Sistem Komunikasi

Sistem Komunikasi

oleh Warsun Najib

04/19/23 Sistem Komunikasi 2

Pengantar

Telekomunikasi = komunikasi yang dipisahkan oleh jarak

Ada 3 komponen utama : Isyarat Komunikasi Kanal Komunikasi Jaringan Komunikasi


Isyarat Komunikasi


Isyarat Komunikasi Sistem Komunikasi harus menyediakan transmisi

yang bagus dengan frekuensi2 di mana spectrum-nya signifikan.

Kanal komunikasi harus bisa melewatkan isyarat dengan range frekuensi 300 Hz s/d 3.4 kHz agar isyarat suara (manusia) dapat terkirim

Sistem telepon menggunakanLPF 4kHz (Low-pass)

Komponen-komponen frekuensi di dalam gelombang isyarat menjadi Lebar Bidang Isyarat


Isyarat Komunikasi Isyarat Musik mengandung komponen-komponen

frekuensi signifikan dari 20Hz sampai 20kHz. Untuk mengirimkan isyarat musik diperlukan LPF dengan

Bandwidth 15kHz Isyarat data biner, interval terkecil antara 0-1, T,

dinamakan interval pengisyaratan (signaling interval) 1/T dinamakan kecepatan isyarat (signaling rate) dengan

satuan bits per second [bps]. Lebar bidang transmisi pada sistem data biner secara

proporsional mendekati bit rate [High data rate = Wider BW of channel]


Kanal Komunikasi

Ada 2 kanal komunikasi yang dominan dipakai saat ini

Sistem gelombang terpandu, menggunakan media fisik, seperti kable Twisted Pair Cable [ UTP, STP, 10BaseT] Coaxial Cable [thin-coax, thick-coax] Optical Fibers [OC-3, OC-12, OC-48]

Sistem radio, dengan gelombang elektromagnetis


Kanal Komunikasi

VLF Very Low Frequency Less than 30kHz

LF Low Frequency 300kHz

MF Medium Frequency 3MHz

HF High Frequency 30MHz

VHF Very High Frequency 300MHz

UHF Ultra High Frequency 3GHz

SHF Super High Frequency 30GHz

EHF Extra High Frequency 300GHz


Kabel Twisted Pair Total pelemahan (total attenuation) karena kabel

tergantung pada panjangnya Pelemahan (isyarat) juga tgt pada frekuensi, akan

meningkat dengan bertambahnya frekuensi. Untuk mempertahankan agar pelemahan isyarat tetap rendah, maka diperlukan equalizer [LPF: Low Pass Filter]

Untuk single pair cable [telephone cable], induktor-induktor dipakai untuk meningkatkan karakteristik transmisi untuk mendapatkan frekuensi respone yang lebih baik

Pada multi-pair cable, cross-coupling dapat menambah kapasitasi antara 2 konduktor. Hal ini menyebabkan suatu isyarat dapat terganggu karena ada kabel lain.


Kabel Coaxial Dipakai untuk komunikasi yang memerlukan

Bandwidth yang lebih besar dari padaTwisted pair

Pelemahan (attenuation) meningkat kira-kira mendekati akar kuadrat frekuensi, sehingga untuk jarak yang jauh dibutuhkan penapisan

Efek kulit (“Skin Effect”), yaitu banyak isyarat dgn frekuensi yang tinggi akan hilang di dekat permukaan. Dibutuhkan penambahan ukuran kabel untuk BW yang lebih besar

BW Isyarat bisa mencapai 60MHz [140Mbps digital transmission]

Crosstalk sangat berkurang pada kabel coaxial


Fiber Optis

Advantages Disadvantages

1. Economic : high BW, lowtransmission loss

2. Dimension : small size, lowweight

3. Chemical/Physicalproperties : resist chemicalcorrosion, high temperaturecapacity

4. Reliability : insensitive toelectromagnetic

5. Raw Material : abundant andcheap [sand]

Optical sources detectors,connector, couplers, andtest equipment are relativelycomplex and expensive atpresent


Fiber Optis [1] Yang paling sederhana : inti fiber [diameter

sekitar 50 micron] dengan index bias n1, dilapisi fiber dengan index bias yang lebih kecil n2

Cahaya berjalan sepanjang inti

n1 sin1 = n2 sin2

sin1 = n2sin2 n1


Snell’s Law of Refraction Critical angle = sin-1 n2/n1

n1 sin1 = n2 sin2

sin1 = n2sin2 n1

Where 1 = c and 2 =90


Total Internal Reflection


Multimode Step Index The pulse spreading places a limit on the

transmission rate over a fiber of given length Hence, the effective bandwidth of MMS fiber

depends on length and on the index difference [n1-n2] and it is normally used for a short haul systems


Multimode Graded Index The fiber bandwidth is greatly improved with the

parabolic refractive index profile which gradually reduces from a maximum at the center to the core-cladding interface

The pulse spreading is very much reduced compared with a step index fiber because all rays paths having almost the same propagation delay


Single Mode Step Index Single mode step index is known as “Monomode” By making a core diameter and index difference

sufficiently small, there will be only one possible mode travel in the fiber.

