Gsm- Lecture 2012 Print

7/30/2019 Gsm- Lecture 2012 Print

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HLR (Home Location Register)It is a permanent databaseabout Mobile subscribers genreally 1 per GSM

operator in a area.

For every incoming call the current location of the subscriber is foundfrom HLR which queries the serving VLR for routing information

It contains

1. List of all subscribers in the circle

2. IMSI

3. Service Subscription information

4. Billing Information

5. Information about current location6. Service restrictions

7. Supplementary services

8. Mobile terminal charceteristics


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VLR (Visiting Location Register)

It is a temprory database, one VLR per MSC. It contains information about

subcribers who are currently in the sevice are covered by MSC/VLR

It contains information about

1. Current location information about the MS(Location area ,cell identitities)

2. TMSI (Temporary mobile station identity)

3. Features currently activated


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Equipment Identity Register

Maintains information about authenticate terminals so that fraudulent stolen ,

non-type-approved terminals can be denied service

Maintains White gray and black lists that may be consulted by networl for

checking authenticity of terminal requesting service


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Logical Channels

1. Traffic Channel (TCH)

2. Control Channels (CCH)


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TCH

CCH

Speech ------ Full Rate(22.8 Kb/s)

Half Rate 11.4 KBp/s

Data-------------- 9 .8 Kb/s

4.6 Bb/s

2.4 Kb/s

BCCH FRREQUENCY CONTROL(FCCH)

(Broadcast) SYNCHRONISATION (SCH)

Speech

DCCH Fast Associated (FACCH)Slow assiciated (SACCH)

Stand Alone(SDCCH)

CCCH Paging (PCH)

(Common) Access Grant (AGCH)

Random Accee (RACH)


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GSM Traffic Channels

Full Rate : User data in one TS per

frame

Half Rate : User data in one TS per

alternate frame


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Full Rate Speech and Data Channels

1. Full Rate Speech channel TCH/FS : Raw data rate 13 kbps. Coded datarate 22.8 kbps

2. Full Rate data channel TCH/F9.6: Raw data rate 9.6 kbps. Coded data

rate 22.8 kbps

3. Full Rate Dta channel for 4.8 Kbps- carries raw user data at 4.8 Kbbps +Forward error correcting coding data is sent at 22.8 Kbps

4. Full Rate Dta channel for 2.4 Kbps- carries raw user data at 4.8 Kbbps

with additional forward error correction data is sent at 22.8 Kbps


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Half Rate Speech and DataChannels

1. Half Rate Speech Channel TCH/HS sampling rate half of the full rate:: Raw data

rate : 6.5 kbps :Channel coding is added to digitised speech Coded data rate : 11.

kbps.

Half Rate Data channel : TCH/H4.8. carries raw user with additional forward errorcorrection applied , The data rate 4.8 kbps is sent at 11.4 kbps

Half Rate data channel :TCH/H2.4- carriers raw user data at 2.4 Kbps, additional

forward error correction , the data is sent at 11.4 Kbps


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GSM Control Channels

Broadcast Channel : BCH

Common Control Channel:CCH

Dedicated Control Channel:DCCH

B d t C t l Ch l BCH


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Broadcast Control Channels:BCH

Operates in Forward link.

Transmits data in first Time slot of certain GSM frames

34 ARFCNS / Forward Link

Data transmission in To time slot(a) Broadcast control channel BCCH- Unidirectional BS to MS

(b) Frequency correction channel FCCH Accurate tuning to BS

(c) Synchronization channel - SCCH Frame Synchronisation


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Broadcast Control Channel(BCCH)

Forward control Channel.Brodcasts information such as

1. Cell and network identity

2. Operating charecteristics of cell(Current control channel,

channel avaliabilty,congestion)3. List of channels currently in use within the cell

4. Frame 2 to 5 in control multi frame contains BCCH data


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Frequency Correction Channel(FCCH)

Occupies TS0 of every First Frame Frame 0. Repated every 10

Frames in the Control chgannel multiframe.

Allows susbcriber unit to synchronize its internal frequency to

exact frequency of Base sation


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FCCH Data Burst

3Start

bits

142 bits of all

zeroes

3 stopbits

8.25 bitsguard

period


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Synchronization Channel(SCH)

Broadcast in TS0 of a Frame Immediately after every FCCH frame

1. Use to identify the serving Base station so that ech mobile unit can

synchronize with it

2. The frame number is sent with Base station identity code.

3. Since different users may be at different distances fro BS it is

necessary to adjust timing information so that received signal is

synchronized with BS clock.

