GSM System - Part I

8/8/2019 GSM System - Part I

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GSM

SYSTEM

Texas Instruments France

Part I


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Introduction System structure Air I/F (Um) time and frequency structure GSM channels MS strategies

- cell selection

- idle mode

- connection establishment

- dedicated mode

MS system partitioning- control unit

- processing unit

Agenda


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Introduction


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GSM overview GSM services (Global System for Mobile communication)

Voice call and related supplementary services (call hold, calling line identity) Fax/data services

SMS / SMS-CB

Wireless capability Radio frequency (GMSK modulation)

Channel quality management (measurements, interferences)

Protection / Error correction (channel encoding)

Authentication - Ciphering

Mobility Cellular network structure

Handover

Location update

Roaming

Multiplexing TDMA FDMA


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Network structure

BS: base station (BTS)BC: base station control center

PSTN: public switched telephone network

ISDN: integrated services device network

PSTN/ISDN


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MSC: Mobile Switching Center

BSC: Base Station Controller

BTS: Base Transceiver Station

HLR: Home Location Register

AVC: Authentication Center

NMC: Network Management Center

ADC: Administration Center

OMC: Operation and Maintenance Center

EIR: Equipment Identity Register

System structure


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System structure (cont.) MS (Mobile Station)

MT (Mobile Termination)

TE (Terminal Equipment)

BSS (Base Station Subsystem):

radio resource management

BTS: manage reception/transmission on the

physical layer

Modulation/demodulation

Equalization (Extract desired signal from

unwanted reflections)

Encoding/decoding

Error correction

TDMA/FDMA multiplexing

Encryption

Measurement of the physical path

BSC: manage radio resource

Channel allocation

Power control (according to BTS

measurements)

Handover management


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System structure (cont.) NSS (Network Sub System):

call establishment / mobility MSC

Communication establishment

SMS transmission Handover

Gateway can be used for routing to fixedphone network (GMSC)

Gateway for SMS (SM-GMSC)

VLR (Visitor Location Register) Memorize subscribers data in a specific

geographic area

Same data as HLR but restricted to thegeographic area

Temporary identity (TMSI)

HLR (Home Location Register) PLMNsubscribers database

IMSI (international subscriber identity)

MSISDN (subscriber phone number)

Profile (services)

Localization (VLR number where thesubscriber is recorded)


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Air interface


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Frequency structure


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Frequency structure


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GSM bandsGSM 450 Band (35 carriers):

- 450,4 MHz to 457,6 MHz: mobile transmit, base receive;

- 460,4 MHz to 467,6 MHz base transmit, mobile receive.

GSM 480 Band (35 carriers):

- 478,8 MHz to 486 MHz: mobile transmit, base receive;

- 488,8 MHz to 496 MHz base transmit, mobile receive.

GSM 850 Band (124 carriers):

- 824 MHz to 849 MHz: mobile transmit, base receive;

- 869 MHz to 894 MHz: base transmit, mobile receive.

Standard or primary GSM 900 Band, P-GSM (124 carriers):


- 935 MHz to 960 MHz: base transmit, mobile receive.Extended GSM 900 Band, E-GSM (includes Standard GSM 900 band), (174 carriers):



Railways GSM 900 Band, R-GSM (includes Standard and Extended GSM 900 Band), (194 carriers):



DCS 1 800 Band (374 carriers):

- 1 710 MHz to 1 785 MHz: mobile transmit, base receive;- 1 805 MHz to 1 880 MHz: base transmit, mobile receive.

PCS 1 900 Band (299 carriers):

- 1 850 MHz to 1 910 MHz: mobile transmit, base receive;

- 1 930 MHz to 1 990 MHz base transmit, mobile receive.

GSM 05.05


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0 1 2 3 4 5 6 2 0 4 2 2 0 4 3 2 0 4 4 2 0 4 5 2 0 4 6 2 0 4 7

0 1 2 3

0 1

1 ( 2 6 - f ra m e ) m u l t if r a m e = 2 6 T D M A f r a m e s ( 1 2 0 m s )

( = 5 1 ( 2 6 - f r a m e ) m u l t i f r a m e s o r 2 6 ( 5 1 - f r a m e ) m u l t i f r a m e s )

4 7 4 8 4 9 5 0

2 4 2 5

1 ( 5 1 - fr a m e ) m u l t i f ra m e = 5 1 T D M A f r a m e s ( 3 0 6 0 / 1 3 m s )

