecommunication system & data communication system. " TX " stands for Trans and "Rx" stands for Receive. For any Information or data to be communicated over a network there is a node that Generate or transmit that information , The sender node point which is transferring information is "TX" or Transmitter point & the Destination node point that has to receive that information is " Rx" or receiver point.

The term g'(t) used in different places for different purposes, as for as lecture number 29 and previous are concerned, I didn’t find this term, kindly give reference of lecture or from where you have read this. Feel free to ask any question regarding lectures or the subject.



What is Asynchronous?

In general, asynchronous (pronounced ay-SIHN-kro-nuhs, from Greek asyn-, meaning "not with," and chronos, meaning "time") is an adjective describing objects or events that are not coordinated in time. In information technology, the term has several different usages. In telecommunication signaling within a network or between networks, an asynchronous signal is one that is transmitted at a different clock rate than another signal. (plesiochronous signals are almost but not quite in synchronization - and a method is used to adjust them - and synchronous signals are those that run at the same clock rate. For example, a telephone conversation is asynchronous because both parties can talk whenever they like. If the communication were synchronous, each party would be required to wait a specified interval before speaking.

What is Bit Stuffing?

Bit stuffing is the insertion of one or more bits into a transmission unit as a way to provide signaling information to a receiver. The receiver knows how to detect and remove or disregard the stuffed bits. Example of bit stuffing, a standard HDLC packet begins and ends with 01111110. To make sure this sequence doesn't appear again before the end of the packet, a 0 is inserted after every five consecutive 1s.

Why do we need Inverse Multiplexing?

•

• • • •

• • •

An organization wants to send data, voice and video each of which requires a different data rate To send voice it needs 64Kbps, To send data, it needs 128 Kbps link To send video it may need 1.544 Mbps link It can lease a 1.544 Mbps line from a common carrier and only use it fully for sometime Or it can lease several separate channels of lower data rates Voice can be sent over any of these channels Data & Video can be broken into smaller portions using Inverse Multiplexing and TX

What is a transceiver?


Transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry.


What is DSU in terms of digital service?

DSU changes rate of digital data created by the subscriber device to 56kbps and encodes it in the format used by service provider. it used in dialing process.DSU is more expensive than modem it has better speed better quality and less susceptibility to noise

What is FTTC?

"Fiber to the curb" (FTTC) refers to the installation and use of optical fiber cable directly to the curbs near homes or any business environment as a replacement for "plain old telephone service" (POTS). Think of removing all the telephone lines you see in your neighborhood and replacing them with optical fiber lines. Such wiring would give us extremely high bandwidth and make possible movies-on-demand and online multimedia presentations arriving without noticeable delay.

What is Bandwidth?

Bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component.

In computer networks, bandwidth is often used as a synonym for data transfer rate - the amount of data that can be carried from one point to another in a given time period (usually a second). This kind of bandwidth is usually expressed in bits (of data) per second (bps).

What are the telephone services provided?

There are two main telephone services.

1. Analog Service 2. Digital Service

Analog Services further has two important types of services.

1. Switched Analog Service


2. Leased Analog Service

CS601 Data communication Lecture no 32

Difference of topology and multiplexing

The physical arrangement of a network; the way in which cables are arranged in order to connect computers in a network is called the topology. In telecommunications and computer networks, multiplexing (also known as muxing) is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire. It originated in telegraphy, and is now widely applied in communications.

What are T Line and E Line?

T line and E line both are digital lines designed to transfer digital data. The concept behind T- line and E-line is same except that Europeans use a version of T-line called E-lines which have different capacity than T-lines

What are the fractional T Lines?

Many subscribers don’t need the entire capacity of the T-line. A small business may need only one-fourth of the capacity of T-line. if four business of same size lie in the same building, they can share T-line.DSU/CSU allow the capacity of T-line to be interleaved in to four channels


What are the Types of Errors?

There are two types of errors

1- Single bit error

2- Burst error

Single Bit & Burst

• In a single-bit error, a 0 is changed to a 1 or a 1 to a 0

Prepared by: Irfan Khan www.virtualians.pk

• •

In a burst error multiple bits are changed For Example, a 0.01 second burst of impulse noise on a TX with a data rate of 1200 bps might change all or some of 12 bits of information

Error Detection

• • • •

Even if we know what type of errors can occur, will we recognize one when we see it? If we have a copy of the intended TX for comparison, of course we will But what if we don’t have a copy of the original Then we will have no way of knowing we have received an error until we have decoded the TX and failed to make sense of it

What are the types of redundancy checks?

There are four types of redundancy checks used in data communication;

1. 2. 3. 4.

Vertical Redundancy check (VCR) Longitudinal Redundancy check(LRC) Cyclic Redundancy check(CRC) Checksum

What is CRC?

Short for cyclic redundancy check, a common technique for detecting data transmission errors. Transmitted messages are divided into predetermined lengths that are divided by a fixed divisor. According to the calculation, the remainder number is appended onto and sent with the message. When the message is received, the computer recalculates the remainder and compares it to the transmitted remainder. If the numbers do not match, an error is detected.

What is Checksum?

A simple error detection scheme in which each transmitted message is accompanied by a numerical value based on the number of set bits in the message. The receiving station then applies the same formula to the message and checks to make sure the accompanying numerical value is the same. If not, the receiver can assume that the message has been garbled.


What is an Error?


An error is the change or the mismatching take place between the data unit sent by transmitter and the data unit received by the receiver e.g. 10101010 sent by sender 10101011 received by receiver. Here is an error of 1 bit.

Define Error Control.

Error control refers to mechanisms to detect and correct errors that occur in the transmission of frames. The most common techniques for error control are based on some or all of the following:

1, Error detection

2. Positive acknowledgement

3. Retransmission after time-out

4. Negative acknowledgement and retransmission.

These mechanisms are also referred as automatic repeat request (ARC)).

Explain about error correction.

Error correction is the mechanism by which we can make changes in the received erroneous data to make it free from error.

The two most common error correction mechanisms are:

(i) Error correction by Retransmission.

(ii) Forward Error Correction.

What are the different types of error? /How a single bit error does differ from a burst error?

A single bit error is an isolated error condition that alters one bit but does not affect nearby bits. On the other hand A burst error is a contiguous sequence of bits in which the first and last bits and any number of intermediate bits are received in error. A single bit can occur in the preserve of while noise, when a slight random deterioration of single-to-noise ratio is sufficient to confuse the receiver’s decision of a single bit. On the other hand burst errors are more common and more difficult to deal with. Burst error can be caused by impulse noise.


Discuss the concept of redundancy in error detection.

It is a most common and powerful technique for the detection of errors. In this technique extra bits are added. But instead of repeating the entire data stream, a shorter group of bits may be appended to the end of each unit. The technique is called redundancy because the extra bits are redundant to the information. They are discarded as soon as the accuracy of transmission has been determined.

Explain the bases of log base 2 and log base 10 ?

In mathematics, a logarithm of a given number to a given base is the power to which you need to raise the base in order to get the number.

Example for base 10

The logarithm of 1000 to the common base 10 = 3 or log(base 10) of 1000 = 3.

Because 10 raised to a power of 3 is 1000 or 10^3 = 1000

Example for base 2

The Logarithm of 8 to common base 2 = 3 or log(base 2) of 8 = 3 Because 2 raised to a power of 3 is 8 or 2^3 = 8.


What is Single bit error?

Single bit error is the error where only one bit of the transmitted data unit is changed from 0 to 1 and 1 to 0. While in burst error two or more bits are changed from 0 to 1 and from 1 to 0. The burst error is un detectable by checksum.

What are timer and sequence number?


TIMER, if sender was not able to get acknowledgment in the particular time than, it sends the buffered data once again to receiver. When sender starts to send the data, it starts timer.

SEQUENCE NUMBER, from this the sender sends the data with the specific sequence number so after receiving the data, receiver sends the data with that sequence number, and here at sender side it also expect the acknowledgment of the same sequence number.

What is Hamming Code?

It is a single bit error correction method using redundant bits.

In this method redundant bits are included with the original data those bits are arrange such that different incorrect bits produce different error results, that incorrect bits can be identified. Once the bit is identified, the receiver can reverse its value and correct the error. Hamming code can be applied to any length of data unit and uses the relationships between the data and the redundancy bits.

What is Flow Control?

Flow Control is one important design issue for the Data Link Layer that controls the flow of data between sender and receiver.

How does the CRC Checker work?

A sequence of redundant bits called CRC or CRC remainder is appended to the end of the data unit, so that the resulting data unit becomes exactly divisible by a second predetermined binary number

At its destination, the data unit is divided by the same number

If at this step, there is no remainder, the incoming data unit is assumed to be intact and is therefore accepted

A remainder indicates that a data unit has been damaged and therefore must be rejected



What are Framing and Flow control?

In computer networking, a frame is a data packet on the Layer 2 of the OSI model. A frame is "the unit of transmission in a link layer protocol, and consists of a link-layer header followed by a packet." Examples are Ethernet frames (maximum 1500 byte plus overhead), PPP frames and V.42 modem frames. In computer networking, flow control is the process of managing the rate of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with data from transmitting node

Differentiate between Hamming Code and Hamming Distance?

In telecommunication, a Hamming code is a linear error-correcting code named after its inventor, Richard Hamming. Hamming codes can detect up to two simultaneous bit errors, and correct single-bit errors; thus, reliable communication is possible when the Hamming distance between the transmitted and received bit patterns is less than or equal to one. By contrast, the simple parity code cannot correct errors, and can only detect an odd number of errors. The Hamming distance between two strings of equal length is the number of positions at which the corresponding symbols are different. Put another way, it measures the minimum number of substitutions required to change one string into the other, or the number of errors that transformed one string into the other


How we calculate the binary division of these no.01001101 divided by 111.

In binary division we will concatenate 0 in quotient left most side until divisor is less than from dividend, if dividend portion is greater than or equal to divisor then we will subtract divisor from dividend portion and will concatenate 1 in quotient left side and so on. For see your required binary example step by step please follows this link


Add your divisor and dividend in below filed of link and then click on start next statement for understand step by step.

What is TTP?

TTP stands for Time triggered protocol which is an open and modular control system platform technology that supports the design of upgradeable, reusable and easy-to-integrate systems As a fault-tolerant time-triggered protocol, TTP provides autonomous fault-tolerant message transport at known times and with minimal jitter by employing a TDMA (Time-Division Multiple Access) strategy on replicated communication channels. TTP offers fault-tolerant clock synchronization that establishes the global time base without relying on a central time server. TTP provides a membership service to inform every correct node about the consistency of data transmission. This mechanism can be viewed as a distributed acknowledgment service that informs the application promptly if an error in the communication system has occurred. If state consistency is lost, the application is notified immediately. Additionally, TTP includes the service of clique avoidance to detect faults outside the fault hypothesis, which cannot be tolerated at the protocol level.


Explain Sliding Window.

