GSM Sudacad.pptx

8/14/2019 GSM Sudacad.pptx

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SUDACADGSM


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History Early 1980s, country isolated analog cellular telephone

systems (interoperability problem) 1982, CEPT (Conference of European Post and

Telecommunications ) established a WG to develop a newpublic land mobile system to span Europe

GSM: Groupe Speciale Mobile (French)

Proposed criteria Good speech quality Low cost for terminals and service International roaming Handheld terminals

Support for introduction of new services Spectral efficiency Compatibility with ISDN


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History (cont)

1989, GSM development responsibility transferred to the

European Telecommunications Standards Institute (ETSI)

1990, GSM phase 1 published

1991, first commercial service launched WG language changed from French to English, and GSM

became Global System for Mobile Communications

1994, phase 2 data/fax services launched

1995, phase 2 standard completed


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Technology

GSM uses a TDMA/FDMA combination

More channels of communication are available

All channels are digital

GSM uses higher frequency bands Provides additional capacity

And higher subscribers densities

GSM is capable for international roamingthrough agreements between GSM operatorsworldwide


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GSM Frequency Bands

450 MHz

Upgrade of older analog cellular systems in Scandinavia

900 MHz

Original band used everywhere except NA and most of SA 1800 MHz

New band used everywhere except NA and most of SA

1900 MHz

PCS band used in NA and most of SA


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Cells and frequency reuse

Service area divided into cells

Available frequencies divided into groups

Frequency used per cell

Same frequency reused in other far away cell

200 kHz, time shared by 8 users

Uplink and downlink separated


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Technical data Multiple Access Method

TDMA / FDMA

BS to MS frequencies

935960 MHz

MS to BS frequencies

890915

Duplexing

FDD

Channel spacing

200 kHz

Modulation

GMSK

Portable TX power, maximum /average (mW)

1000 / 125

Power control

handset and BSS

Speech coding and rate (kbps)

RPE-LTP / 13

Speech Channels per RF channel

8

Channel rate (kbps)

270.833

Channel coding Rate

1/2 convolutional

Frame duration (ms)

4.615

Duplex spacing 45 MHz


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Network Elements

Mobile Station (MS)

Base Transceiver Station (BTS)

Base Station Controller (BSC)

Base Station Subsystem (BSS) Mobile Switching Center (MSC)

Equipment Identity Register (EIR)

Authentication Center (AuC)

Home Location Register (HLR) Visitor Location Register (VLR)

Network and Switching Subsystem (NSS)


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SIM

Subscriber Identity Module or Smart Card

Contains a computer chip and some non-volatile memory

Inserted into a slot in the base of the handset

The memory held info include Subscriber identity number

Telephone number

Original network to which the subscriber belongs

Can be moved from one handset to another

A handset reads the info off the smart card and transmitsit to the network


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MS

Mobile Station

Starting point of a mobile wireless network

Can contain

Mobile Terminal (MT)

GSM cellular handset

Terminal Equipment (TE)

PC or Personal Digital Assistant (PDA)

Can be

Two devices (MT & TE) interconnected with a P-t-P interface

A single device with both functions integrated


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BTS

Base Transceiver Station

A subscriber call request is sent by the MS tothe BTS

Includes all the necessary radio equipment forradio transmission within a cell Antennas, signal processing devices, amplifiers

Responsible for

Establishing the link to the MS Modulating/Demodulating radio signals between

the MS and the BTS


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BSC

Base Station Controller

The controlling component of the radio network

Manages the BTSs

Reserves radio frequencies for communications

Handles the handoff between BTSs when an MS roams

from one cell to another

Responsible for paging the MS for incoming calls


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BSS

Base Station Subsystem

A GSM network is comprised of many BSSs

Each BSS is controlled by a BSC

The BSS performs the necessary functions for

Monitoring radio connections to the MS

Voice coding/decoding

Rate adaptation to/from the wireless network

A BSS can contain several BTSs


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MSC

Mobile Switching Center

A digital switch that sets up connections to

the other MSCs and to the BSCs

The MSCs form the wired (fixed) backbone of

a GSM network and can switch calls to the

PSTN

An MSC can connect to several BSCs


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EIR

Equipment Identity Register

A database that stores the internationalmobile equipment identities (IMEIs) of all the

