GSM : The Network Layer

Rasha Morsi

Presentation Outline:

Radio Resource ManagementMobility ManagementCall Control ManagementLayer 3 ProtocolsMobile Application Part protocolFlow chart of initial mobile acquisitionMobile originating call procedureStructure of a layer 3 messageWhy GSM?

Acronyms:

TMSI : Temporary Mobile Subscriber Identity IMSI : International Mobile Subscriber IdentityBCCH : Broadcast Control ChannelFCCH : Frequency Correction ChannelSCH : Synchronization ChannelRACH : Random Access ChannelAGCH :Access Grant ChannelSDCH : Stand-alone Dedicated Control ChannelSABM: Set Asynchronous Balanced Mode

Acronyms (cnt’d)

RIL3 : Radio Interface Layer 3MAP : Mobile Application PartUA : Unnumbered AcknowledgeTUP : Telephone User Part ISUP : ISDN User PartTCH : Tariff ChannelACCH: Associated Control ChannelBSSMAP : BSS Management PartSCCP : Signaling Connection Control Part

RIL3 - CC

RIL3 - MM

CCM

MM

RR

Air (Um)Interface

A-bisInterface

MAP/EMAP/G

MAP/D

DATP

RIL3 - RR RSM BSSMAP

Distribution Protocol TCAPComponent Sublayer

Transaction Sublayer

A B C,D HLR/AuC

MAP/C

PresentationLayer *Session Layer *TransportLayer *

NetworkLayer

Physical

Data Link LAP-D

Radio

LAP-D

64kbps ch

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

MTP3

MTP2

MTP1

TUP,ISUP

OSI

LAYERS

Radio Resource Management sublayer:

Tasks closely related to physical layermanagement of frequency spectrumeverything to do with maintaining a clear

channel between PLMN and MSHandles all procedures necessary to establish,

maintain and release dedicated radio channels

Procedures in RR layer:

Channel assignment Channel releaseChannel change and handoverChange of channel frequencies, hopping sequences

(algorithms) and frequency tablesMeasurement reports from the MSPower control and Time advanceModification of channel modes (speech and data)Cipher mode setting

Mobility Management sublayer:

Cope with all effects of handling a mobile user that are not directly related to radio functions. Support of user mobility, registration, and

management of mobility data checking user and equipment identity user allowed to use services ? Extra services

allowed ? Support user confidentiality ( register user under a

TMSI)

Provision of user security Provision of an MM connection, based on existing

RR connection, to the CM sublayer

Procedures in MM sublayer :

Location UpdatePeriodic updatingAuthentication procedure IMSI attach procedure (on power up a MS will present

its IMSI to network and get a TMSI) IMSI detach ( on power off of a MS, detach procedure to

tell network it is no longer in service.TMSI reallocation Identification

Call Control sublayer:

Manages all functions necessary for circuit-switched call control in GSM PLMN

includes several protocol entities e.g. CC itself, SS, and SMS.

CC entity includes control functions defined for ISDN network signaling, messages and procedures required for

call establishment,

call clearing

call information etc.

Procedures in CC sublayer:

Call establishment for mobile-originated callsCall establishment for mobile-terminated callsChanges of transmission mode during an

ongoing call ( incall modification)Call reestablishment after interruption of an

MM connectionDual-tone multifrequency (DTMF) control

procedure for DTMF transmission

RIL3 - CC

RIL3 - MM

CCM

MM

RR

Air (Um)Interface

A-bisInterface

MAP/EMAP/G

MAP/D

DATP

RIL3 - RR RSM BSSMAP

Distribution Protocol TCAPComponent Sublayer

Transaction Sublayer

A B C,D HLR/AuC

MAP/C

PresentationLayer *Session Layer *TransportLayer *

NetworkLayer

Physical

Data Link LAP-D

Radio

LAP-D

64kbps ch

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

MTP3

MTP2

MTP1

TUP,ISUP

OSI

LAYERS

Layer 3 Protocols:

RIL3-RR : establish and release radio connections between an

MS and various BSCs for the duration of a call despite user movements.

Provide system information broadcasting provide inter- and intracell change of channels provides ciphering mode setting

Radio Subsystem Management ( RSM ): provides RR functions between BTS and BSC

Direct Transfer Application Part ( DTAP ): provide RR messages between MS and MSC

BSSMAP : provides RR messages between BSC and MSC

Distribution Protocol : distinguishes between BSSMAP and DTAP

RIL3-MM : deals with MS location management

location updatingIMSI attachperiodic updating

security aspects of systemauthenticationTMSI reallocationidentification.

RIL3-CC : deals with mobile originating and terminating call

establishment SS SMS

All MM and CC functions reside in MSC. All CC and MM messages are not interpreted by either the BTS or BSC. An RR connection needs to be established before communication between MS and MSC can occur.

RIL3 - CC

RIL3 - MM

CCM

MM

RR

Air (Um)Interface

A-bisInterface

MAP/EMAP/G

MAP/D

DATP

RIL3 - RR RSM BSSMAP

Distribution Protocol TCAPComponent Sublayer

Transaction Sublayer

A B C,D HLR/AuC

MAP/C

PresentationLayer *Session Layer *TransportLayer *

NetworkLayer

Physical

Data Link LAP-D

Radio

LAP-D

64kbps ch

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

SCCP

MTP3

MTP2

MTP1

MTP3

MTP2

MTP1

TUP,ISUP

OSI

LAYERS

Transaction Capabilities Application Part (TCAP) : provides correlation between individual operations

and structured exchanges building up a complete transaction.