Sliding windows, a technique also known as windowing, is used by the Internet's Transmission Control Protocol (TCP) as a method of controlling the flow of packets between two computers or network hosts method of for transferring data over network.TCP sends data across a network in packets and requires the receiving device to send an acknowledgement, or ACK, when it successfully has received the packet, which signals the sending device that another packet can be sent. Window size is the number of data packets that can be sent without waiting for an ACK. With the sliding window method, the receiving device can send a single acknowledgement message for multiple packets of data sent in one window. Within that acknowledgement message is information about the receiving device, which tells the sending device to increase or decrease the number of packets in the next transmission (this is where the "sliding" in the name comes in).

What is the term STP?

STP is Shielded Twisted Pair. It is the same as UTP but has a shield inside to protect from electrical interference. It should be used in Token Ring networks or in Ethernet networks where electrical interference may be a problem. STP cables have a conducting shield made of metallic foil encasing the twisted wire pairs, which blocks out electromagnetic interference, allowing it to carry data at a faster rate of speed.


What is EOT?

EOT stands for end of transmission; this frame is sent to let the transmitter know that the session should be terminated.

Explain Flow control.

Flow control is the process of adjusting the flow of data from one device to another or between nodes on a network to ensure that the receiving device or node can handle all of the incoming data. This is particularly important where the sending device or node is capable of sending data much faster than the receiving device or node can receive it. Too much data arriving before a device or node can handle it causes data overflow, resulting in data loss and possible retransmission.

What is Stop-and-Wait Flow Control?

The simplest form of flow control is stop-and-wait and works as follows. A source entity transmits a frame. After reception, the destination entity indicates its willingness to accept another frame by sending back an acknowledgement to the frame just received. The source must wait until it receives the acknowledgement before sending the next frame. The destination can thus stop flow of data by just withholding the acknowledgement. This method fails if the message transmitted is very large.


What is Asynchronous?

Asynchronous is a communications protocol that transmits one byte of information at a time at irregular time intervals. A start bit (one character) precedes the byte (eight characters) of information, and a stop bit (one character) follows the byte. The sending and receiving computers do not have to be synchronized

Explain sliding Window ARQ

A sliding window protocol is a feature of packet-based data transmission protocols. Sliding window protocols are used where reliable in-order delivery of packets is required, such as in the Data Link Layer (OSI model) as well as in the Transmission Control Protocol (TCP). Conceptually, each portion of the transmission (packets in most data link layers, but bytes in


CS101 Introduction of computing

www.Virtualins .pk Prepared by Imran Baloch

TCP) is assigned a unique consecutive sequence number, and the receiver uses the numbers to place received packets in the correct order, discarding duplicate packets and identifying missing ones. The problem with this is that there is no limit of the size of the sequence numbers that can be required. By placing limits on the number of packets that can be transmitted or received at any given time, a sliding window protocol allows an unlimited number of packets to be communicated using fixed-size sequence numbers.

Explain XMODEM?

XMODEM like most file transfer protocols, breaks up the original data into a series of Packet that are sent to the receiver, along with additional information allowing the receiver to determine whether that packet was correctly received. Files were transferred one packet at a time. When received, the packet's checksum was calculated by the receiver and compared to the one received from the sender at the end of the packet. If the two matched, the receiver sent an ACK (Acknowledgment) message back to the sender, which then sent the next packet in sequence. If there was a problem with the checksum, the receiver instead sent a NAK (Negative acknowledgment) If a NAK was received, the sender would re-send the packet, and continued to try several times, normally ten, before aborting the transfer.

What is Go-Back-N ARQ?

Retransmission begins with the last unacknowledged frame even if subsequent frames have arrived correctly .duplicate frames are discarded. Go-back-n ARQ -- Receiver must get Frames in correct order


How is CRC superior to the two dimensional parity ?

CRC is based on binary division. In CRC, instead of adding bits to achieve a desired parity, a sequence of redundant bits, called the CRC or the CRC remainder, is appended to the end of a data unit so that the resulting data unit becomes exactly divisible by a second, predetermined binary number. At its destination, the incoming data unit is divided by the same number. If at this step there is no remainder the data unit is assumed to be intact and is therefore accepted. A remainder indicates that the data unit has been damaged in transit and therefore must be rejected.

The redundancy bits used by CRC are derived by dividing the data unit by a predetermined divisor; the remainder is the CRC. To be valid, a CRC must have two qualities: It must have exactly one less bit than the divisor, and appending into the end of the data string must make the resulting bit sequence exactly divisible by the divisor.




A CRC checker functions exactly as the generator does. After receiving the data appended with the CRC, it does the samemodulo-2 division. If the remainder is all 0’s, the CRC is dropped and the data are accepted: otherwise, the received stream of bits is discarded and data are resent. We assume that there is no error. The remainder is therefore all 0’s, and the data are accepted.

What is Purpose of the hamming code?

Hamming code is a method of error detection and correction in which every string of four bits is replaced with a string of seven bits. The last three added bits are parity checking bits that the receiving device uses to check for and correct any errors. Hamming code will detect any double errors but can only correct a single error. This method of error correction is best suited for situations in which randomly occurring errors are likely, not for errors that come in bursts.

What is the difference between even parity and odd parity?

"Parity" refers to the number of 1s in a given binary number. Odd parity means there are an odd number of 1s and even parity means that there is an even number of 1s.

How can a parity bit detect a damaged data unit?

In order to reduce errors parity bit is introduced (even parity or odd parity) Suppose If we want to transmit 11010101 using even parity at transmitter then there are total five 1's so as we are using even parity to make the total no. of 1's even the Parity bit required is '1' if we are using odd parity bit required is 0.This parity bit appended to the data as 110101011,and this 110101011 is transmitted in place of 11010101(actual data),So as according to the parity used (even r odd) the receiver counts the no. of 1's in the received data. Process follows as at receiver if received data is 110101011 the parity using here is even so on counting no. of 1's by receiver they are even so No error if odd No. of 1's appear then there is an error Using the Even Parity Using even parity, add the parity bit 1 and the set of data becomes: 11010001 (8 bits of data including the parity bit), total number of 1s becomes even. Using the Odd Parity Using odd parity, total number is already odd, parity bit is set to 0 and the set of data becomes: 01010001 (8 bits of data including the parity bit).





What is HDLC?

HDLC (High-level Data Link Control) is a group of protocols or rules for transmitting data between points (sometimes called nodes). In HDLC, data is organized into a unit (called aflame) and sent across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent. HDLC is one of the most commonly-used protocols in what is layer 2 of the industry communication reference model called Open Systems Interconnection (OSI).

What is difference between L RC and VRC?. VRC:: A redundant bit called parity bit is appended to every data unit so that total number of 1’s in the unit becomes even including the parity bit If 1100001 value is Transmitted, totals number of 1’s is 3 an odd number. Before transmission, this data unit is passed through a parity generator, which counts the 1’s and appends the parity bit (1) to the end ,the total number of 1’s is now 4, an even number, this data is sent across network. When it reaches its destination, Receiver puts all 8 bits through an even parity checking function, if the RX sees 11100001, it counts four ones, an even number and the data unit passes. If the data got corrupted during transit and let say if instead of 11100001, receiver receives 11100101. Then when the parity checker counts the 1’s, it gets 5 an odd number receiver knows that an error has occurred and rejects the whole unit. LRC:: In LRC, a block of bits is organized in a table (rows and columns) For example instead of sending 32 bits, we organize them in a table made of 4 rows and 8 columns We then calculate the Parity bit for each column and create a new row of 8 bits which are the parity bits for the whole block, Keep in mind that the first parity bit in the 5th row is calculated based on all the first bits The second parity bit is calculated based on all the second bits and so on We then attach the 8 parity bits to the original data and send them to the receiver




What are the advantages of bit oriented protocols?

Bit oriented protocols are more useful than character oriented protocols. The most important data link bit oriented protocol is HDLC protocol. A synchronous communications protocol requiring only a single bit to communicate a command signal to the target station. It transmit information without regard to character boundaries and thus handle all types of information images.


What are an AUI cables?

An AUI cable can be used to provide the connection between an external MAU and the Ethernet interface. Each station is linked to its corresponding transceiver by an AUI cable also called the Transceiver cable An AUI is a 15 wire cable with plug that performs the physical layer interface functions b/w the station and the transceiver An AUI has a max. Length of 50 meters and it terminates in a 15-pin DB- 15 connector The AUI cable carries three data signals between the Ethernet interface and MAU: transmit data (from the Ethernet interface to the network), receive data (from the network to the interface), and a collision presence signal (from the network to the interface). Each signal is sent over a pair of wires. Another pair of wires are used to carry 12 volt DC power from the Ethernet interface to the MAU. The standard AUI cable is relatively thick (approx. 1cm or 0.4 inch diameter), and may be up to 50 meters (164 feet) long.

What is Spike?

Spike is a medal conductor, behaves like a connector. Spike is attached to wires inside the transceiver. When the cable is pressed into the well, the spike pierces the jacket and sheathing layer and makes electrical connection between transceiver and cable. This type of connector is often called vampire tab because it bits the cable

What is a Local-Area Network (LAN)?

A local-area network (LAN) is a computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings, however, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN).


Define BNC T-connectors?




BNC T-connectors (used with the 10Base-2 system) are female devices for connecting two cables to a Network interface card. A BNC barrel connector allows connecting two cables together. BNC connectors can also be used to connect some Monitors, which increases the accuracy of the signals sent from the Video Adaptor , N is the number of multiple devices attached to 10 Mbps hub .

What is FDDI?

FDDI defines three time registers to control circulation of the token and distribute link –access opportunities among the nodes equitably. Each station has three registers. The registers hold time values that control the operation of the ring. These values are set when the ring is initialized and do not vary in the course of operation. The registers are called synchronous allocation (SA), target token rotation time (TTRT), and absolute maximum time (AMT).

What are the advantages of Ethernet?

• • •

Reduced cost and ease of installation Because the cable is lighter weight and more flexible than that used in Thicket

What are the disadvantaged of Ethernet?

• •

Short Range (185 m as opposed to 500 m) Smaller Capacity (thinner cable accommodates fewer stations)


What is the Difference b/w "FDDI" & "Multi Station Access Unit?

MSAU is combination of automatic switches with maximum of 8.While FDDI is new standard with higher data rate than MSAU.

What is Station Procedure?

• • • • • •

THT is set to the difference between TTRT and TRT THT = TTRT - TRT TRT is reset to zero (TRT = 0) The station sends S-frames during the time in SA The station sends A-frames as long as THT ³ 0 Release the token

Prepared by: Irfan Khan www.virtualians.pk


What is MAU?

• Individual automatic switches are combined in to a hub • One MAU can support up to 8 stations • Although it looks like a star, it is in fact a ring


What are Repeaters?

Network repeaters regenerate incoming electrical, wireless or optical signals. With physical media like Ethernet or Wi-Fi, data transmissions can only span a limited distance before the quality of the signal degrades. Repeaters attempt to preserve signal integrity and extend the distance over which data can safely travel.

Actual network devices that serve as repeaters usually have some other name. Active hubs, for example, are repeaters. Active hubs are sometimes also called "multiport repeaters," but more commonly they are just "hubs." Other types of "passive hubs" are not repeaters. In Wi-Fi, access points function as repeaters only when operating in so-called "repeater mode."

What are routers?