MSs in the network The IMEI is an equipment identifier assigned

by the manufacturer of the MS

The EIR provide security features such asblocking calls from handsets that have beenstolen


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HLR

Home Location Register

The central database for all users to register tothe GSM network

Stores subscribers static information such as International mobile subscriber identity (IMSI)

Subscribed services

Subscriber authentication key It also stores dynamic subscriber info such as

the current location of the mobile subscriber


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AuC

Authentication Center

A database associated with the HLR

Contains

The algorithms for subscribers authentication

The necessary encryption keys to safeguard

the user input


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VLR

Visitor Location Register

A distributed database that temporarily storesinformation about the MSs that are active in thegeographic area for which the VLR is responsible

A VLR is associated with each MSC in the network When a new subscriber roams into a location area, the

VLR copies subscriber info from the HLR to its localdatabase

This HLR-VLR relationship avoids Frequent HLR database update

Long distance signaling of the user info

Hence allowing faster access to subscriber info


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GSM database

The HLR, VLR, and AuC comprise themanagement database that support roaming(including international roaming) in the GSM

network They authenticate calls while the GSM

subscribers roam between the privatenetwork and the PLMN

They store subscriber identities, currentlocation area, and subscription levels


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NSS

Network and Switching Subsystem

The heart of the GSM system

Connects the wireless network to the standard

wired network

Responsible for calls handoff between BSSs

Perform services such as

Charging

Accounting

Roaming


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GSM network structure

MSC/VLR

HLR

GMSC/VLR

BSC

AuC

NSS

EIR

BTS

BSC

MS

cell

cell

MSBTS

BSS

Other Network


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GSM Interfaces (1)

Various interfaces used for communication betweennetwork elements

A separate interface exists between each pair ofelements

Each interface requires its own set of protocols

Communication over the interfaces occurs in a sequentialmanner

MS to BTS, BTS to BSC, BSC to MSC

And also to the different databases

Communication may traverse multiple MSCs

GMSC is the gateway towards other networks


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Interfaces (2) Um

Air interface (MS to BTS)

Traffic

Voice: 13kbps, Data: 9.6kbps

Signaling

Link Access Procedure-D mobile (LAPDm)

Abis

BTS to BSC

Traffic

16kbps

Signaling LAP-D signaling protocol


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Interfaces (3)

TRAU: Transcoder Rate Adapter Unit

BSC to MSC

A interface

Traffic

Translates between the 16 kbps on the BTS sideand the 64 kbps on the GMSC side

Signaling SS7 protocol, which defines call set-up and call

services across the interface


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Interfaces (4) B

MSC-VLR

No traffic

Signaling

MAP: Mobile Application Part of the SS7 stack

C

MSC-HLR

No traffic

Signaling MAP


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Interfaces (5) D

HLR-VLR

No traffic

Signaling: MAP

E

MSC-MSC

Traffic: 64 kbps

Signaling: MAP, ISUP


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Interfaces (6) F MSC-EIR

No traffic Signaling

G VLR-VLR

No traffic Signaling: MAP

H HLR-AuC

No traffic


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Interface to external world

GMSC-PSTN

GMSC-ISDN

GMSC-PDN

Traffic

64 kbps

Signaling ISUP

TUP


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GSM interfaces

MS

BSC GMSC

MSC

PSTN, ISDN, PDNUm

B

C

D

A

BTS

VLR

E

VLR HLR

Abis

AuC

F

H

EIR

B

D


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Representation of Cells

Ideal cells Fictitious cells


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Cell size and capacity

Cell size determines number of cells availableto cover geographic area and (with frequencyreuse) the total capacity available to all users