Thus, pulse spreading due to mode velocity difference cannot exist

Single mode step index is the fastest mode in optical communications

Due to the complexity of manufacturing, a single mode fiber optic cable is therefore the most expensive one


Radio Systems An Electromagnetic wave is launched from an

antenna at the transmitter, propagates through the atmosphere an on arrival at the receiver is picked up by a receiving antenna

There are five major of Radio Propagation1. Line of Sight2. Surface wave or ground wave3. Ionospheric propagation4. Tropospheric scattering5. Free space propagation


Radio Propagation Line of Sight

Omidirectional : is used for radio broadcasting Directional : is used for point-to-point radio communication

link Surface wave or ground wave

A radio wave travels along the curvature of the earth as a result of currents flowing in the ground

Ionospheric propagation Radio wave can refracted by the ionosphere and returned to

the surface some considerable distance from the transmitter This is because the gradual reduction with height of the

refractive index in the ionosphere


Radio Propagation[1]

Tropospheric scattering Radio wave can be scattered by small particles in the

lower atmosphere to provide over-the-horizon radio communication

Free space propagation Wave can propagate without the aid of a physical

medium and this is how light from the Sun and other stars reaches the Earth


Communication Network The communication cannot takes place in

isolation Each source and destination needs to have at

least one connection to make communication happen

The classic example of communication network are : Switched Network

Data NetworkIntegrated Services Digital Network


Switched Network Switching center can produce all possible connections

required but not all at once The statistical properties of the communication requirements

of large user groups are relied upon to effect an economy The idea of a star connection can be extended to form a star

of star hierarchical network the central switching center must provide a sufficient

communication capacity to allow several message to be transferred simultaneously

This is achieved by combining several message waveforms to form a single composite message and it is known as “multiplexing” [TDM,FDM]


Data Network A protocol is a well-defined procedure to enable

terminals/nodes to communicate effectively over a network

Ring topology allows any nodes to communicate to any other nodes by simply applying signals to the ring

This is essence of a form of Local Area Network [LAN] used for computer/data communications

Data circulate around the ring in packets, a packet being a sequence of binary digits

Protocol layering of OSI reference model provide the standard for data network


Protocol Layering (OSI layer)

Layer1 Physical [Please]Layer2 Data link [Do]Layer3 Network [Not]Layer4 Transport [Throw]Layer5 Session [Sausage]Layer6 Presentation [Pizza]Layer7 Application [Away]


ISDN Increasingly, both signal transmission and switching

are implemented using digital data technique and the telephone systems are rapidly moving to a digital network

In addition, there is an increasing need for the wide range of telecommunication services ; data transmission, E-mail, Video conference, etc.

In combining a various services and make these available on a single network with the continued expansion of digital transmission and switching

It gives rise to form an Integrated Services Digital Network [ISDN]


Satuan ukuran

Common Unit of measurement : Gain & Loss

Current Gain [Ai] = Io/IiVoltage Gain [Av] = Vo/ViPower Gain [G] = Po/PiPower Loss [L] = Pi/Po

Note: G = 1/L or L = 1/G


Satuan Decibels [dB] Power Gain expressed in dB : G[dB] = 10 log G

Gain G = Po/Pi G [dB]

100 +20 dB

1 0 dB

0.1 -10 dB


Satuan dBw dan dBm dBw dan dBm digunakan untuk menyatakan

daya absolute dengan referensi 1 Watt (untuk dBw) dan 1milliwatt (untuk dBm)

Contoh, daya P=100W Dinyatakan dlm dBw = 10 log P/P[ref]

= 10 log 100W/1W= 20 dBw

Dinyatakan dlm dBm = 10 log P/P[ref]= 10 log 100W/1mW= 10 log 105mW/1mW= 50 dBm

Catatan : dBm = dBw + 30

Sistem Komunikasi

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