Routers are small physical devices that join multiple networks together. Technically, a router is a Layer 3 gateway device, meaning that it connects two or more networks and that the router operates at the network layer of the OSI model.

Home networks typically use a wireless or wired Internet protocol(IP) router, IP being the most common OSI network layer protocol. An IP router such as a DSL or cable modem broadband routers joins the home's local network area (LAN) to the wide- area network (WAN)of the Internet.

What does TCP/IP do?

Transmission Control Protocol/Internet Protocol (TCP/IP) is a suite of standard protocols for connecting computers across networks. TCP/IP enables Windows-based computers to connect and share information with other Microsoft and non-Microsoft systems.

Who does this feature apply to?

All users who use TCP/IP to connect and communicate information over a network should be aware of the changes incorporated in Windows Server 2003 Service Pack 1.


What new functionality is added to this feature in Windows Server 2003 Service Pack 1?

SYN attack protection is enabled by default

When a SYN attack is detected, TCP/IP in Windows Server 2003 and Windows XP lowers the number of retransmissions of the SYN-ACK segment and does not allocate memory or table entry resources for the connection until the TCP three-way handshake has been completed..

What is the Purpose of Encoding Techniques?

Encoding techniques are used to represent these 0s,1,s on some voltages level (into signals).So when you are asked to encode it, represent these 0's,1's on time lines graphs using specific schema (NRZ-I,NRZ-L etc.).


FINALTERM EXAMINATION

Spring 2010

CS601- Data Communication

Question No: 31 ( Marks: 2 )

What are the advantages of a multipoint connection over a point-to-point connection?

Answer:

Point-to-point connection is limited to two devices, where else more than two devices share a

single link in multipoint connection. Multipoint connection can be used for fail-over and

reliability.


What's the name of the telephone service in which there is no need of dialing.

Answer:

DSS (digital data service) is the telephone service in which there is no need of dialing.


Which type of frames are present in BSC frames?

Answer:

There are two types of frames that are present in BSC.

1. 1. Control Frames and

2. 2. Data Frames


What methods of line discipline are used for peer to peer and primary secondary

communication?

Answer:

Line discipline is done in two ways:

1. ENQ/ACK (Enquiry Acknowledgement)

This is used for peer to peer communication.

1. Poll/ Select

This method is used for primary secondary communication.


How does the checksum checker know that the received data unit is undamaged? [3]

Answer:

Checksum Checker or generator:

The sender subdivides data units into equal segments of ‘n’ bits(16 bits)

1. These segments are added together using one’s complement.

2. The total (sum) is then complemented and appended to the end of the original data unit as

redundancy bits called CHECKSUM.

3. The extended data unit is transmitted across the network.

4. The receiver subdivides data unit and adds all segments together and complement the result.

5. If the intended data unit is intact, total value found by adding the data segments and the checksum

field should be zero.

6. 6. If the result is not zero, the packet contains an error & the receiver rejects it.


Which one has more overhead, a repeater or a bridge? Explain your answer. [3]

Answer:

A bridge has more overhead than a repeater. A bridge processes the packet at two

layers ; a repeater processes a frame at only one layer. A bridge needs to search a

table and find the forwarding port as well as to regenerate the signal; a repeater

only regenerates the signal. In other words, a bridge is also a repeater (and more); a

repeater is not a bridge.


Write down disadvantages of Ring Topology.

Answer:

Disadvantages of Ring Topology

Unidirectional Traffic

A break in a ring that is a disabled station can disable the entire network

Can be solved by using:

Dual Ring or

A switch capable of closing off the Break


How parity bits are counted in VRC error detection method technique in case of odd parity

generator?

Answer:

For example:

We want to TX the binary data unit 1100001

Adding together the number of 1’s gives us 3, an odd number

Before TX, we pass the data unit through a parity generator, which counts the 1’s and appends the

parity bit (1) to the end

The total number of 1’s is now 4, an even number

The system now transfers the entire expanded across the network link

When it reaches its destination, the RX puts all 8 bits through an even parity checking function

If the RX sees 11100001, it counts four ones, an even number and the data unit passes

When the parity checker counts the 1’s, it gets 5 an odd number

The receiver knows that an error has occurred somewhere and therefore rejects the whole unit

Some systems may also use ODD parity checking

The principal is the same as even parity


How are lost acknowledgment and a lost frame handled at the sender site? [5]

Answer:

At some error rates (16%-20%) the protocol hung up in an infinite loop, while it worked fine for

other error rates. On examining the code it was determined that this problem resulted from

improper variable initialization. On these certain error rates the pseudo-random number generator

caused the very first frame sent to be lost or damaged. The receiver used a variable to keep track

of the last in sequence frame received. This was erroneously initialized to 0. Therefore if the first

frame got lost (sequence no 0), when the receiver received the second frame (sequence number 1)

it sent an acknowledgment for the last in sequence frame received, which had been initialized to

0. Therefore the sender received an acknowledgement for sequence number 0 and moved its

window up accordingly. It caused everything to get out of synch, and caused the protocol to go

into infinite loop. This was resolved by initializing the variable to remember the last in sequence

frame received to an out of range sequence number.


Explain Protocol Data Unit (PDU)?

Answer:

Protocol data unit (PDU) is an OSI term that refers generically to a group of information added or

removed by a particular layer of the OSI model. In specific terms, an LxPDU implies the data and

headers defined by layer x. Each layer uses the PDU to communicate and exchange information.

The PDU information is only read by the peer layer on the receiving device and then stripped off,

and data is handed over to the next upper layer.

FINALTERM EXAMINATION Spring 2009

CS601- Data Communication


What are the conditions for the polynomial used by the CRC generator?

CRC generator:

CRC generator (the divisor) is most often represented not as a1’s and 0’s but as an algebraic

polynomial.

conditions for the polynomial:

it should have following properties:

It should not be divisible by “x”.

It should not be divisible by “x+1”.

The first condition guarantees that all burst error of a length equal to degree of the polynomial is

detected.

The 2nd condition guarantees that all burst error affecting an odd number of bits are detected.


What are intelligent modems?

Intelligent modems:

A modem that responds to commands and can accept new instructions during online

transmission. It was originally developed by Hayes.

Example:

• Automatic answering,

• Dialing etc.


What is the basic purpose of Router?

Basic purpose of Router:

"A router is a device that extracts the destination of a packet it receives, selects the best path to

that destination, and forwards data packets to the next device along this path. They connect

networks together;

a LAN to a WAN for example, to access the Internet.

"A more precise definition of a router is a computer networking device that interconnects separate

logical subnets."



The fractional T Lines:

Many subscribers don’t need the entire capacity of the T-line.

For example,

A small business may need only one-fourth of the capacity of T-line. if four business of same

size lie in the same building, they can share T-line.DSU/CSU allow the capacity of T-line to be

interleaved in to four channels


What are the light sources used for optic fiber?

• light sources used for optic fiber:

• The light source can weather be an LED or ILD

• LED (Light emitting diode) cheaper but provide unfocused light that strikes the boundaries of

channel at uncontrollable angles.

• Limited to short distance use.

• LASSER

• Can be focused to a narrow range allowing control over angle of incidence.


What is Multi Access Unit (MAU) in Token Ring?

Multi Access Unit (MAU) in Token Ring:

• Individual automatic switches are combined in to a hub

• One MAU can support up to 8 stations.

• Although it looks like a star, it is in fact a ring.


Give characteristics of Dual Ring, if necessary then draw the diagram. [5]

Characteristics of Dual Ring:

A network topology in which two concentric rings connect each node on a network instead of one

network ring that is used in a ring topology. Typically, the secondary ring in a dual-ring topology

is redundant. It is used as a backup in case the primary ring fails. In these configurations, data

moves in opposite directions around the rings. Each ring is independent of the other until the

primary ring fails and the two rings are connected to continue the flow of data traffic.

What the receiver will receive if the checksum method is applied to the following

bit.

10101001 00111001

Ans:

the receiver will receive the checksum method is applied to the following bit.

10101001 00111001

10101001 00111001

Sum of 2 bits are

10101001

00111001

-------------------

11100010

00011101 1's complement

1

--------------

00011110 2's complement

-----------

10101001 00111001 ==> 00011110

So the data transmitted which will receiver get:

10101001 00111001 00011110


What is rafraction in terms of optic fiber? Give one example.

Refraction:

Light travels in a straight line as long as it is moving through a single uniform structure If a ray

of light traveling through one substance enters another (more or less dense) substance, its speed

changes abruptly causing the ray to change direction. This phenomenon is called Refraction.

Refraction in terms of optic fiber:

the propagation of light in an optical fiber which in its simplest form consists of a circular core of

uniform refractive index surrounded by a cladding of slightly lower refractive index. The light is

launched into the entrance face of the fiber.

The light is propagated by the total internal reflection at the interface between core and cladding.

However the rays incident at angles larger than a certain angle, called the cut-off angle, suffer

both refraction and reflection at the interface between the core and the cladding.

They, therefore, are not guided. Due to this the optical fiber has a numerical aperture. The

numerical aperture is given by the square root of (n12-n2

2). Typical values of numerical aperture

lie between 0.1 and 0.3.

The refractive indices of the core and the cladding are n1 and n 2 respectively. The fiber is

normally in air (n0=1) but could also be in a medium of refractive index n0.


What are the asynchronous protocols in data link layer? Discuss in detail with examples. [10

marks]

Asynchronous protocols in data link layer:

Asynchronous communication at the data link layer or higher protocol layers is known as

statistical multiplexing or packet mode communication,

For example :

Asynchronous transfer mode (ATM). In this case the asynchronously transferred blocks are called

data packets,

Async protocols in Data link layer is called statistical multiplexing. for example ATM cells.

The opposite is circuit switched communication, which provides constant bit rate, for example

ISDN and SONET/SDH.

The packets may be encapsulated in a data frame, with a frame synchronization bit sequence

indicating the start of the frame, and sometimes also a bit synchronization bit sequence, typically

01010101, for identification of the bit transition times. Note that at the physical layer, this is

considered as synchronous serial communication.

Examples of packet mode data link protocols that can be/are transferred using synchronous serial

communication are the

• HDLC,

• Ethernet,

• PPP and

• USB protocols.

what are the conditions for the polynomial used by the CRC generator?

Ans:

• The first condition guarantees that all burst errors of a length equal to the degree of the

polynomial are detected

• The 2nd

guarantees that all burst errors affecting an odd number of bits are detected

writes down names of the control frames in XMODEM?

Control frames from the receiver

ACK: Acknowledgement

NAK: Error or start of transmission

CAN: Aborts the transmission

defines frequency Modulation? [3]

Frequency Modulation (FM)

Frequency of carrier signal is changed according to the amplitude of modulating

signal

o Amplitude and Phase of the carrier signal remain constant

Q#4 what is ETX in data frames of data link layer? [3]

Q#5 what are three types of digital service? [3]

ANs: Three types of digital service are:

Switched / 56 Service: It is digital version of Analog switched line Allows data rates of up to 56

Kbps. Both parties must subscribe to the service. Digital Data Service (DDS) : Digital version of analog leased line. Max. data rate of 64Kbps. Like switched/56, DDS requires a DSU (Digital service unit). Digital Signal (DS) Service: Telephone companies saw the need to de velop a hierarchy of

digital services much like those used for Analog services.