Capacity within cell limited by availablebandwidth and operational requirements

Each network operator has to size cells to

handle expected traffic demand


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Cell structure

Implements space division multiplex: base station covers a certaintransmission area (cell)

Mobile stations communicate only via the base station

Advantages of cell structures:

higher capacity, higher number of users

less transmission power needed more robust, decentralized

base station deals with interference, transmission area etc. locally

Problems:

fixed network needed for the base stations

handover (changing from one cell to another) necessary

interference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on the country side(GSM) - even less for higher frequencies


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Capacity of a Cellular System

Frequency Re-Use Distance

The K factor or the cluster size

Cellular coverage or Signal to interference

ratio

Sectoring


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i

j

1

2

3

4

5

6

7

Frequency re-use distance is based on the cluster size K

The cluster size is specified in terms of the offset of the center of a cluster from the

center of the adjacent cluster

K = i2 + ij + j2

K= 22+ 2*1 + 12

K = 4 + 2 + 1

K = 7

D = 3K * R

D = 4.58R

1

2

35

6

7

D

R

The K factor and Frequency Re-Use Distance


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K = i2 + ij + j2

K= 22+ 2*0 + 02

K = 4 + 0 + 0

K = 4

D = 3K * R

D = 3.46R i

D

R

The Frequency Re-Use for K = 4


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1

2

3

4

1

1

1

1

1

12

2

2

2

2

3

3

3

3

3

4

4

4

4

4

4

3

2

Cell Structure for K = 4


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1

11

1

2 2

22

3

3

3

3

4

4 4

45

5 5

5

6

6 6

6

7

7

7

7

8 8

889

99

9

10

1010

10

1111

1111

1212

12 12

Cell Structure for K = 12


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Increasing cellular system capacity

Cell sectoring

Directional antennas subdivide cell into 3 or 6

sectors

Might also increase cell capacity by factor of 3 or 6


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Increasing cellular system capacity

Cell splitting

Decrease transmission power in base and mobile

Results in more and smaller cells

Reuse frequencies in non-contiguous cell groups

Example: cell radius leads 4 fold capacity

increase


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Tri-Sector antenna for a cell


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Highway

TownSuburb

Rural

Cell Distribution in a Network


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f h f


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One Cell = 288 traffic channels

72 Cell = 1728 traffic channels

246 Cell = 5904 traffic channels

Re-use of the frequency

8 X 36 = 288

8 X (72/12 X 36) = 1728


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Concept of TDMA Frames and Channels

f

t

c

GSM combines FDM and TDM: bandwidth is subdividedinto channels of 200khz, shared by up to eight stations,

assigning slots for transmission on demand.


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GSM uses paired radio channels

0 124 0 124

890MHz 915MHz 935MHz 960MHz


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/


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1 2 3 4 5 6 7 8

higher GSM frame structures

935-960 MHz

124 channels (200 kHz)

downlink

890-915 MHz124 channels (200 kHz)

uplink

time

GSM TDMA frame

GSM time-slot (normal burst)

4.615 ms

546.5 s577 s

guard

space

guard

spacetail user data TrainingS S user data tail

3 bits 57 bits 26 bits 57 bits1 1 3

GSM - TDMA/FDMA

LOGICAL CHANNELS


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TRAFFIC SIGNALLING

FULL RATE

Bm 22.8 Kb/S

HALF RATE

Lm 11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNEL

SCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNEL

PCH -- PAGING CHANNEL

RACH -- RANDOM ACCESS CHANNEL

AGCH -- ACCESS GRANTED CHANNEL

SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL

SACCH -- SLOW ASSOCIATED CONTROL CHANNEL

FACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLY

BOTH UP &

DOWNLINKS

B d Ch l BCH


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Broadcast Channel - BCH

Broadcast control channel (BCCH) is a base tomobile channel which provides general informationabout the network, the cell in which the mobile iscurrently located and the adjacent cells

Frequency correction channel (FCCH) is a base tomobile channel which provides information forcarrier synchronization

Synchronization channel (SCH) is a base to mobilechannel which carries information for framesynchronization and identification of the basestation transceiver