0 1 2 3 4 6 4 7 4 8 4 9 5 00 1 2 3 4 2 2 2 3 2 4 2 5

0 1 2 3 4 5 6 7

1 T D M A f r a m e = 8 t i m e s l o t s ( 1 2 0 / 2 6 o r 4 , 6 1 5 m s )

1 t i m e s l o t = 1 5 6 , 2 5 b i t d u r a t i o n s ( 1 5 / 2 6 o r 0 , 5 7 7 m s )

( 1 b i t d u r a t i o n = 4 8 / 1 3 o r 3 , 6 9 s )

T B E n c r y p t e d b i t s T r a i n i n g s e q u e n c e E n c r y p t e d b i t s T B G P8 , 2 535 82 65 83

F i x e d b i t s T B G P

8 . 2 53

T B E n c r y p t e d b i t s E n c r y p t e d b i t s T B G P

8 , 2 53 96 43

T B E n c r y p t e d b i t s T B G P6 8 , 2 533 64 18

33 9

1 4 2

N o r m a l b u r s t ( N B )

F r e q u e n c y c o r r e c t io n b u r s t ( F B )

A c c e s s b u r s t ( A B )

( T B : T a i l b i t s - G P : G u a r d p e r io d )

1 s u p e r f r a m e = 1 3 2 6 T D M A f r a m e s ( 6 , 1 2 s )

S y n c h r o n i z a t io n s e q u e n c e

S y n c h r o n i z a t io n s e q u e n c e

S y n c h r o n i z a t io n b u r s t ( S B )

1 h y p e r f r a m e = 2 0 4 8 s u p e r f r a m e s = 2 7 1 5 6 4 8 T D M A f r a m e s ( 3 h 2 8 m n 5 3 s 7 6 0 m s )

3

T B

Time structure


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Time structure

TDMA frame Time base of GSM system

Duration = 4.615 ms

FN (Frame Number) used as a counter for schedulingof transmission on the physical channel

FN = [0MAX_FN] 0MAX_FN is an hyperframe (3h 28 mn 53 s)

1 Hyperframe = 2048 Superframes Super-frame

1 superframe = 26 x multiframe 51 (MF51) 1 superframe = 51 x multiframe 26 (MF26)

Multi-frame

Used as basis for multiplexing logical channels

MF51 used for beacon / signaling purpose

MF26 used for traffic purpose (voice / data)

1 TDMA frame = 8 timeslots (multiplexing of different

channels) One channel can only use one slot per TDMA

Timeslot = 577 us

Element transmitted during one timeslot = burst


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Time structure

FB (Frequency Burst): Emitted by the network to allow mobiles synchronize on frequency Fixed value (0) bearer transmission

SB (Synchronization Burst): Emitted by the network to allow mobiles synchronize on time Big fixed sequence permitting timing synchronization

Includes system date (FN) and BSIC (Base Station Identity Code)

AB (Access Burst): Transmitted by the mobile to request resources to the network Transmission time inferior to slot duration (due to propagation time)

Small content (only 8 bits are encoded)

NB (Normal Burst): Used for general transmission purpose (control messages speech data)

Training sequence used for time adjustment during the communication Control messages sent in one block always using 4 x NB


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Transmission on the physical channel

f

f10 1 2


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GSM channels


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GSM Logical channels

FCCH: Frequency burst (fixed values) used for frequency correction

SCH: Synchro burst used for time synchronization (frame number and fine tuning)

network and BTS identification (BSIC code)

BCCH broadcast following kind of information (NB)

Cell selection mode

Area identifier (location update)

Random access rules

Control channel organization

CBCH organization

Neighbor cells description

CCCH channels

PCH: MS calling using the TMSI (call, SMS, authentication) (NB)

AGCH: Radio resource allocation (NB)

RACH: Request from the MS (AB)

CBCH: Information broadcast


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GSM Logical channels: dedicated channels

TCH: traffic channel (voice/data) (NB)

SDCCH: low rate channel dedicated to signaling (NB)

SACCH (Slow Associated Control Channel) (NB)

Timing advance update mechanism

Transmit power control

Radio link quality control

Measurement reporting (for handover)

FACCH (Fast Associated Control Channel) (NB)

Handover execution


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F i g u r e 8 a : T D M A f r a m e m a p p i n g f o r F C C H + S C H + B C C H + C C C H

GSM 05.02Beacon frequency

FCH Frequency Channel (downlink)SCH Synchronization Channel (downlink)BCCH Broadcast Control Channel (downlink)CCCH Common Control Channel