Q#6 what is selective Reject ARQ in sliding window? [5]

Q#7 Give methods of Analog to Analog conversion? [2]

Analog To Analog Conversion Methods

Amplitude Modulation (AM)

o Amplitude of carrier signal is changed according to the amplitude of

modulating signal

o Frequency and phase of the carrier remain the same

� Frequency Modulation (FM)

o Frequency of carrier signal is changed according to the amplitude of modulating

signal

o Amplitude and Phase of the carrier signal remain constant

Phase modulation (PM) o Simpler hardware requirements o Phase is modulated with the

amplitude o Amplitude & Frequency of the carrier signal remain constant

Q#8 whether in Asynchronous or Synchronous TDM, addressing in used?

Answer: (Page 158) Addressing is used only in Asynchronous TDM

Q#9 what is the basic purpose of Router?

Basic purpose of Router: A router is a device that extracts the destination of a packet it receives,

selects the best path to that destination, and forwards data packets to the next device along this path.

They connect networks together; a LAN to a WAN for example, to access the Internet

Q#10 what are the possible consequences of replacing a bridge with a routes?


What are the advantages of a multipoint connection over a point-to-point connection?

Answer:

Point-to-point connection is limited to two devices, where else more than two devices share a

single link in multipoint connection. Multipoint connection can be used for fail-over and

reliability.


What's the name of the telephone service in which there is no need of dialing.

Answer:

DSS (digital data service) is the telephone service in which there is no need of dialing.


Which type of frames are present in BSC frames?

Answer:

There are two types of frames that are present in BSC.

1. 1. Control Frames and

2. 2. Data Frames


What methods of line discipline are used for peer to peer and primary secondary

communication?

Answer:

Line discipline is done in two ways:

1. ENQ/ACK (Enquiry Acknowledgement)

This is used for peer to peer communication.

2.Poll/ Select

This method is used for primary secondary communication.


How does the checksum checker know that the received data unit is undamaged? [3]

Answer:

Checksum Checker or generator:

The sender subdivides data units into equal segments of ‘n’ bits(16 bits)

1. These segments are added together using one’s complement.

2. The total (sum) is then complemented and appended to the end of the original data unit as

redundancy bits called CHECKSUM.

3. The extended data unit is transmitted across the network.

4. The receiver subdivides data unit and adds all segments together and complement the result.

5. If the intended data unit is intact, total value found by adding the data segments and the checksum

field should be zero.

6. If the result is not zero, the packet contains an error & the receiver rejects it.


Which one has more overhead, a repeater or a bridge? Explain your answer. [3]

Answer:

A bridge has more overhead than a repeater. A bridge processes the packet at two

layers ; a repeater processes a frame at only one layer. A bridge needs to search a

table and find the forwarding port as well as to regenerate the signal; a repeater

only regenerates the signal. In other words, a bridge is also a repeater (and more); a

repeater is not a bridge.


Write down disadvantages of Ring Topology.

Answer:

Disadvantages of Ring Topology

• Unidirectional Traffic

• A break in a ring that is a disabled station can disable the entire network

• Can be solved by using:

• Dual Ring or

• A switch capable of closing off the Break


How parity bits are counted in VRC error detection method technique in case of odd parity

generator?

Answer:

For example:

We want to TX the binary data unit 1100001

Adding together the number of 1’s gives us 3, an odd number

Before TX, we pass the data unit through a parity generator, which counts the 1’s and appends the

parity bit (1) to the end

The total number of 1’s is now 4, an even number

The system now transfers the entire expanded across the network link

When it reaches its destination, the RX puts all 8 bits through an even parity checking function

If the RX sees 11100001, it counts four ones, an even number and the data unit passes

When the parity checker counts the 1’s, it gets 5 an odd number

The receiver knows that an error has occurred somewhere and therefore rejects the whole unit

Some systems may also use ODD parity checking

The principal is the same as even parity


How are lost acknowledgment and a lost frame handled at the sender site? [5]

Answer:

At some error rates (16%-20%) the protocol hung up in an infinite loop, while it worked fine for

other error rates. On examining the code it was determined that this problem resulted from

improper variable initialization. On these certain error rates the pseudo-random number generator

caused the very first frame sent to be lost or damaged. The receiver used a variable to keep track

of the last in sequence frame received. This was erroneously initialized to 0. Therefore if the first

frame got lost (sequence no 0), when the receiver received the second frame (sequence number 1)

it sent an acknowledgment for the last in sequence frame received, which had been initialized to

0. Therefore the sender received an acknowledgement for sequence number 0 and moved its

window up accordingly. It caused everything to get out of synch, and caused the protocol to go

into infinite loop. This was resolved by initializing the variable to remember the last in sequence

frame received to an out of range sequence number.


Explain Protocol Data Unit (PDU)?

Answer:

Protocol data unit (PDU) is an OSI term that refers generically to a group of information added or

removed by a particular layer of the OSI model. In specific terms, an LxPDU implies the data and

headers defined by layer x. Each layer uses the PDU to communicate and exchange information.

The PDU information is only read by the peer layer on the receiving device and then stripped off,

and data is handed over to the next upper layer.


What is the formula to calculate the number of redundancy bits required to correct a bit error in a

given number of data bits? [2]

Messages(frames) consist of m data (message) bits, yielding an n=(m+r)-bit codeword.


What is R G rating of coaxial cable?

Different coaxial cable designs are categorized by their Radio government (

RG ) ratings

Each cable defined by RG rating is adapted for a specialized function:

RG-8

• Used in Thick Ethernet

RG-9


RG-11


RG-58

• Used in Thin Ethernet

RG-59

• Used for TV


What are the advantages of thin ethernet?

The advantages of thin Ethernet are :

• reduced cost and

• ease of installation

Because the cable is lighter weight and more flexible than that used in Thicknet


What is the difference between a unicast, multicast, and broadcast address? [3]

Three methods can be used to transmit packets over a network: unicast, multicast, and broadcast.

Unicast involves communication between a single sender and a single receiver. This is a type of

point-to-point transmission; since the packet is transmitted to one destination at a time.

Multicast is used to send packets to a group of addresses, represented by a "group address." In

this case, packets are transmitted from a single sender to multiple receivers. Since the same data

packet can be sent to multiple nodes by sending just one copy of the data, the load of the sender

and the overall load of the network are both reduced.

Broadcast involves sending packets to all nodes on a network simultaneously. This type of

transmission is used to establish communication with another host, and for DHCP type methods

of assigning IP addresses.


T lines are designed for Digital data how they can be used for Analog Transmission ?

T Lines are digital lines designed for digital data however; they can also be used for analog

transmission (Telephone connections). Analog signals are first sampled and the Time

Multiplexed.


What are the three types of Guided Media?

Guided Media, are those media that provide a conduit from one device to another. Three types are

1. Twisted pair cable

2. Coaxial cable

3. Fiber-optic Cable


Why do we need Inverse Multiplexing? [5]

Data & Video can be broken into smaller portions using Inverse Multiplexing and TX. An inverse

multiplexer (often abbreviated to "inverse mux" or "imux") allows a data stream to be broken into

multiple lower data rate communication links. An inverse multiplexer differs from

a demultiplexer in that each of the low rate links coming from it is related to the others and they

all work together to carry their respective parts of the same higher rate data stream. By contrast,

the output streams from a demultiplexer may be completely independent from each other and the

demultiplexer does not have to understand them in any way.

This is the opposite of a multiplexer which creates one high speed link from multiple low speed

ones.

It can lease a 1.544 Mbps line from a common carrier and only use it fully for

sometime

Or it can lease several separate channels of lower data rates

Voice can be sent over any of these channels


Describe method of checksum briefly?

The sender subdivides data units into equal segments of ‘n’ bits(16 bits).These segments are

added together using one’s complement. The total (sum) is then complemented and appended to

the end of the original data unit as redundancy bits called CHECKSUM. The extended data unit is

transmitted across the network. The receiver subdivides data unit as above and adds all segments

together and complement the result. If the intended data unit is intact, total value found by adding

the data segments and the checksum field should be zero. If the result is not zero, the packet

contains an error & the receiver rejects it


Explain Asynchronous Time Division Multiplexing in detail? Also discuss its advantages over

synchronous TDM?

Asynchronous time-division multiplexing (ATDM) is a method of sending information that

resembles normal TDM, except that time slots are allocated as needed dynamically rather than

preassigned to specific transmitters. ATDM is more intelligent and has better bandwidth

efficiency than TDM.

Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in which

two or more signals or bit streams are transferred apparently simultaneously as sub-channels in

one communication channel, but are physically taking turns on the channel. The time domain is

divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte

or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2,

etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the

cycle starts all over again with a new frame, starting with the second sample, byte or data block

from sub-channel 1, etc.

asynchronous time-division multiplexing comprising receive circuits (CRl/i) supplying cells

received via input links, transmit circuits (CTl/j) transmitting retransmitted cells on output links, a

buffer memory (MT) storing the received cells and delivering the cells to be retransmitted and a

buffer memory addressing device (SMT) including a write address source (SAE) and a read

address source (fsl/j).

The switching unit further comprises a write disabling circuit (pi) conditioned by a signal (adl)

derived from the content of at least one received cell or a signal (tle) derived from the absence of

any received cell and supplying a disabling signal (spi) and the address source includes a

disabling device (pac, pal) influenced by the disabling signal (spi) so that no memory location is

then occupied in the buffer memory (MT).

Advantages asynchronous TDM:

In asynchronous TDM, the timeslots are not fixed. They are assigned dynamically as needed.

In order to reduce the communications costs in time-sharing systems and multicomputer

communication systems, multiplexing techniques have been introduced to increase channel

utilization. A commonly used technique is Synchronous Time Division Multiplexing (STDM). In

Synchronous Time Division Multiplexing, for example, consider the transmission of messages

from terminals to computer, each terminal is assigned a fixed time duration. After one user's time

duration has elapsed, the channel is switched to another user. With synchronous operation,

buffering is limited to one character per user line, and addressing is usually not required. The

STDM technique, however, has certain disadvantages. As shown in Figure 1, it is inefficient in

capacity and cost to permanently assign a segment of bandwidth that is utilized only for a portion

of the time. A more flexible system that efficiently uses the transmission facility on an

"instantaneous time-shared" basis could be used instead. The objective would be to switch from

one user to another user whenever the one user is idle, and to asynchronously time multiplex the

data. With such an arrangement, each user would be granted access to the channel only when he

has a message to transmit. This is known as an Asynchronous Time Division Multiplexing

System (ATDM). A segment of a typical ATDM data stream is shown in Figure 2. The crucial

attributes of such a multiplexing technique are:

1. An address is required for each transmitted message, and

2. Buffering is required to handle the random message arrivals.

Question 1: [5]

Match the following to one of the five layers.

Answer:

a. Route determination--------------------Network Layer

b. Flow control--------------------Data Link Layer & Transport Layer

c. Interface to physical world------------------------ Application Layer

d. Provide access to the network for the end users--------------- Application Layer

e. Packet switching--------------------- Session Layer

Question 2: [5]

If the throughput at the connection between the device and transmission medium is 5 Kbps,

how long does it take to send 400,000 bits out of this device?