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Common Control Channel - CCH

Paging channel (PCH) is a base to mobile channel used

to alert a mobile to a call originating from the network

Random access channel (RACH) is a mobile to base

channel used to request for dedicated resources Access grant channel (AGCH) is a base to mobile

which is used to assign dedicated resources (SDCCH or

TCH)


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Dedicated Control Channel - DCCH

Stand-alone dedicated control channel (SDCCH)

is a bi-directional channel allocated to a specific

mobile for exchange of location update

information and call set up information


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Dedicated Control Channel - DCCH

Slow associated control channel (SACCH) is a bi-directionalchannel used for exchanging control information between base

and a mobile during the progress of a call set up procedure. The

SACCH is associated with a particular traffic channel or stand

alone dedicated control channel Fast associated control channel (FACCH) is a bi-directional

channel which is used for exchange of time critical information

between mobile and base station during the progress of a call.

The FACCH transmits control information by stealing capacityfrom the associated TCH

LOGICAL CHANNELS


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TRAFFIC SIGNALLING

FULL RATE

Bm 22.8 Kb/S

HALF RATE

Lm 11.4 Kb/S

BROADCAST COMMON CONTROL DEDICATED CONTROL

FCCH SCH BCCH

PCHRACH

AGCH

SDCCH SACCH FACCH

FCCH -- FREQUENCY CORRECTION CHANNEL

SCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNEL

PCH -- PAGING CHANNEL

RACH -- RANDOM ACCESS CHANNEL

AGCH -- ACCESS GRANTED CHANNEL

SDCCH -- STAND ALONE DEDICATED CONTROL CHANNEL

SACCH -- SLOW ASSOCIATED CONTROL CHANNEL

FACCH -- FAST ASSOCIATED CONTROL CHANNEL

DOWN LINK ONLY

UPLINK ONLY

BOTH UP &

DOWNLINKS

Location update from the mobile


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Mobile looks for BCCH after switching on

RACH send channel request

AGCH receive SDCCH

SDCCH authenticate

SDCCH switch to cipher mode

SDCCH request for location updating

SDCCH authenticate response

SDCCH cipher mode acknowledge

SDCCH allocate TMSI

SDCCH acknowledge new TMSI

SDCCH switch idle update mode

Call establishment from a mobile


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Mobile looks for BCCH after switching on

RACH send channel request

AGCH receive SDCCH

SDCCH do the authentication and TMSI allocation

SDCCH require traffic channel assignment

SDCCH send call establishment request

SDCCH send the setup message and desired number

FACCH switch to traffic channel and send ack (steal bits)

FACCH receive alert signal ringing sound

FACCH acknowledge connect message and use TCH

TCH conversation continues

FACCH receive connect message


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GSM speech coding

AIR INTERFACE

UPLINK

890

-915

MHz

DOWNLI

NK935

-960M

Hz

MOBILE

BASE TRANSCEIVER STATION

h


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Transmit Path

BS Side

8 bit A-Law

to

13 bit UniformRPE/LTP speech Encoder

To Channel Coder 13Kbps

8 K sps

MS Side

LPF A/DRPE/LTP speech Encoder

To Channel Coder 13Kbps

8 K sps,

Sampling Rate - 8K

Encoding - 13 bit Encoding (104 Kbps)

RPE/LTP - Regular Pulse Excitation/Long Term Prediction

RPE/LTP converts the 104 Kbps stream to 13 Kbps


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GSM Speech Coding


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GSM Speech Coding

Speech is divided into 20 millisecond samples,

each of which is encoded as 260 bits, giving a

total bit rate of 13 kbps.

Regular pulse excited -- linear predictive coder(RPE--LPC) with a long term predictor loop is

the speech coding algorithm.

The 260 bits are divided into three classes:


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e 60 b ts a e d ded to t ee c asses

Class Ia 50 bits - most sensitive to bit errors.

Class Ib 132 bits - moderately sensitive to bit errors.