PCH Paging Channel (downlink)AGCH Access Grant Channel (downlink)RACH Random Access Channel (uplink)

SDCCH/4: Stand-alone Dedicated Control Channel (up&downlink)


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Other frequencies (traffic)

SDCCH/8 Stand-alone Dedicated Control Channel (up&downlink)

TCH/F Traffic Channel Full Rate (up&downlink)

TCH/H Traffic Channel Half Rate (up&downlink)

SACCHSlow Associated Control Channel (up&downlink)

FACCHFast Associated Control Channel (up&downlink)

CBCH Cell Broadcast Control Channel (downlink)


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MS Strategies


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Cell selection

Monitor the entire GSM band

Select the strongest carrier (up to 30)

Detect the Frequency Burst from this carrier (FCH)

Apply Frequency Correction

Detect the Synchronization Burst from this carrier (SCH)

Apply Time-base Correction

Read the system information (BCCH) If the Cell is suitable, MS camps on the cell (Serving Cell)

and start Idle mode

Purpose: Select the best suitable cell as the Serving Cell


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Monitor the Paging Sub-Channel (PCH)

Monitor the Serving Cell System Information (BCCH)

Monitor the Cell Broadcast messages (CBCH) (optional)

Monitor the Serving Cell beacon receive power level

Purpose: maintain a good link with the Serving Cell in order

to get normal services from the BTS (call) when requested

Activity from Serving Cell

Idle mode


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Idle mode

Monitor the receive power level of a list of neighbor cells (BA list)

Select the 6 strongest carriers Detect the Frequency Burst from those carriers (FCH)

(Re-)Detect the Synchronization Burst from those carriers (SCH)

Read the system information from those carriers (BCCH)

Activity from neighbor cells


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Idle mode

Service quality maintenance

Monitor the link degradation (count the bad data blocks read from PCH)

Maintain a list of 6 suitable neighbor cells Apply a degradation criteria to decide to abandon the current Serving

Cell and select one of the 6 suitable neighbors as the new Serving Cell


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Idle mode

Location Update

Purpose: Establishment of a full duplex communication (over SDCCH) to

notify the network that the MS is moving within a new Location Area.

This procedure is done prior to camping on a cell in normal services.

MS calls the network (Mobile Originated)

A signaling channel is established (SDCCH)

MS registration is performed

MS is told whether the Cell can grant normal service


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Location update

RACH

AGCH

SDCCH


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MS BSS VLR HLR AUC

Send Parameters

Send Parameters Result(RAND, SRES, Kc)

MSC

Authenticate

Authentication Request

Authentication Response

Authentication Complete

Service Accept (conditional)

Service Reject (if not accepted)

Authentication Complete

Authentication


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HLR

Home

Location

Register

MSC/VLR Mobile

SIM

cardA3

A8

Store

Kc

BSS

KiRAND (128 bits) RAND

(128 bits)

SRES=

SRESc?

Acces

Granted

Yes

NO Deny Access

SEND

to BTSTransfer CipherKey to BTSKcCipher Key

64 bits

Then, A5 algorithm used to cipher data using Kc key

Authentication process


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Connection establishment

Mobile Originated Call

Purpose: the MS user dial a phone number to call a distant (establish a

speech communication for instance). As a pre-requisite, the MS must be in

Idle mode in normal services.

Send Channel Request messages to the BTS (RACH)

Keep reading CCCH/BCCH channels looking for Access Grant

BTS responds with an Immediate Assignment message (AGCH).

The description of the allocated full duplex channel is provided

MS stops Connection Establishment mode and enter Dedicated

Mode

MS Leave Idle mode and enter Connection Establishment mode:


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Connection Establishment

RACH

AGCH

SDCCH / TCH

TCH

Mobile Originated Call


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MS BSS MSC/VLR HLR/EIR/AUC PSTN/ISDN

Dial Number

Press SEND

Channel Request

Immediate

Assignment

Service Request

TMSI, Call Setup Service Request

TMSI, Call Setup

RACH

AGCH

SDCCH

Authentication, Ciphering and TMSI assignment

Setup

(call information) Send Call Setup

Information

Check IMEI (optional)

Call CompleteCall Proceeding

Assignment Command (TCH)

Assignment Complete Initial Address Message (IAM)Called party phone rings

Address Complete Message (ACM)Alerting

Answer Message Called party takes

phone off hookConnect (TCH/FACCH)

Connect Acknowledge

Conversation starts


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Mobile Terminated Call

Purpose: the network calls the MS (to establish a speech communication for

instance). As a pre-requisite, the MS must be in Idle mode in normal services.