Answer:

Time taken= 5*0.38146972

Time taken= 1.9073sec

Question 3: [5]

Give the Signal level for each line encoding from following.

Answer:

a.Unipolar

This encoding is called unipolar because it uses only one polarity

This encoding is simple and uses only one technique and this encoding is almost obsolete today,

however study of this encoding make us familiar with many encoding problems, complexities and

techniques.

b. NRZ-L

Non Return to Zero

level of the signal is represented by the data (Bits) it represents.

c. NRZ-I

1 Bit is represented by the inversion of the level.

0 is represented by no change in level

d. RZ

Some time, data contains long strings of 1’s or 0’s, RZ can loose its timing

Signal changes not between the bits but during each bit

The only problem with RZ encoding is that it requires two signal changes to encode one bit and

therefore occupies more BANDWIDTH

e. AMI

(Alternate Mark Inveraion) it the type of bipolar encoding.

By inverting on each occurrence of 1, AMI accomplishes 2 things i.e ;

1-Long sequence of 1’s stay synchronized & 2-The DC component is zero.

What kind of error is undetectable by the checksum? [2]

Error detection method used by the higher layers like VRC,LRC,CRC,checksum is also based on

the concept of redundancy

what are the categories of multiplexing?

There are three catogaries of multiplxing

FDM

TDM

Have a two other catagories

Synchronous and asyrounce

WDM

Question:Compare line decipline methods ENQ/ACK and Poll/ Select?

=>ENQ/ACK coordinates which device may start a transmission and whether or not the intended

recipient is ready and enabled.

=> Using ENQ/ACK, a session can be initiated by either station on a link as long as both are of

equal rank.

=> In both half-duplex and full-duplex transmission, the initiating device establishes the session.

=> In half duplex, the initiator then sends its data while the responder waits. The responder may

take over the link when the initiator is finished or has requested a response.

=> In full duplex, both devices can transmit simultaneously once the session has been established.

POLL/SELECT:

=> The poll/select method of line discipline works with topologies where one device is

designated as a primary station and the other devices are secondary stations.

=> Multipoint systems must coordinate several nodes, not just two.

=> The question to be determined in these cases, therefore, is more than just, are you ready? It is

also, which of the several nodes has the right to use the channel?

Define high frequency [HF] and super high frequency [SHF], which devices uses these

frequencies [3]

High Frequency (HF) : Use ionospheric propagation

These frequencies move into the ionosphere where the density difference reflects them back on

earth .Used for Citizen’s Band Radio, International Broadcasting, Military Communication,

Telephone, Telegraph and Fax

Super High Frequency (SHF) (Figure)

o SHF waves are TX using mostly line-of-sight and some Space propagation

o Used for Terrestrial and Satellite Microwave and Radar Communication


Define Sampling Rate.

Answer: (page82)

The accuracy of any digital reproduction of an analog signal depends upon the o. of samples taken


Write down the types of digital to analog modulation.

Answer: (PAGE 85)

Types of digital to analog modulation

ASK

FSK

PSK

QAM


What is periodic signal?

Answer: (page58)

A signal is called Periodic if it completes a pattern within a measurable time frame called a Period

and then repeats that pattern over identical subsequent Periods


How bandwidth of a Modem is measured?

Answer: (page111)

Bandwidth of a modem is not measured in bytes, it is measured in bits.

Modem Speed –ASK (full-duplex)

Effective bandwidth of a telephone line is 2400 Hz, the maximum buad rate is

also 2400.

Baud rate and bit rate are equal for ASK, so maximum bit rate is also 2400 bps.


Briefly describe Rerturn to Zero (RZ) encoding scheme?

Anwer: (page75)

§ Any time, data contains long strings of 1’s or 0’s, Rx can loose its timing

§ In unipolar, we have seen a good solution is to send a separate timing signal but this solution is

both expensive and full of error

§ A better solution is to somehow include synch in encoded signal somewhat similar to what we did

in NRZ-I but it should work for both strings of 0 & 1

§ One solution is RZ encoding which uses 3 values : Positive, Negative and Zero

§ Signal changes not b/w bits but during each bit

§ Like NRZ-L , +ve voltage means 1 and a –ve voltage means 0, but unlike NRZ- L, half way through

each bit interval, the signal returns to zero

§ A 1 bit is represented by positive to zero and a 0 is represented by negative to zero transition

§ The only problem with RZ encoding is that it requires two signal changes to encode one bit and

therefore occupies more BANDWIDTH

§ But of the 3 alternatives we have discussed, it is most effective value.


Compare Phase Modulation(PM) with Frequency Modulation(FM)?

Answer: (page 96-97)

Phase modulation (PM)

Ø Simpler hardware requirements

Ø Phase is modulated with the amplitude

Ø Amplitude & Frequency of the carrier signal remain constant


Ø Frequency of carrier signal is changed according to the amplitude of modulating signal

Ø Amplitude and Phase of the carrier signal remain constant

FM Bandwidth

Bandwidth of FM signal (modulated signal) = 10 * bandwidth of modulating signal Significant

spectrum of FM audio = 15 KHz

Minimum 150 KHz bandwidth

Write all steps of checksum method. [3]

o The sender subdivides data units into equal segments of ‘n’ bits(16 bits)

o These segments are added together using one’s complement

o The total (sum) is then complemented and appended to the end of the original data unit as

redundancy bits called CHECKSUM

o The extended data unit is transmitted across the network

o The receiver subdivides data unit as above and adds all segments together and complement the

result o If the intended data unit is intact, total value found by adding the data segments and the

checksum field should be zero o If the result is not zero, the packet contains an error & the

receiver rejects it.


What are Guard Bands in Multiplexing?

Ans: Channels must be separated by strips of unused BW (guard bands) to prevent signals from

Overlapping



Many subscribers do not need the entire capacity of the T line.

For example, a small business may need only one-fourth of the capacity of a T-1 line. If four business of same size lie in same building, they can shae a T-1 line. DSU/CSU allow the

capacity of T-1 line to be interleaved into 4 channels.

Question No: ( Marks: 10 )

In how many ways TDM can be implemented? Describe any one with example.

Implementation of TDM TDM can be implemented in two ways:

–Synchronous TDM

–Asynchronous TDM

Synchronous TDM

o The term synchronous has a different from that used in other areas of

telecommunication

o Here synchronous means that MUX allocates exactly the same time slot to each device

at all device whether or not the device has any thing to transmit

� Synchronous TDM Example

o Time slot A for example is assigned to device A alone and cannot be used by any other

device

o Each time its allocated time slot comes up a device has the opportunity to send a

portion of its data

o If a device is unable to transmit or does not have data to send time slot remains empty

. What is coaxial cable?

Instead of having two wires, coaxial cable has a central core conductor of solid or stranded

wire (usually copper) enclosed in an insulating sheath

o This is in turn encased in an outer conductor of metal foil, braid or a combination of the

two

o The outer metallic wrapping serves both as a shield against Noise and as the second

conductor which completes the circuit

o This outer conductor is also enclosed in an insulating sheath and the whole cable is

protected by a plastic cover


What is the formula to calculate the number of redundancy bits required to correct a bit error

in a given number of data bits? [2]



What is R G rating of coaxial cable? http://www.vustudents.net


RG ) ratings


RG-8


RG-9


RG-11


RG-58


RG-59

• Used for TV









Three methods can be used to transmit packets over a network: unicast,

multicast, and broadcast.

Unicast involves communication between a single sender and a single receiver.

This is a type of point-to-point transmission; since the packet is transmitted to

one destination at a time.

Multicast is used to send packets to a group of addresses, represented by a

"group address." In this case, packets are transmitted from a single sender to

multiple receivers. Since the same data packet can be sent to multiple nodes by

sending just one copy of the data, the load of the sender and the overall load of

the network are both reduced.

Broadcast involves sending packets to all nodes on a network simultaneously.

This type of transmission is used to establish communication with another host,

and for DHCP type methods of assigning IP addresses.





Multiplexed.



Guided Media, are those media that provide a conduit from one device to another. Three

types are


2. Coaxial cable




Data & Video can be broken into smaller portions using Inverse Multiplexing and TX. An

inverse multiplexer (often abbreviated to "inverse mux" or "imux") allows a data stream to be

broken into multiple lower data rate communication links. An inverse multiplexer differs

from a demultiplexer in that each of the low rate links coming from it is related to the others

and they all work together to carry their respective parts of the same higher rate data stream.

By contrast, the output streams from a demultiplexer may be completely independent from

each other and the demultiplexer does not have to understand them in any way.

This is the opposite of a multiplexer which creates one high speed link from multiple low

speed ones.


sometime






added together using one’s complement. The total (sum) is then complemented and appended

to the end of the original data unit as redundancy bits called CHECKSUM. The extended data

unit is transmitted across the network. The receiver subdivides data unit as above and adds all

segments together and complement the result. If the intended data unit is intact, total value

found by adding the data segments and the checksum field should be zero. If the result is not

zero, the packet contains an error & the receiver rejects it


Explain Asynchronous Time Division Multiplexing in detail? Also discuss its advantages

over synchronous TDM?


resembles normal TDM, except that time slots are allocated as needed dynamically rather

than preassigned to specific transmitters. ATDM is more intelligent and has better bandwidth


Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in

which two or more signals or bit streams are transferred apparently simultaneously as sub-

channels in one communication channel, but are physically taking turns on the channel. The

time domain is divided into several recurrent timeslots of fixed length, one for each sub-

channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-

channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel.

After the last sub-channel the cycle starts all over again with a new frame, starting with the

second sample, byte or data block from sub-channel 1, etc.


received via input links, transmit circuits (CTl/j) transmitting retransmitted cells on output

links, a buffer memory (MT) storing the received cells and delivering the cells to be

retransmitted and a buffer memory addressing device (SMT) including a write address source

(SAE) and a read address source (fsl/j).

The switching unit further comprises a write disabling circuit (pi) conditioned by a signal

(adl) derived from the content of at least one received cell or a signal (tle) derived from the

absence of any received cell and supplying a disabling signal (spi) and the address source

includes a disabling device (pac, pal) influenced by the disabling signal (spi) so that no

memory location is then occupied in the buffer memory (MT).





utilization. A commonly used technique is Synchronous Time Division Multiplexing

(STDM). In Synchronous Time Division Multiplexing, for example, consider the

transmission of messages from terminals to computer, each terminal is assigned a fixed time

duration. After one user's time duration has elapsed, the channel is switched to another user.

With synchronous operation, buffering is limited to one character per user line, and

addressing is usually not required. The STDM technique, however, has certain disadvantages.

As shown in Figure 1, it is inefficient in capacity and cost to permanently assign a segment of

bandwidth that is utilized only for a portion of the time. A more flexible system that

efficiently uses the transmission facility on an "instantaneous time-shared" basis could be

used instead. The objective would be to switch from one user to another user whenever the

one user is idle, and to asynchronously time multiplex the data. With such an arrangement,

each user would be granted access to the channel only when he has a message to transmit.