4. BS issues coarse timing advancement commands to MS via SCH

5. SCH is transmitted once every 10 Frames


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SCH Burst

3start

bits

39 data bits64Training

bits

39 Data bits 3stop

bits

8.25 bitsGuard

period


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Control Multi-frame

0

F

1

S

2

B

3

B

4

B

5

B

6

C

10

F

11

S

20

F

21

S

49

C

50

I

F : FCCH burst(BCH) I : Idle

S : SCH burst (BCH)

B : BCCH burst(BCH)

C : PCH/AGCH burst(CCCH)


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Common Control Channels

(CCCHs)1. Paging Channels(PCH) (Forward Link/Down Link/)2. Random Access Channels(RACH) (Reverse Link/Uplink)

3. Access Grant Channel(AGCH) (Forward Link/Down Link/)


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Paging Channel(PCH)

Provides paging signals from BS to all mobiles in a cell

Notifies subcsriber regarding incoming call

Transmit IMSI of Target Subscriber on an incoming call

Broadcast ASCII text messages to all subscribers as a part of the SMS feature.


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Random Access Channel(RACH)

Reverse Link. Uses all the 51 control frames.

1. Used by subcriber to acknowledges page freom PCH

2. Used by Mobiles to originate a call

3. In establishing service BS responds to RACH by allocating channel andassigning a standalone dedicated channel (SDCCH) for signalling during call.


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Access Grant Channel(AGCH)

Used by BS to provide forward link communication to mobile

Assign Dedicated resources to mobile

Instructs the mobile to operate in a particular physical channel in response to a

RACH sent by a mobile station in a previous CCCH frame.

Used by BS to respond to RACH sent by mobile station

It is the final CCH sent by BS before user is moved off Control Channel

D di d C l Ch l


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Dedicated Control Channels(DCCHs)

There are 3 types of dedicated Control channels

1) Stand alone dedicated control channel

2) Slow associated control channel

3) Fast associated control channelThey are Bi-directional. DCCH can exist in any time slot and in any

frame except in TS0 of BCH ARFCN


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Stand alone dedicated control

channel(SDCCH)Carries signaling data between base station and mobile .

SDCCH ensures that MS and Base station remain connected while

the BS and MSC verify subscriber unit and allocate resources.

SDCCH is an intermediate channel which accepts newly connected

call from the BCHand hold the traffic while waiting for Base station

to allocate TCH

SDCCH is used to send authentication and alert messages.

Sl A i t d C t l


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Slow Associated ControlChannel(SACCH)

1 .. 12 S .13 24 I/S0

SACCH is always associated with TCH or SDCCH.

Each ARFCN carries SACCH data for all its cuurent users

On the Forward Link/Downlink: used to send slow but

continuously cahnging information such as transmit Power

level instructions, Timing Advance Instructions

On the Reverselink/Uplink Link : SACCH carries information

of RSSI(Received Signal Strength Info) , quality of TCH,BCH

measurement results of Neighboring cell.

SACCH is transmitted on 13th

Frame and 26th

Frame.


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Fast Associated ControlChannel(FACCH)

57 1 5713 26 8.253

Stealing Flags

Carries urgent messagescontains same type of inforamtion as

SDCCH.

FACCH is assigned whenever SDCCH has not been dedicated

for a user and there is urgent message such as handoff request

FACCH gains access to aTime slot by setting frames from the

traffic channel.This is done by stting Stealing bits.If stealing

bita are set the TS contains FACCH data


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Location Updating

1. MS sends location update request to

VLR new

BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

Old

MSC

New

HLR

VLROld

VLR

New

11

1

2

34

4 41

BTS

i i


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BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

Old

MSC

New

HLR

VLROld

VLR

New

11

1

2

34

4 41

BTS

Location Updating

2. VLR sends to HLR the address of

the VLR new and IMSI of MS.This

updating of HLR not required if the

new area is served by the same VLR


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BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

Old

MSC

New

HLR

VLROld

VLR

New

11

1

2

34

4 41

BTS

3.Service and security related data for

the MS is downloaded to the new VLR

Location Updating


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BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

Old

MSC

New

HLR

VLROld

VLR

New

11

1

2

34

4 41

BTS

4. The MS is sent an acknowledgement

of successful location update.

Location Updating


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BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

Old

MSC

New

HLR

VLROld

VLR

New

11

1

2

34

4 41

BTS

5. The HLR requests the old VLR to

delete the data relating to the MS

Location Updating5


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Mobile Call Origination

MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

1

1

1

1.The MS sends the dialed number

indicating service requested to the

MSC


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MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

2 2

2. The MSC checks from the VLR if the MS

is allowed the requested service.