Transaction sublayer : manages transactions on an end-to-end basis.

Component sublayer : correlates commands and responses within a dialog.

MAP Protocol :

Handles non-call related signaling between different network parts

designed to interact with MSC, VLR/HLR, AUC, and GMSC so they can communicate with each other.

Query - responsecontains a number of Application System Elements

(ASE)

all messages contain either mandatory or optional application parameters.

Functions can be grouped into two categories: MM and

basic service support.

MAP Protocol Connections :RIL3

BSS

BSSMAP

MAP/B

VLR

MSC EIR

HLR

GMSC

SMSGateway

MAP/C

MAP/C

MAP/I

MAP/D

MAP/F

VLR

MSC

MAP/H

MAP/B

MAP/EMAP/G

MAP Communication between peer-to-peer nodes :

ASE1

ASE n

ASE 1

ASE n

MAP

MAP

TCAP TCAP

Physical Connection

Flow Chart of Initial Mobile Acquisition:

Power onScan channels,monitor RFlevels

Select channel with highestRF level among control ch.s

Scan ch. For frequency correction burst (FCCH)

IsFCCH detected?

NO

YES

Scan ch. For timing sync.Burst ( SCH )

Select ch. With next highestRF level from control list

isSCH detected ?

NO

YES

Read data from BCCH ch and determine if the ch is a control ch ( BCCH )

isthe currentBCCH incl.

YES

NOFrom the ch data update thecontrol ch list

Camp on BCCH and start decoding

Mobile Originating Call Procedure:MS BSS MSC VLR HLR/EIR/AUC PSTN/ISDN

Press send RIL3-RR Ch. RequestRACH

RIL3-RR IMM SABMAGCH

< identity of message >SDCH (Up.Lk)

UASDCH (Dn. Lk.)

Service Request

TMSI, Call Setup

SDCH (Up. Lk.)

Service Request

TMSI, Call Setup

MAP/B Service Request, TMSI,Call Setup

Authentication, Ciphering, and TMSI assignment

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

RIL3-CC Setup ( Call Information ) MAP/B Send Callsetup information

Call complete MAP/B

RIL3-CCAssignment cmd

RIL3-CC assign.

complete

SDCH (Up.Lk.)

RIL3-CC assigncomplete

TUP/ISUP Initial Address Message ( IAM )

“ TUP/ISUPAnswer Signal”

TUP/ISUP Address Complete Message ( ACM )RIL3-CC alerting

Answer MessageRIL3-CC connectTCH/ACCH

RIL3-CC Connect Ack.

Conversation Starts

RIL3-CC call ProceedingSubscriberPhone Rings

Structure of a layer 3 message:

CRC

Address field

Control field

Length indicator field

Information field

1

2

3

4

.

.

.

101112

Layer 2 Format type B

8 7 6 5 4 3 2 1 Octet Bit 8 7 6 5 4 3 2 1 Octet

Information Elements “ mandatory ”

Information Elements “ optional ”

0 Message Type

TIflag

TI Protocol Discriminator

Structure of a GSM layer 3 message

Transaction Identifier ( TI ) :

Used to distinguish between possible (multiple) parallel CC connections and between various transactions taking place over these simultaneous CC connections.

TI not relevant for RR and MM connections ( coded “ 0000” )

TI flag : indicates originator of a CC message ‘0’ by originator and coded ‘1’ by its peer entity

originator fills in TI value. Kept and serves as a label during transaction

Protocol Discriminator ( PD )

Links layer 3 protocol to the entity the message is addressed to.

Identifies 6 protocolsProtocol PD Binary

RRM 0110MM 0101CC 0011SMS 1001SS 1011Test procedure 1111All other values reserved

Message Type :

MT indicates the function of layer 3 message.Uses lower 6 bits ( 64 possible message types)

for a protocol type indicated by PD.MT is part of a set of messages in a protocolBit 8 = 0 ( reserved )Bit 7 = send sequence variable and may be

used for MM and CC messages.

Information Elements (IE):

Depending on MT, may or may not have one or more IE

Types of IE : Mandatory fixed length

( MF ) Mandatory variable length

( MV) Optional Fixed Length (OF) OV

Information Elements “ mandatory ”

Information Elements “ optional ”

0 Message Type

TIflag

TI Protocol Discriminator

Structure of a GSM layer 3 message

Why GSM ?

DigitalHigh audio quality and link integrityHigh spectral efficiencyIdentical system in all countriesIntersystem RoamingHigh degree of flexibility ( open architecture

that will allow new services to be introduced in the future.

Economy in both sparsely and heavily populated areas

Integration with ISDNOther security featuresSMS and use of facsimile featuresEasy to introduce the systemLow-cost infrastructure

References:

GSM System Engineering, Asha Mehrotra,Artech House Publishers, 1997

An Introduction To GSM, Redl, Weber, Oliphant, Artech House Publishers, 1995