This is known as an Asynchronous Time Division Multiplexing System (ATDM). A segment

of a typical ATDM data stream is shown in Figure 2. The crucial attributes of such a

multiplexing technique are:


2. Buffering is required to handle the random message arrivals.

What is the critical angle, explain?

We have a beam of light moving from a more dense to a less dense medium

o We gradually increase the angle of incidence measured from vertical axis

o As angle of incidence increases, so does the angle of refraction

o The angle at which refracted line lies on the horizontal axis is called

o Critical Angle

How can we use Hamming Code to Correct Burst Errors?

Hamming code (the bits that are power of 2 are check bits, others are message bits, each check

bit forces the parity of some collection of bits, including itself)

Can only correct single bit error. Uses kr check bits to make blocks of km data bits immune to a

single burst error of length k or less (called interleaving).

How many types of Redundancy Checks exist, Write Names and describe any one of

them?

Types of Redundancy Checks

o There are 4 types of redundancy checks used in data communication

Vertical Redundancy Check (VRC)

Longitudinal Redundancy Check (LRC)

–Cyclic Redundancy Check (CRC)

–Checksum

Vertical Redundancy Check (VRC)

o Most common and least expensive

o Also called Parity Check

o A redundant bit called parity bit is appended to every data unit so that total number of

1’s in the unit becomes even including the parity bit


What is the formula to calculate the number of redundancy bits required to correct a bit error

in a given number of data bits? [2]



What is R G rating of coaxial cable?


RG ) ratings


RG-8


RG-9


RG-11


RG-58


RG-59

• Used for TV









Three methods can be used to transmit packets over a network: unicast,

multicast, and broadcast.

Unicast involves communication between a single sender and a single receiver.

This is a type of point-to-point transmission; since the packet is transmitted to

one destination at a time.

Multicast is used to send packets to a group of addresses, represented by a

"group address." In this case, packets are transmitted from a single sender to

multiple receivers. Since the same data packet can be sent to multiple nodes by

sending just one copy of the data, the load of the sender and the overall load of

the network are both reduced.

Broadcast involves sending packets to all nodes on a network simultaneously.

This type of transmission is used to establish communication with another host,

and for DHCP type methods of assigning IP addresses.





Multiplexed.



Guided Media, are those media that provide a conduit from one device to another. Three

types are


5. Coaxial cable




Data & Video can be broken into smaller portions using Inverse Multiplexing and TX. An

inverse multiplexer (often abbreviated to "inverse mux" or "imux") allows a data stream to be

broken into multiple lower data rate communication links. An inverse multiplexer differs

from a demultiplexer in that each of the low rate links coming from it is related to the others

and they all work together to carry their respective parts of the same higher rate data stream.

By contrast, the output streams from a demultiplexer may be completely independent from

each other and the demultiplexer does not have to understand them in any way.

This is the opposite of a multiplexer which creates one high speed link from multiple low

speed ones.


sometime






added together using one’s complement. The total (sum) is then complemented and appended

to the end of the original data unit as redundancy bits called CHECKSUM. The extended data

unit is transmitted across the network. The receiver subdivides data unit as above and adds all

segments together and complement the result. If the intended data unit is intact, total value

found by adding the data segments and the checksum field should be zero. If the result is not

zero, the packet contains an error & the receiver rejects it


Explain Asynchronous Time Division Multiplexing in detail? Also discuss its advantages

over synchronous TDM?


resembles normal TDM, except that time slots are allocated as needed dynamically rather

than preassigned to specific transmitters. ATDM is more intelligent and has better bandwidth


Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing in

which two or more signals or bit streams are transferred apparently simultaneously as sub-

channels in one communication channel, but are physically taking turns on the channel. The

time domain is divided into several recurrent timeslots of fixed length, one for each sub-

channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-

channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel.

After the last sub-channel the cycle starts all over again with a new frame, starting with the

second sample, byte or data block from sub-channel 1, etc.


received via input links, transmit circuits (CTl/j) transmitting retransmitted cells on output

links, a buffer memory (MT) storing the received cells and delivering the cells to be

retransmitted and a buffer memory addressing device (SMT) including a write address source

(SAE) and a read address source (fsl/j).

The switching unit further comprises a write disabling circuit (pi) conditioned by a signal

(adl) derived from the content of at least one received cell or a signal (tle) derived from the

absence of any received cell and supplying a disabling signal (spi) and the address source

includes a disabling device (pac, pal) influenced by the disabling signal (spi) so that no

memory location is then occupied in the buffer memory (MT).





utilization. A commonly used technique is Synchronous Time Division Multiplexing

(STDM). In Synchronous Time Division Multiplexing, for example, consider the

transmission of messages from terminals to computer, each terminal is assigned a fixed time

duration. After one user's time duration has elapsed, the channel is switched to another user.

With synchronous operation, buffering is limited to one character per user line, and

addressing is usually not required. The STDM technique, however, has certain disadvantages.

As shown in Figure 1, it is inefficient in capacity and cost to permanently assign a segment of

bandwidth that is utilized only for a portion of the time. A more flexible system that

efficiently uses the transmission facility on an "instantaneous time-shared" basis could be

used instead. The objective would be to switch from one user to another user whenever the

one user is idle, and to asynchronously time multiplex the data. With such an arrangement,

each user would be granted access to the channel only when he has a message to transmit.

This is known as an Asynchronous Time Division Multiplexing System (ATDM). A segment

of a typical ATDM data stream is shown in Figure 2. The crucial attributes of such a

multiplexing technique are:


2. Buffering is required to handle the random message arrivals


Write down three characteristics of Sine Wave.

Solution: p_61

Amplitude

Period / Frequency

Phase


A Sine wave has a frequency of 6 Hz. What is its period?

Solution: p_63

T = 1/ f

T = 1/6

=0.17

Period: Amount of time (in seconds) a signal need to complete one cycle

Frequency: Number of cycles completed in one second


Give similarities between 4 QAM & 8 QAM.

Solution: p_96

In both case no. of amplitude shifts is more than the no. of phase shifts

Because amplitude changes are susceptible to Noise, number of phase

shifts used by QAM is always larger that the amplitude shifts


CS601_MIDTERM_SOLVE_PAPER ( COMPOSED BY SADIA ALI SADII

What is digital signal? Give an example

Solution: p_58-59

A digital signal is discrete. It can have only a limited number of defined

values, often as simple as 1s and 0s

The transition of a digital signal from value to value is instantaneous like a

light being switched ON and OFF

Example:

Data stored in the memory of a computer in the form of 1s and 0s. it is

usually converted to a digital signal when it is transferred from one

position to the other inside or outside the computer.


Difference between Phase Modulation(PM) with Frequency Modulation(FM)?

Solution:

Lecture # 19, p_99


Write down the criteria or technique that we follow for assigning integral values to

sampled signals and also its conversion into binary format. Explain it?

Solution: p_83-84

Lecture # 16 Quantization

Explain redundancy in terms of error detection mechanism.

Ans :handouts pg no 170

. What is differential Manchester encoding scheme?

Differential Manchester

o Inversion at the middle of the bit interval is used for Synchronization but presence or

absence of an additional transition at the beginning of bit interval is used to identify a

bit

o A transition means binary 0 & no transition means binary 1

o Requires 2 signal changes to represent binary 0 but only one to represent binary 1

Write down the four architectures of LAN?

Architectures for LANS

Dominated by 4 architectures:

o Ethernet, Token Bus, Token Ring � Standards of IEEE and a part of its Project 802

o Fiber Distributed Data Interface (FDDI) � ANSI Standard

What is Network Virtual Terminal (NVT)?

NVT is a software version of a physical terminal and allows a user to log on to plication created

emulation of terminal at the remote host. To do so the application created emulation of terminal at

the remote host. (hand outs pg no 53)

. What are the basic types of Telephone Network?

A telephone network is a telecommunications network used for telephone calls between two or more

parties.

There are a number of different types of telephone network:

• A fixed line network where the telephones must be directly wired into a single telephone

exchange. This is known as the public switched telephone network or PSTN.

• A wireless network where the telephones are mobile and can move around anywhere within

the coverage area.

• A private network where a closed group of telephones are connected primarily to each other and

use a gateway to reach the outside world. This is usually used inside companies and call

centres and is called a private branch exchange (PBX)

What are the uses of Extremely High Frequency?

• • Use space propagation

• o Used for Radar , Satellite and Experimental Communication

What is meant by Transmission Impairments?

Ans: Transmission media are not perfect .What is sent is not what is received.

There are three types of transmission impairments

1) Attenuation 2) Distortion

3) Noise


Name and discuss briefly the bits in the HDLC control field. [5]

Control field in the I-Frame and S-Frame is two bytes long to allow seven bits of sending and

receiving sequence

However the control field in the U-Frame is still one byte.

Information field contains the user’s data in an I-Frame and Network Management

information in a U-Frame

o An S-Frame has no information field

o Its length can vary from one network to another but remains fixed within each network

o It is possible to send Control information in the information field of the I-Frame along

with data.

o This process is called Piggybacking


What is satellite communication?

Satellite Communication o Satellite TX is much like line-of-sight transmission in which one of the stations is the satellite

orbiting around the earth

o The principle is similar to the terrestrial microwave with a satellite acting as a Super tall

antenna and Repeater

o Although in satellite TX, signals must still travel in straight line, the limitations imposed on

distance by curvature of earth are reduced

o In this way satellites can span Continents and oceans with one bounce off the satellite

o Satellite can provide TX capability to and from any location on earth no matter how remote

o This advantage makes high quality communication available to underdeveloped parts of the

worked at almost no cost

o Satellites themselves are very expensive but leasing a freq or time on one can be cheap

FAQs

Question: What do we mean by the term session?

Answer: Session is the period of time a user interfaces with an application. The user session

begins when the user accesses the application and ends when the user quits the

application.

Question: How can I define Data communication?

Answer: Data Communication is the exchange of data (in the form of 0’s and 1’s) between

two devices (computers) via some form of the transmission medium. Data

communication is communication between computers. Data communication

concerns the exchange of digital data between computers. Nowadays, data

communication between computers also supports digital speech telephone and

videophone, thus supporting direct communication between people.

Question: How could it be more secure using distributed processing?

Answer: In this case security is achieved by providing the user with limited access. It means

that user can only perform those tasks which are allowed by system designer. Its'

simple example is bank's ATM. Where user can't perform operations on database

other than provided.

Question: What does the terms transit and response time mean? Also tell me about the topic peak load

periods.

Answer: Transit time is the time taken by the message to travel from source to destination.

Response time is the time of response (acknowledgement) from destination to

source. Peak load periods are those periods of time in which number of users on a

network are more than normal time.

Question: What are the advantages of networking?

Answer: • Files can be stored on a central computer (the file server) allowing data to be

shared throughout an organization. • Files can be backed up more easily when they

are all on a central fileserver rather than when they are scattered across a number of

independent workstations. • Networks also allow security to be established, ensuring

that the network users may only have access to certain files and applications. •

Software and resources can be centrally managed. • Network versions of software

often allow for their speedy installation on workstations from the file server. •

Expensive devices such as laser printers or scanners can be shared. • Users can

access their files from any workstation

Question: What are different factors to choose a network topology?

