Model of GSM

Rasha Morsi

Presentation outline:

AcronymsExample of communication over 3 layers GSM as an OSI related modelFunctional Layers of GSMMore Protocols!Protocol overviewNext presentationReferences

Acronyms

MS : Mobile Station BTS : Base Transceiver Station BSC : Base Station Controller MSC : Mobile-services Switching Center. VLR : Visitor Location Register HLR : Home Location Register AUC : Authentication center EIR : Equipment Identity Register GMSC: Gateway MSC

Example of communication over 3 layers [1]:

Layer 3

Layer 2

Layer 1

Brazil Germany

CookCook

CaptainCaptain

Radio Operator

RadioOperator

RIL3 - CC

RIL3 - MM

CCM

MMRR

APPL LI AC YA ET RION

MS BTSAir (Um)Interface

A-bisInterface

BSC

MAP/EMAP/G

MAP/D

AnchorMSC/VLR

RelayMSC/VLR

DATPRIL3 - RR RSM BSSMAP

Distribution Protocol TCAPComponent Sublayer

Transaction Sublayer

A B C,D HLR/AuCGMSCSMS gateway

MAP/C

PresentationLayer *Session Layer *TransportLayer *

NetworkLayer

PhysicalData Link LAP-D

Radio

LAP-D

64kbps ch

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

PSTM/ISDN

MTP3

MTP2

MTP1

TUP,ISUP

OSI

LAYERS

Functional Layers of GSM:

Three layers : Physical Data Link Layer 3

Physical Layer:

lowest layer of radio interfaceprovides functions to transfer bit streams over

the physical radio links

Data Link Layer:

provides a reliable dedicated signaling link between the MS and BS

Layer 2 protocol is based on an adaptation of ISDN link access procedure (LAP-Dm)

standard LAP-D used between BTS and BSCMessage Transfer Part (MTP) of SS7 is used between

BSC - MSC to provide a reliable data link service.Same Protocol ( MTP1) is kept between MSCs, MSC-

HLR/AUC,AUC-GMSC and GMSC-PSTN.

RIL3 - CC

RIL3 - MM

CCM

MMRR

APPL LI AC YA ET RION

MS BTSAir (Um)Interface

A-bisInterface

BSC

MAP/EMAP/G

MAP/D

AnchorMSC/VLR

RelayMSC/VLR

DATPRIL3 - RR RSM BSSMAP

Distribution Protocol TCAPComponent Sublayer

Transaction Sublayer

A B HLR/AuCC,D

GMSCSMS gateway

MAP/C

PresentationLayer *Session Layer *TransportLayer *

NetworkLayer

PhysicalData Link LAP-D

Radio

LAP-D

64kbps ch

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

PSTN/ISDN

MTP3

MTP2

MTP1

TUP,ISUP

OSI

LAYERS

Layer 3 :Radio Resource Management (RR), Mobility Management

(MM), Call Control Management (CCM or CM).Overall objectives:

establishment, operation, and release of a dedicated radio channel connection (RR).

Location update, authentication, and Temporary Mobile Subscriber Identity (TMSI) reallocation (MM)

establishment, maintenance, and termination of a circuit-switched call (CCM)

Supplementary Service (SS) support Short Message Service (SMS) support

Radio Resource Management (RR): RR protocol provides control functions for the

operation of common and dedicated channels. RIL3 RR :

protocols establishes and releases radio connections between MS-various BSCs for the duration of the call.

Provides system information broadcastinginter- and intracell change of channels

Radio Subsystem Management (RSM) protocol provides functions between the BTS-BSC.

Direct Transfer Application Part (DATP) protocols provide RR messages between the MS-MSC.

The BSS Management Application Part (BSSMAP) protocols provide RR messages between the BSC-MSC

The distinction between DTAP and BSSMAP is provided by a small “distribution” protocol below them.

Mobility Management (MM): defines dialog between MS and the network. Deals with MS location, security functions

(authentication and ciphering key management)

provides connection management services to the Call Control Layer.

Call Control Management Layer (CM): CM protocol controls end-to-end call

establishment ( both mobile originating and terminating)and all functions relating to call management.

Includes independent entities : SMS, SS protocols

More Protocols!

In addition, there are other protocols: MTP3 and Signaling Connection Control Part

(SCCP) used above layer 2 between BSCs - MSCs and MSCs - different databases.

Transaction Capabilities Application Part (TCAP) protocol : sits above SSCP supports transactions between 2 nodes of networkmanages transaction on an end-to-end basis

Mobile Application Part (MAP) protocol :between MSC, VLR, HLR, and AUC in form of query

and response messages.Protocols designated as MAP/B through MAP/H

Protocol Overview:

LAP-D: LAPDm used on the air interface(MS - BTS) LAPD used on A-bis interface(BTS - BSC)

MTP: is the basic layer information exchange MTP1, MTP2 provide the means of protected

message exchange between adjacent nodes of a signaling network

LAPDm Frame Structure

CRC CRC

Address fieldAddress field

Control field Control field

Length indicator field

Fill BitsInformation field

Octet no.

1

2

3

10.12

.

.

.

Bit no. 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1 Octet no.

1

2

3

4

.

.

.

101112

Format type A Format type B

[2]

Network Tx: 00101011Mobile Tx : 11111111 or 00101011

( 8-, 16-, 24-)

LAPD frame structure:Octet no.

Fill Bits

Length indicator field 1

2.....

i

Bit no. 8 7 6 5 4 3 2 1

Length indicator field

Information field

Fill Bits

Octet no.

1

2..i

i +1...k

A-bis format type B-bis format type

[2]

Next Presentation :

Detailed Layer 3 Protocols in relation to mobility management

References :

GSM System Engineering , Asha Mehrotra, Artech House Publishers, 1997An Introduction to GSM , Redl, Weber, and

Oliphant, Artech House Publishers, 1995