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GSM : The Network Layer

Rasha Morsi

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Presentation Outline:

Radio Resource Management

Mobility Management

Call Control Management

Layer 3 Protocols

Mobile Application Part protocol

Flow chart of initial mobile acquisition Mobile originating call procedure

Structure of a layer 3 message

Why GSM?

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Acronyms:

TMSI : Temporary Mobile Subscriber Identity

IMSI : International Mobile Subscriber Identity

BCCH : Broadcast Control Channel

FCCH : Frequency Correction Channel

SCH : Synchronization Channel

RACH : Random Access Channel AGCH :Access Grant Channel

SDCH : Stand-alone Dedicated Control Channel

SABM: Set Asynchronous Balanced Mode

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Acronyms (cntd)

RIL3 : Radio Interface Layer 3

MAP : Mobile Application Part

UA : Unnumbered Acknowledge

TUP : Telephone UserPart

ISUP : ISDN UserPart

TCH : Tariff Channel ACCH: Associated Control Channel

BSSMAP : BSS Management Part

SCCP : Signaling Connection Control Part

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RIL3 - CC

RIL3 - MM

CCM

MM

RR

Air (Um)

Interface

A-bis

Interface

MAP/EMAP/G

MAP/D

DATP

RIL3 - RR RSM BSSMAP

Distribution ProtocolTCAP

Component Sublayer

Transaction Sublayer

A B C,D HLR /AuC

MAP/C

Presentation

Layer *

Session

Layer *

Transport

Layer *

Network

Layer

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

O

S

I

L

A

Y

E

R

S

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Radio Resource Management sublayer:

Tasks closely related to physical layer

management of frequency spectrum

everything to do with maintaining a clear

channel between PLMN and MS

Handles all procedures necessary to establish,

maintain and release dedicated radio channels

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Procedures in RR layer:

Channel assignment

Channel release

Channel change and handover

Change of channel frequencies, hopping sequences

(algorithms) and frequency tables

Measurement reports from the MS

Power control and Time advance

Modification of channel modes (speech and data)

Cipher mode setting

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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 servicesallowed ?

Support user confidentiality ( register user under a

TMSI)

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Provision of user security

Provision of an MM connection, based on existing

RR connection, to the CM sublayer

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Procedures in MM sublayer :

Location Update

Periodic updating

Authentication 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

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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.

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Procedures in CC sublayer:

Call establishment for mobile-originated calls

Call establishment for mobile-terminated calls

Changes of transmission mode during an

ongoing call ( incall modification)

Call reestablishment after interruption of an

MM connection

Dual-tone multifrequency (DTMF) control

procedure for DTMF transmission

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RIL3 - CC

RIL3 - MM

CCM

MM

RR

Air (Um)

Interface

A-bis

Interface

MAP/E

MAP/G

MAP/D

DATP

RIL3 - RR RSM BSSMAP

Distribution ProtocolTCAP

Component Sublayer

Transaction Sublayer

A B C,D HLR /AuC

MAP/C

Presentation

Layer *

Session

Layer *

Transport

Layer *

Network

Layer

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

O

S

I

L

A

Y

E

R

S

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Layer 3 Protocols:

RIL3-RR :

establish and release radio connections between an

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

Provide system information broadcasting

provide inter- and intracell change of channels

provides ciphering mode setting

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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

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RIL3-MM :

deals with MS location management

location updating

IMSI attach

periodic updating

security aspects of system

authentication

TMSI reallocation

identification.

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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.

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RIL3 - CC

RIL3 - MM

CCM

MM

RR

Air (Um)

Interface

A-bis

Interface

MAP/E

MAP/G

MAP/D

DATP

RIL3 - RR RSM BSSMAP

Distribution ProtocolTCAP

Component Sublayer

Transaction Sublayer

A B C,D HLR /AuC

MAP/C

Presentation

Layer *

Session

Layer *

Transport

Layer *

Network

Layer

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

O

S

I

L

A

Y

E

R

S

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Transaction Capabilities Application Part

(TCAP) :

provides correlation between individual operationsand 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.

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MAPProtocol :

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 - response

contains a number of Application System Elements

(ASE)

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all messages contain either mandatory or optional

application parameters.

Functions can be grouped into two categories: MM and

basic service support.

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MAPProtocol Connections :RIL3

BSS

BSSMAP

MAP/B

VLR

MSCEIR

HLR

GMSC

SMS

Gateway

MAP/C

MAP/C

MAP/I

MAP/D

MAP/F

VLR

MSC

MAP/H

MAP/B

MAP/EMAP/G

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MAP Communication between peer-

to-peer nodes :

ASE1

ASE n

ASE1

ASE n

MAPMAP

TCAP TCAP

Physical Connection

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Flow Chart of Initial Mobile Acquisition:

Power onScan channels,

monitor RF

levels

Select channel with highest

RF level among control ch.s

Scan ch. For frequency

correction burst (FCCH)

Is

FCCH detected?

NO

YES

Scan ch. For timing sync.

Burst ( SCH )

Select ch. With next highest

RF level from control list

isSCH detected ?

NO

YES

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

isthe current

BCCH incl.

YES

NOFrom the ch data update the

control ch list

Camp on BCCH and start

decoding

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

Press send RIL3-RR Ch. Request

RACH

RIL3-RR IMM SABM

AGCH

< 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

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

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

setup information

Call complete MAP/B

RIL3-CC

Assignment cmd

RIL3-CC assign.

complete

SDCH (Up.Lk.)

RIL3-CC assign

complete

TUP/ISUP Initial Address Message ( IAM )

TUP/ISUP

Answer

Signal

TUP/ISUPAddress Complete Message ( ACM )RIL3-CC alerting

Answer MessageRIL3-CC connectTCH/ACCH

RIL3-CC Connect Ack.

Conversation Starts

RIL3-CC call Proceeding

SubscriberPhone

Rings

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Structure of a layer 3 message:

CRC

Address field

Control field

Length indicator field

Information field

1

2

3

4

.

.

.

10

11

12

Layer2 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

TI

flagTI Protocol

Discriminator

Structure of a GSM layer 3 message

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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

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Protocol Discriminator ( PD )

Links layer 3 protocol to the entity the

message is addressed to.

Identifies 6 protocolsProtocol PD Binary

RRM 0110

MM 0101

CC 0011SMS 1001

SS 1011

Test procedure 1111

All other values reserved

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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 protocol

Bit 8 = 0 ( reserved )

Bit 7 = send sequence variable and may be

used for MM and CC messages.

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Information Elements (IE):

Depending on MT, may

or may not have one or

more IE

Types of IE :

Mandatory fixed length

( MF )

Mandatory variablelength ( MV)

Optional Fixed Length

(OF)

OV

Information Elements

mandatory

Information Elements

optional

0 Message Type

TI

flagTI Protocol

Discriminator

Structure of a GSM layer 3 message

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Why GSM ?

Digital

High audio quality and link integrity

High spectral efficiency

Identical system in all countries

Intersystem Roaming

High degree of flexibility ( open architecture

that will allow new services to be introduced

in the future.

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Economy in both sparsely and heavily

populated areas

Integration with ISDN

Other security features

SMS and use of facsimile features

Easy to introduce the system

Low-cost infrastructure

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References:

GSM System Engineering, Asha Mehrotra,

Artech House Publishers, 1997

An Introduction To GSM, Redl, Weber,

Oliphant, Artech House Publishers, 1995