Class II 78 bits - least sensitive to bit errors.

Class Ia bits have a 3 bit cyclic redundancy code added for error

detection = 50+3 bits.

132 class Ib bits with 4 bit tail sequence = 132 + 4 = 136.

Class Ia + class Ib = 53+136=189, input into a 1/2 rate convolution

encoder of constraint length 4. Each input bit is encoded as twooutput bits, based on a combination of the previous 4 input bits. The

convolution encoder thus outputs 378 bits, to which are added the 78

remaining class II bits.

Thus every 20 ms speech sample is encoded as 456 bits, giving a bit

rate of 22.8 kbps.


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GSM Protocol Suite


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BTS

Radio interface

HLR

MSC

VLR

BSC

RR

MM + CM

SS

Link Layer


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Link Layer

LAPDm is used between MS and BTS

LAPD is used between BTS-BSC

MTP2 is used between BSC-MSC/VLR/HLR


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Network Layer

To distinguish between CC, SS, MM and RR protocoldiscriminator (PD) is used as network address.

CC call control management MS-MSC.

SS supplementary services management MS-MSC/HLR.

MM mobility management(location management, securitymanagement) MS-MSC/VLR.

RR radio resource management MS-BSC.

Messages pertaining to different transaction are

distinguished by a transaction identifier (TI).


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Application Layer protocols

BSSMAP between BSC and MSC DTAP messages between MS and MSC.

All messages on the A interface bear a discriminationflag, indicating whether the message is a BSSMAP or

a DTAP. DTAP messages carry DLCI(information on type of

link on the radio interface) to distinguish what isrelated to CC or SMS.

MAP protocol is the one between neighbor MSCs.MAP is also used between MSC and HLR.

GSM Functional Architecture and Principal Interfaces


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Q.921

Radio Interface

Q.931

Q.921

MAP

TCAP

CCS7 MTP

CCS7 SCCP

Mobile Application Part

Q931 BSSAP

SCCP

CCS7 MTP

A Interface

A-Bis Interface

Um

Base Station System

GSM Functional Architecture and Principal Interfaces

GSM protocol layers for signaling


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GSM protocol layers for signaling

CM

MM

RR

MM

LAPDm

radio

LAPDm

radio

LAPD

PCM

RR BTSM

CM

LAPD

PCM

RRBTSM

16/64 kbit/s

Um Abis A

SS7

PCM

SS7

PCM

64 kbit/s /2.048 Mbit/s

MS BTS BSC MSC

BSSAPBSSAP

Protocols involved in the radio interface


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Protocols involved in the radio interface

Level 1-Physical

TDMA frame

Logical channels multiplexing

Level 2-LAPDm(modified from LAPD)

No flag

No error retransmission mechanism due to real time constraints

Level 3-Radio Interface Layer (RIL3) involves three sub layers

RR: paging, power control, ciphering execution, handover

MM: security, location IMSI attach/detach

CM: Call Control(CC), Supplementary Services(SS), Short Message

Services(SMS),


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LAPDm on radio interface


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In LAPDm the use of flags is avoided.

LAPDm maximum length is 21 octets ofinformation. It makes use of more bit to

distinguish last frame of a message. No frame check sequence for LAPDm, it uses

the error detecting performance of thetransmission coding scheme offered by thephysical layer


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The acknowledgement for the next expected frame in theindicator N(R ).

On radio interface two independent flows(one for signaling,and one for SMS) can exist simultaneously.

These two flows are distinguished by a link identifier calledthe SAPI(service access point identifier).

LAPDm SAPI=0 for signaling and SAPI=3 for SMS.

SAP1=0 for radio signaling, SAPI=62 for OAM and SAPI=63 forlayer 2 management on the Abis interface.

There is no need of a TEI, because there is no need todistinguish the different mobile stations, which is done bydistinguishing the different radio channels.

Protocols involved in the A-bis interface


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Protocols involved in the A bis interface

Level 1-PCM transmission (E1 or T1) Speech encoded at 16kbit/s and sub multiplexed in

64kbit/s time slots.