Answer: Working conditions of network, working scenarios, arrangement of links and

devices. Depending upon our network what actually we are dealing with and in

which geographical area our devices are present, small area available or have large

area. All these factors are considered when choosing a Topology.

Question: What is the relative status of the devices to be linked?

Answer: The geographical area in which our devices are present is the relative status of

devices.

Question: What is the definition of Hub?

Answer: Hub is a common connection point for devices in a network. Hubs are commonly

used to connect segments of a LAN. A hub contains multiple ports. When a packet

arrives at one port, it is copied to the other ports so that all segments of the LAN can

see all packets. A passive hub serves simply as a conduit for the data, enabling it to

go from one device (or segment) to another. So-called intelligent hubs include

additional features that enable an administrator to monitor the traffic passing

through the hub and to configure each port in the hub. Intelligent hubs are also

called manageable hubs. A third type of hub, called a switching hub, actually reads

the destination address of each packet and then forwards the packet to the correct

port.

Question: What do we mean by "TAP"?

Answer: A tap is a connection to a coaxial cable in which a hole is drilled through the outer

shield of the cable so that a clamp can be connected to the inner conductor of the

cable. Instead of cutting the cable and attaching connectors to both ends of the

severed coaxial cable,

Question: Give some detail about "fault tolerance".

Answer: Fault tolerance means the system will not fail because any one component fails. The

system also should provide recovery from multiple failures. Components are often

over engineered or purposely underutilized to ensure that while performance may be

affected during an outage the system will perform within predictable, acceptable

bounds

Question: What is PDU in OSI Model?

Answer: Normally a communication request originates at the highest layer (Application

Layer). The request is passed down through the lower layers in the form of a packet

called a protocol data unit (PDU). Layers in the protocol stack communicate with

their adjacent layers via one or more Service Access Points (SAP). Each succeeding

layer in the stack adds its own information to the PDU that will be read by its

counterpart (peer) layer on the receiving system. Once the data arrives at the lower

layers, the PDU is encoded into data frames and placed onto the cable for

transmission. The data frames make their way to the receiving system and the entire

process is reversed as the PDU makes its way up the protocol stack. As it moves up

the stack, each layer "unwrap" the PDU and receives the information from its peer

layer on the sending system.

Question: What is the difference between radio and microwave?

Answer: Although there is no clear-cut demarcation between radio and microwave,

electromagnetic waves ranging in frequencies between 3 KHz and 1 GHz are

normally called radio waves; waves ranging in frequencies between 1 GHz and 300

GHz are called microwaves. Radio waves are used for multicast communications,

such as radio and television systems.

Question: What is the difference between internet and intranet?

Answer: Internet The Internet is a system of linked networks that are worldwide in scope and

facilitate data communication services such as remote login, file transfer, electronic

mail, the World Wide Web and newsgroups. With the meteoric rise in demand for

connectivity, the Internet has become a communications highway for millions of

users. The Internet was initially restricted to military and academic institutions, but

now it is a full-fledged conduit for any and all forms of information and commerce.

Internet websites now provide personal, educational, political and economic

resources to every corner of the planet. Intranet A network based on TCP/IP

protocols (an internet) belonging to an organization, usually a corporation,

accessible only by the organization's members, employees, or others with

authorization. An intranet's Web sites look and act just like any other Web sites, but

the firewall surrounding an intranet fends off unauthorized access. Like the Internet

itself, intranets are used to share information. Secure intranets are now the fastestgrowing

segment of the Internet because they are much less expensive to build and

manage than private networks based on proprietary protocols.

Question: What is the difference between hub and switch?

Answer: On the outside, hubs and switches appear very similar in that they both have a

number of RJ-45 jacks for connecting devices. Inside, however, they work very

differently. To understand why switches provide so much more functionality than

hubs, you must understand a fundamental limitation of (non-switched) Ethernet:

there can only be one device transmitting on a segment at any given time. If two or

more devices attempt to transmit at the same time, a collision occurs. (In fact, an

Ethernet segment where only one conversation can occur is called a collision

domain.) After a collision, all devices must retransmit. As you can imagine, as the

number of devices on an Ethernet segment increases, the probability for collisions

increase. Because devices must spend more time retransmitting data, the network is

perceived to be slow. Before the advent of switches, a network could be divided into

segments with a device called a bridge. Bridges have two Ethernet ports. As traffic

flows through a network, a bridge learns which devices (identified by the MAC or

"hardware" address) are on each side. The bridge then makes decisions to forward or

not forward each packet to the other side based on where the destination device is

located. A bridge thus divides a network into two collision domains, allowing two

independent "conversations" to occur. If a bridge is placed intelligently (e.g.,

separating two departments and their respective file servers), they can improve

network efficiency. Hubs do no processing on network traffic--they simply repeat

the incoming signal to all available ports. On a switch, every port acts as a bridge. If

each switch port is connected to a single device, each device can, in principle, act

independently of every other device. For example, consider a switch with the

following devices attached: computer 1 computer 2 computer 3 printer file server

uplink to the Internet In this case, computer 1 could be printing a document, while

computer 2 connects to a files server, while computer 3 accesses the Internet.

Because the switch intelligently forwards traffic only to the devices involved, there

can be multiple independent simultaneous conversations.

Question: How different layers of OSI model are implemented?

Answer: The OSI 7 layers model has clear characteristics. Layers 7 through 4 deal with end

to end communications between data source and destinations. Layers 3 to 1 deal

with communications between network devices. On the other hand, the seven layers

of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and

lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with

application issues and generally are implemented only in software. The highest

layer, the application layer, is closest to the end user. The lower layers of the OSI

model handle data transport issues. The physical layer and the data link layer are

implemented in hardware and software. The lowest layer, the physical layer, is

closest to the physical network medium (the wires, for example) and is responsible

for placing data on the medium.

Question: What is the difference between WAP and GPRS?

Answer: Wireless Application Protocol, a secure specification that allows users to access

information instantly via handheld wireless devices such as mobile phones. Wireless

Application Protocol (or WAP) is envisioned as a comprehensive and scalable

protocol designed to use with mobile phones using Short Message Services (SMS),

General Packet Radio Services (GPRS), CDMA and GSM.

Question: What are routers, repeaters and bridges?

Answer: Router: Routers relay packets among multiple interconnection networks. Router is

device that determines the next network point to which a data packet should be

forwarded enroute toward its destination. Routers create or maintain a table of the

available routes and use this information to determine the best route for a given data

packet.

Question: What do we mean by capacity of the channel?

Answer: Channel capacity: The maximum possible information transfer rate through a

channel, subject to specified constraints.

Question: What is remote data communication?

Answer: Data Communication is considered remote, if the devices are farther apart.

Question: Explain VPN (Virtual Private Network

Answer: A virtual private network (VPN) is a way to use a public telecommunication

infrastructure, such as the Internet, to provide remote offices or individual users with

secure access to their organization's network. A virtual private network can be

contrasted with an expensive system of owned or leased lines that can only be used

by one organization. The goal of a VPN is to provide the organization with the same

capabilities, but at a much lower cost.

Question: Define encryption of data.

Answer: Encryption of data: The translation of data into a secret code. Encryption is the most

effective way to achieve data security. To read an encrypted file, you must have

access to a secret key or password that enables you to decrypt it. Unencrypted data is

called plain text; encrypted data is referred to as cipher text.

Question: Can we implement OSI in wireless networks?

Answer: Wireless Application Protocol (WAP), a secure specification that allows users to

access information via handheld wireless devices, specifies architecture based on

layers that follow the OSI model fairly closely. WAP defines network architecture

for content delivery over wireless networks. Central to the design of WAP is a

network stack based on the OSI model. WAP implements several new networking

protocols that perform functions similar to the well-known Web protocols HTTP,

TCP, and SSL.

Question: What is distributed processing?

Answer: It refers to any of a variety of computer systems that use more than one computer,

or

processor, to run an application. This includes parallel processing, in which a single

computer uses more than one CPU to execute programs. More often, however,

distributed processing refers to local-area networks (LANs) designed so that a single

program can run simultaneously at various sites. Most distributed processing

systems contain sophisticated software that detects idle CPUs on the network and

parcels out programs to utilize them. Another form of distributed processing

involves distributed databases, databases in which the data is stored across two or

more computer systems. The database system keeps track of where the data is so

that the distributed nature of the database is not apparent to users. CS601 Subjecive

What is zero frequency & how it effect on signal? Done with cs601

If a frequency does not change at all and maintain constant level of voltage the entire time, it is

called zero frequency. This is also called DC Component I.e. a component with zero frequency. When

a signal contains a DC Component, it cannot travel through a Tx. Medium that cannot handle DC

components.

If a signal does not change, it will never complete any cycles, and frequency is no. of cycles in 1

second so Freq = 0

OR

zero frequency means a signal has no change in one second. When a signal contains zero frequency,

it cannot travel through a Tx. Medium that cannot handle it.

Consider a dialup modem for modulating digital data to analog signals suggest the most efficient

technique used for modulation?

Quadratue Amplitude Modulation(QAM) is the most efficient of these options and is the

mechanism used in all modern modems

Despite of having digital signals why we need to have digital to analog conversion of signal?

Explain with example.

We might want to send a digital signal coming out of computer through a medium designed for

analog signals

Example: When you Tx data from one computer to the other using a public telephone line. Original

data is digital but because telephone wires carry analog signal, original data must be converted.

Sketch the Manchester’s encoding for the stream “01100”.

011 0 0

Amplitude

Manchester

Time

Consider a carrier signal whose phase & frequency remain constant varying amplitude, which kind

of modulation will take place?

ASK (Amplitude Shift Keying) will take place when the peak amplitude of a signal remains constant

during each bit duration. Its value depends on bits (0 or 1).

Calculate bit rate & bund rate for an analog signal transmitting 500 signals per sec, where each

signal element carries 5 bit.

Baud rate = Number of Signal elements

Baud rate = 500 bauds/sec

Bit rate = Baud rate * number of bits per Signal element

Bit rate = 500 * 5 = 2500 bps

Frequency is constant what effect on signal?

It is one of the two extreme frequencies, if a signal does not change atll and maintain a constant

voltage level all the time. In such cases, frequency is going to be zero. If a signal does not change, it

will never complete any cycles, and frequency is no. of cycles in 1 second so Freq = 0

Types of transmission medium

1. Twisted wire 300 bps – 10 Mbps low

2. Microwave 256kbps – 100 Mbps low

3. Coaxial cable 56 kbps – 200 Mbps low

4. Fiber optic cable 500 kbps – 10 Gbps High

Why RZ requires more bandwidth than NRZ?

beacuse it requires two signal changes to encode one bit and therefore occupies more

BANDWIDTH.

In mobile communication which kind of Signal conversion mechanism is used and why?

In which scheme the bandwidth seems to be higher?

RZ scheme.

A company uses NRZ scheme what problems it will face?

Calculate bandwidth in KHz if signal per cycle = 6sec.