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MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

3

3. If the call is allowed the MSC routes the call

to GMSC


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MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

4

4. The GMSC routes the call to the local

exchange of called user


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MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS 5

5. The LE applies ringing to the called

terminal


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MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

6

6. Answer back ring tone from the called

terminal to LE


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MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

7777

7

7. Answer back signal is routed to MS through MSC

which also completes the speech path to the MS


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Mobile Call Termination

MSC GMSC

VLRLE

PSTN

Telephone

BSC

BTS

1.PSTN user dials the MSISDN of the called user

in GSM

1

2

3,

74

9

5

8

6

10

1111

11

12


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2. lE routes the call to the GMSC of the called GSM user3. The GMSC uses the dialled MSISDN number to determine the serving

HLRfor the GSM user and interrogates it to obtain the requiredrouting number

4. The HLR requests the current serving VLR for the called party MSNfor a MSRN(MS Roaming Number) so that call can be correctly routedthrough MSC

5.The VLR passes the MSRN to HLR

6 HLR passes the MSRN to the GMSC

7 Using MSRN the GMSC routes the call to the serving MSC

8 MSC interrogates the VLR for the current loaction area identity(LAI) forthe MS

9 VLR provides current LAI for MS to MSC

10 MSC pages the MS via appropriate BSS.The MS responds to the page

and sets up the necessary signalling links11. When BSS has established necessary radio links.MSC is informed

and call is delivered to MS

12. When MS answers the call connection is completed to aclling PSTNuser

A h i i d


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Authentication andEncryption Sequence

BSC MSC

VLRHLR

BTS

1

2

1. A terminal location update VLR sends IMSI to HLR.

2. HLR returns security triplets(RAND,SRES,Kc)


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Authentication Sequence

BSC MSC

VLRHLR

BTS

3

33

3

3. VLR sends RAND to MS


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BSC MSC

VLRHLR

BTS

4

4 4

4

4. MS calculates SRES and returns to

VLR. It also calculates cipher key Kc


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BSC MSC

VLRHLR

BTS

5

55

5. If the SRES returned by the MS matches with

the stored SRES, VLR sends the cipher key Kc

to BTS which uses Kc for ciphering the radio

path.


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BTS

B

BTSA

BSC

A

BSC

B

MSC A

MSC B

HLR

VLR A

VLR B

INTER MSC HANDOVER

1

234

5

6

7

1. BSC A informs MSC A tha t MS needs


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handover from BTS A to BTS B

2. MSCA informs MSC B that handover fromBTS A to BTS B is underway

3. MSC A commands BSC A/BTS A toproceed with handover to BTS B

4. BTS A commands MS to change to aspecified channel on BTS BB

5. MS informs BTS B that it is on the specifiedchannel on BTS B

6. BTS B informs BSC A/MSC A that handoveris complete

7. MSC B informs MSC A that handover toBTS B is complete

S h di i GSM


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Speech coding in GSM

Signal Processing

RPE/LT

E

Channel

Encoder

Interleav

er

A/D Modulat

or

DuplexerSegment

ation

RPE/LT

EDecoder

Channel

Decoder

D/A DeModul

ator

DuplexerDeSegme

ntation

DeInterleaver

Mobile Station

Base Station

8kHz sampling

13 bits/sample

160 sampless

13 bits/sample

260 bits/frame

13 bits/sample

22.8 Kb/s

450 bits/frame 33.85 Kb/s per user

270.8 Kb/s per burst

Speech coding in GSM


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Speech coding in GSM Digitised speech is passed at 64 Kbps though a speech coder which

compresses the 64 Kbps PCM speech to 13 Kbps data rate.

The transcoder models the vocal tract of the user and generates a set of Filterparameters that are used to represent a segment of speech (20 ms) and only

the Filter parameters and impulse input to the filter are transmitted on the radiointerface

GSM transcoder also permits detection of silent periods in speech sampleduring which the transmitter is turned off to save battery power

The Transcoded speech is error protected by passing it through a channelencoder, which utilizes both parity code and convolution code.

Channel encoding increases bit rate from 13 Kbps (260 Kb/20 ms speech) 22.8Kbps(456bits/20 ms) for GSM full rate coder and (11.4Kbps ,16 slots per frame)for GSM half rate coder.

The interleaved data is then modulated by means of Gaussian Minimum shiftkeying(GMSK) and passed through Duplexer which provides filtering to isolatetransmit and receive signals

GSM speech coding leads to better speech quality. It introduces additionaldelayed. Dealy introduced by speech coding(20 ms) , interleaving(37 ms) , A/Dconversion(8 ms) , and processing for transmission and switching (15 ms) addup to 80 ms

For connection to PSTN this 80 ms delay is in addition to existing PSTN delay.Therefore echo controlling devices become neccesssary for GSM calls that usePSTN

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