MS leaves Idle mode and enters Connection Establishment mode:

Send Channel Request messages to the BTS (RACH)

Keep reading CCCH/BCCH channels looking for Access Grant

BTS responds with an Immediate Assignment message (AGCH). Thedescription of the allocated full duplex channel is provided

MS stops Connection Establishment mode and enter Dedicated Mode

MS receives a Paging Request message (PCH)

Connection establishment


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Connection Establishment

RACH

AGCH

SDCCH / TCH

TCH

Mobile Terminated Call


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Two types of dedicated channels:

SDCCH (with associated SACCH / FACCH)

TCH/F or TCH/H (with associated SACCH / FACCH)

Data bits Data bitsTraining Sequence

Tail Tail

GuardPeriod

Stealing Flags

57 bits 57 bits26 bits3 31 1 8,25

Dedicated mode


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Dedicated mode

Stand Alone Control Channel(SDCCH)

Signaling outside a call: in some cases, there is a need to

establish a connection between an MS and the Network solely for

signaling matters (location updating or short message service for

instance).

Signaling in connection with a call: SDCCH is used to manage the

call establishment (Immediate Assignment).


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Dedicated mode

Used for carrying the main binary data flow.

Traffic Channel TCH (/F or /H)

Full Rate Speech

Enhanced Full Rate Speech

Full Rate Data 14400

Full Rate Data 9600

Full Rate Data 4800

Full Rate Data 2400

Half Rate speech AMR (Adaptive Multi Rate)


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Dedicated mode

Traffic Channel TCH (/F or /H)

T T T T T T T T T T T T A T T T T T T T T T T T T -

T t T t T t T t T t T t A T t T t T t T t T t T a

( a )

( b )

2 6 f r a m e s = 1 2 0 m s

( a ) c a s e o f o n e f u l l r a t e T C H

T , t : T D M A f r a m e f o r T C H A , a : T D M A f r a m e f o r S A C C H / T- : i d l e T D M A f r a m e

( b ) c a s e o f t w o h a l f r a t e T C H s

t


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Dedicated mode

Associated Control Channels

Slow Associated control Channel (SACCH): (2 per second)

Used in uplink to report the neighbor cell measurement data needed for the

decision concerning handover

Used in downlink to carry the system information containing the list of

neighbor cells to be monitored Used in downlink to carry physical layer information:

txpwr, timing advance

Fast Associated control Channel (FACCH): (every 20ms if needed)

Used in uplink or downlink to manage call establishment progress,

subscriber authentication or handover for instance.

No real channel dedicated to FACCH. It is carried by TCH channel (user

data are stolen from TCH and used for signaling).


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Timing advance algorithm


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Dedicated mode

Service quality maintenance

MS continuously monitors the downlink link degradation

(percentage of bad information bit received by the MS).

BTS continuously monitors the uplink link degradation(percentage of bad information bit received by the BTS).

MS maintains a list of 6 suitable neighbor cells

MS reports to the BTS the downlink quality and the 6 suitable

neighbor base-station identities.

BTS decides whether the channel quality is acceptable. If not,

BTS arranges a Handover to another channel or another BTS.


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Dedicated mode

Monitor the receive power level of a list of neighbor cells (BA list)

Select the 6 strongest carriers

Detect the Frequency Burst from those carriers (FCH)

(Re-)Detect the Synchronization Burst from those carriers (SCH)

Dedicated Mode

Activity from neighbor cells


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MS System partitioning


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P t l l


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MMI

CM

MM

RR

DL

PL

L3

L2

L1

Protocol layers

C t l it


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Control unit MMI (Man-Machine Interface)

Keyboard management

Display management SIM card management

Layer 3 CM (Call Management)

CC (Call Control)

SMS (Short Message Services)

SS (Supplementary Services) MM (Mobility Management)

Authentication

Location update

IMSI attach / detach

RR (Radio Resource Management) Cell selection / re-selection

Physical channel establishment / release

Ciphering Handover

C t l it


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Control unit

Layer 2 (Data Link Layer) Used for every signaling channels (Except FB/SB/RACH) Frame segmentation and numbering Error detection Acknowledge / Re-transmission Data flow multiplexing (signaling, data)

Physical Layer Task scheduling Burst reception and transmission control

Servo Control management Receiver Adaptive Gain Control (AGC) Time Tracking Timing Advance Adaptive Frequency Control (AFC) Transmit Power Control


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Documents

GSM System - Part I