Which system you will prefer in the polar type, if there is a large no. of computers in a network.

calculate bandwidth for AM in MHz if frequency and baud rate are equal in this case what will be

frequency starting from 8

-Bandwidth= 2*8 =0.016 Mhz (not confirm)

Different Types of Polar and Bipolar

Polar:

1. NRZ

• NRZ-I

• NRZ-L

2. RZ

3. Biphase

• Manchester

• Differential Manchester

Bipolar

1. AMI

2. B8ZS

3. HDB3

Draw binary values 0001110101 in NRZ-I

Amplitude

Time

0 0 0 1 1 1 0 1 0 1

NRZ-I

Calculate the maximum baud rate for FSK Bandwidh is 12,000Hz; the carrier must be saperated by

2000 Hz (f1-f0). Tx is in full duplex mode.

For FSK, if fc1 and fc2 are the carrier frequencies, then:

BW=Baud Rate + (fc1 – fc0)

Baud rate is the same as bit rate

By putting value

12000=Baud rate +(2000) = 12000 - 2000 = 1000 Hz

Define Bandwith and its formula

Bandwidth refers to the range of the component frequencies and Frequency Spectrum refers to

the elements within that range. Bandwidth: of a signal is the width of the frequency spectrum

To calculate Bandwidth, subtract the lowest frequency from the highest frequency

BW= FH-FL

Which is more subject to noise FSK or PSK and write their limitations "2 marks"

The limiting factors of FSK are the physical capabilities of the carrier. Although FSK shifts between two carrier frequencies, it is easier to analyze as two co-existing

frequencies. PSK is not susceptible to the noise degradation that affects ASK, nor to the bandwidth limitations

of FSK. Limitations of PSK:

–PSK is limited by the ability of the equipment to distinguish small differences in phase

–This factor limits its potential bit rate

find the bit rate Bit Rate=Baud Rate * Number of bits per signal element

Five common units of frequency corresponding to time ? "5 marks

Frequency is measured in hertz, There are 5 units used in Hertz

Seconds____Hertz Milliseconds____Kilohertz

Microseconds____Megahertz

Nanoseconds____Gigahertz

Picoseconds____Terahertz

Mc100401285 [email protected]

CS601-Data Communication

Latest Solved subjective from Midterm Papers

May 08,2011

Lectures 1-22 Mc100401285 [email protected] Moaaz Siddiq Latest subjectives

MIDTERM EXAMINATION

Spring 2010


How many types of Bipolar encoding scheme exist?

Answer: (PAGE NO.76)

Given two sine waves A and B, if the frequency of A is twice that of B, then the period of B is

________ that of A.There are three types of bipolar encoding:

1. AMI (Alternate Mark Inversion)

2. B8ZS

3. HDB3


Give methods of Analog to Analog conversion.

Answer: (PAGE 95)

Analog Conversion:

Representation of analog information by an analog signal is Analog –to- analog conversion.


What is frequency spectrum?

Answer: (PAGE 66)

Frequency spectrum is the collection of a l l the components frequencies it contains.



Answer: (PAGE 112)

Bandwidth of a modem is not measured in bytes, it is measured in bits. Modem Speed –ASK

(full-duplex)

Effective bandwidth of a telephone line is 2400 Hz, the maximum baud rate is also 2400.




What do you understand by the term DCE? Explain DCE. [5]

Answer: (PAGE 101)

Any device that transmits or receives signal through network is DCE.

It is any functional unit that transmits or receives data in the form of an analog or digital

signal

through a network.

At the physical layer DCE takes data generated by DTE, converts it to the signal.


Describe different types of NRZ encoding scheme?

Answer:

In NRZ encoding the level of signal is either positive or negative. NRZ is

classified into 2 types:

NRZ-L NRZ-I

NRZ-L:

Level of the signal depends on the type of bit it represents.

Positive voltage usually means the bit is a 1 and a negative voltage usually means the bit

is a 0.

NRZ-I:

The inversion of the level represents a 1 bit. A bit 0 is

represented by no change.

NRZ-I is superior to NRZ-L.(PAGE 73)


MIDTERM EXAMINATION

Spring 2010


Define Sampling Rate.

Answer: (page82)

The accuracy of any digital reproduction of an analog signal depends upon the o. of

samples taken


Write down the types of digital to analog modulation.

Answer: (PAGE 85)

Types of digital to analog modulation

ASK

FSK

PSK

QAM


What is periodic signal?

Answer: (page58)

A signal is called Periodic if it completes a pattern within a measurable time frame called

a Period and then

repeats that pattern over identical subsequent Periods



Answer: (page111)

Bandwidth of a modem is not measured in bytes, it is measured in bits.

Modem Speed –ASK (full-duplex)

Effective bandwidth of a telephone line is 2400 Hz, the maximum buad rate is

also 2400.




Briefly describe Return to Zero (RZ) encoding scheme?

Answer: (page75)

§ Any time, data contains long strings of 1’s or 0’s, Rx can loose its timing

§ In unipolar, we have seen a good solution is to send a separate timing signal but this

solution is both

expensive and full of error

§ A better solution is to somehow include synch in encoded signal somewhat similar to

what we did in

NRZ-I but it should work for both strings of 0 & 1

§ One solution is RZ encoding which uses 3 values : Positive, Negative and Zero

§ Signal changes not b/w bits but during each bit

§ Like NRZ-L , +ve voltage means 1 and a –ve voltage means 0, but unlike NRZ- L, half

way through

each bit interval, the signal returns to zero

§ A 1 bit is represented by positive to zero and a 0 is represented by negative to zero

transition

§ The only problem with RZ encoding is that it requires two signal changes to encode

one bit and therefore

occupies more BANDWIDTH

§ But of the 3 alternatives we have discussed, it is most effective value.


Compare Phase Modulation (PM) with Frequency Modulation (FM)?









FM Bandwidth

Bandwidth of FM signal (modulated signal) = 10 * bandwidth of modulating signal

Significant spectrum of FM

audio = 15 KHz



MIDTERM EXAMINATION

Spring 2010


Write down three characteristics of Sine Wave.

Answer : (page no.59)

v Amplitude

v Period/Frequency

v Phase


A Sine wave has a frequency of 6 Hz. What is its period?

Answer: (page 61)

F= 6 , t= ?

T = 1/f = 1/6 = 0.17 sec


Give similarities between 4 QAM & 8 QAM.

Answer: (page 92)

In both case no. of amplitude shifts is more than the no. of phase shifts

Because amplitude changes are susceptible to Noise , number of phase shifts used by

QAM is always larger than

the amplitude shifts


What is digital signal? Give an example

Answer: (page 57)

A digital signal is discrete. It can have only a limited number of defined values, often as

simple as 1s and 0s

The transition of a digital signal from value to value is instantaneous like a light being

switched ON and OFF.



Difference between Phase Modulation(PM) with Frequency Modulation(FM)?









FM Bandwidth

Bandwidth of FM signal (modulated signal) = 10 * bandwidth of modulating signal

Significant spectrum of FM

audio = 15 KHz



Write down the criteria or technique that we follow for assigning integral values to

sampled signals

and also its conversion into binary format. Explain it?

MIDTERM EXAMINATION

Spring 2010


How does noise affect FSK?

Answer: (P170)

The duration of the noise is normally longer than the duration of a bit which means that when

noise affects data, it affects a set of bits.


Define Bit interval and Bit rate.


Answer: (P68)

Bit Interval and Bit Rate

Most digital signals are aperiodic and thus Period and Frequency are not the appropriate

terms to describe them

Bit Interval (seconds)

Time required to send one single bit

Bit Rate (bps)

Number of bits sent per second


Write down the names of layers present in TCP/IP Protocol Suite?

Answer: (P54)

Physical (physical standards)

Data Link (N/w Interface)

Network (Interconnectivity)

Transport (Transport Functions)

Application (Session, Pres, app of OSI)


The data points of a constellation are at (4,0) and (6,0). Draw the constellation.

Show the amplitude and phase for each point. Is the modulation ASK, PSK, or

QAM? How many bits per baud can one send with this constellation? [10]

Answer:

Hint: 2 amplitudes (4,6) , ASK , bit/baud

MIDTERM EXAMINATION

Spring 2009

Question No: 21 (Marks: 2)

What is simplex mode of transmission?

Answer:

Communication is Unidirectional

Only one of the two stations can transmit

Other can only receive (P34)


What is HDB3.

Answer:

Alteration of AMI adopted in Europe and Japan

Introduces changes into AMI, every time four consecutive zeros are encountered instead

of waiting for eight

zeros as in the case of B8ZS. (P78)



Write a note on Full duplex mode of transmission.

Answer:

Both stations can transmit and receive simultaneously

Two way street with traffic flowing in both directions at the same time

Signals traveling in either direction share the capacity of the link

The sharing can take place in two ways:

Either the link must contain two physically separate transmission paths:

v One for sending and

v One for receiving


In your view which one is best either B8ZS or HDB3 and why? Write down salient

features of both? What is meant by polarity of 1 since the last substitution?

MIDTERM EXAMINATION

Spring 2009


What is purpose of distributed processing?

Answer:

Multiple computers working on a problem can solve a problem faster than a computer

working alone.

(P13)


What is HDB3?

Answer:

Alteration of AMI adopted in Europe and Japan

Introduces changes into AMI, every time four consecutive zeros are encountered instead

of waiting for eight

zeros as in the case of B8ZS. (P78)


Differentiate between the following terms. [10 marks]

(a) Syntax and Semantics of protocol.


Answer:

Syntax:

Communication between peer entities (P21)

Semantics:

Client/server (P21)

(b) Network management and exchange management.

Answer:

Exchange Management

Error Detection and Correction (P17)

Network management (P17)

Security

(c) Monolithic and structured.

Answer: (P21)

Monolithic

Communications is a complex task

To complex for single unit

Structured

Structured design breaks down problem into smaller units

Layered structure


Differentiate between the following terms. a)

Frequency spectrum and bandwidth.

Answer: (P66)

Frequency Spectrum / Bandwidth

Frequency Spectrum: of a signal is the collection of all the component frequencies it

contains

It is shown using a Frequency domain graph

Bandwidth: of a signal is the width of the frequency spectrum

In other words, Bandwidth refers to the range of the component frequencies and

Frequency Spectrum refers to

the elements within that range

(b) Bandwidth and throughput.

Answer: (P144)

Measurement of how fast data can pass through a point o In other words, if we consider any

point in the TX Medium as a wall through which bits pass, then throughput is the number of

bits

that can pass this wall in second


(c) Bit interval and bit rate.

Answer: (P68)

Bit Interval and Bit Rate

Most digital signals are aperiodic and thus Period and Frequency are not the appropriate

terms to describe them

Bit Interval (seconds)

Time required sending one single bit

Bit Rate (bps)

Number of bits sent per second Frequency is constant what effect on signal (2marks)

Frequency of signal is varied to represent binary 1 or 0

o The frequency of the signal during each bit duration is constant and depends on the bit

(0 or 1)

Type of Transmission Medium � Medium defines speed at which data can travel

� Fiber Optic Cable

� 100Mbps and 10 Mbps

� Hardware

� Software

We might want to send a digital signal coming out of computer through a medium

designed for analog signals

o For example, To send data from one place to the other using a Telephone line

o This is called “Digital-to-Analog Conversion” or “Modulating a digital Signal”

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