Data which rate is adapted and synchronized.

Level 2-LAPD protocol, standard HDLC Radio Signaling Link (RSL)

Operation and Maintenance Link (OML).

Level 3-Application Protocol

Radio Subsystem Management (RSM)

Operation and Maintenance procedure (OAM)

Presentation of A-bis Interface


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Presentation of A bis Interface

Messages exchanges between the BTS and BSC. Traffic exchanges

Signaling exchanges

Physical access between BTS and BSC is PCM

digital links of E1(32) or T1(24) TS at 64kbit/s. Speech:

Conveyed in timeslots at 4X16 kbit/s

Data:

Conveyed in timeslots of 4X16 kbit/s. The initial userrate, which may be 300, 1200, is adjusted to 16kbit/s

LAPD message structure


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FLAG ADRESS CONTROL INFORMATION 0260 OCT FCS FLAG

SAPI TEI

N(S) N(R)

LAPD message structure


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LAPD

The length is limited to 260 octets of information.

LAPD has the address of the destination terminal, to

identify the TRX, since this is a point to multipoint

interface. Each TRX in a BTS corresponds to one or several

signaling links. These links are distinguished by TEI

(Terminal Equipment Identities).

SAPI=0, SAPI=3, SAPI=62 for OAM.

Signaling Protocol Model


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Presentation on the A-Interface


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Presentation on the A Interface

BSSMAP- deals with procedures that take place logically between the BSSand

MSC, examples:

Trunk Maintenance,Ciphering, Handover, Voice/Data Trunk

Assignment

DTAP- deals with procedures that take place logically between the MSand

MSC. The BSSdoes not interpret the DTAPinformation, it simply repackages it

and sends it to the MSover the Um Interface. examples:

Location Update,MS originated and terminated Calls, Short Message

Service, User Supplementary Service registration, activation, deactivation

and erasure

I t MSC t ti


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Inter MSC presentation


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O

AM

L

A

P

D

BTS

MTP2

SCCP

MTP3

L

A

P

D

O

AM

RR

DT

A

P

BS

S

M

A

P

BSSAP

BSC

MTP1

MTP3

MTP2

SCCP

MTP2

MTP3

SCCP

BSSAP

DTAP/BSSMAP

T

CA

P

MM

CM M

A

P

NSS

R

R

MM

CM

MS

Um

Interface

A bis

Interface

A

Interface

B S i


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Bearer Services

Telecommunication services to transfer databetween access points

Specification of services up to the terminal interface(OSI layers 1-3)

Different data rates for voice and data (originalstandard)

Data service

Synchronous: 2.4, 4.8 or 9.6 kbit/s

Asynchronous: 300 - 1200 bit/s

Tele Services


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Tele Services Telecommunication services that enable voice communication via

mobile phones.

All these basic services have to obey cellular functions, security

measurements etc.

Offered services.

Mobile telephony

primary goal of GSM was to enable mobile telephony offering thetraditional bandwidth of 3.1 kHz.

Emergency number

common number throughout Europe (112); Mandatory for all

service providers; Free of charge; Connection with the highest

priority (preemption of other connections possible). Multinumbering

several ISDN phone numbers per user possible.

Performance characteristics of GSM


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Communication

mobile, wireless communication; support for voice and data

services

Total mobility

international access, chip-card enables use of access points of

different providers

Worldwide connectivity one number, the network handles localization

High capacity

better frequency efficiency, smaller cells, more customers per cell

High transmission quality

high audio quality and reliability for wireless, uninterrupted

phone calls at higher speeds (e.g., from cars, trains)

Security functions

access control, authentication via chip-card and PIN

Di d t f GSM


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Disadvantages of GSM

No full ISDN bandwidth of 64 kbit/s to the user

Reduced concentration while driving

Electromagnetic radiation

Abuse of private data possible High complexity of the system

Several incompatibilities within the GSM standards


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Thank You

GSM Sudacad.pptx

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