GSM Signaling System

Objectives

At the end of this course, you will be able to:z Know GSM system signaling model z State various types of GSM protocol messagesz Understand GSM system basic signaling procedure

Content

GSM System Signaling Model GSM protocol message Basic Signaling Procedure

GSM Interface

SGSN

AbisUm

BTS

BSC

TRAU

GGSN

MSC/VLR/GMSC

NSMU FSMU

Ater A

OMC

Qx

HLR/AUC

EIR SMCPSTN / PLMN / PSPDN / ISDN

BTS

BTS

BTS

Gb

GSM System Signaling Model

z GSM system signaling model adopts the lowest three layer of OSI seven layer protocol model, from low to high:

Application layerLink layer/Network layer

Physical layer

Physical Layer

The physical layer provides error protection transmission. It defines the electric parameters of transmission.

In ZTE-GSM digital mobile communication systems, The physical layer of Um interface between MS and BTS is Radio link. Abis interface physical layer adopts 75-ohm coaxial cable or 120-ohm symmetrical twisted pair whose rate is 2Mbit/s.

Data Link Layer

The data link layer major functions: frame transmissionerror-free transmission and realize end-to-end bits transfer between two connective entities. Open maintenance and close the connection of two connective entities.

The link layer protocol used by the GSM system at the radio interface is the LAPDm protocol (Dm channel link access procedure). The data link layer of Abis interface between BTS and BSC is LAPD (D channel link access procedure).

Application Layer

Application Layer

CM MM RR

CC SS SMS

Application Layer

Radio Resources (RR) management handles the establishment, maintenance and release of physical channels. Its major functions are performed by BSC. Part of functions are performed by BTS.

Mobility Management (MM) deals with the mobile stations register and the identify of the mobile subscriber, The function are performed by MSC.

The CM Layer is composed of three functional entities: Call Control (CC), deals with the functions to establishmaintenance and release the callShort Message Service support (SMS) and Supplementary Service support (SS).

MobileSwitching

Centre(MSC)

) ) ))))Base

TransceiverStation(BTS)

BaseStation

Controller(BSC)

EquipmentID

Register(EIR)

VisitorLocationRegister(VLR)

HomeLocationRegister(HLR)

Authenti-cationCentre(AUC)

MobileSwitching

Centre(MSC)

Public SwitchedTelephone Network

(PSTN)

MAP/TCAP + ISUP/TUP

To other VLR

ISUP/TUP

DTAP + BSSMAP

MAP/TCAP

MAP/TCAP MAP/TCAP

BTSM

A-bis interfaceA interface

GSM protocol

DTAP

DTAP Direct Transfer Application PartBSSMAP BSS Management Application PartCM Call ManagementMM Mobile ManagementRR Radio Resource Management

SCCP Signalling Connection Control PartMAP Mobile Application PartTCAP Transaction Capability Application PartISUP ISDN User PartMTP Message Transfer Part

CM

MM

RR

Sig. layer 2

Layer 1 (air)

Sig. layer 2

Sig. layer 1

MTP

SCCP

BSSMAPRR

(CM+MM)

MS BSC

MTP MTP

SCCP SCCP

CM

MM

BSSMAP TCAP

MAP

ISUP

MSC

Sig. layer 2

Layer 1 (air)

BTS

(CM)

(MM)

(RR)

(CM)

(MM)

(RR)

(CM)

(MM)

(LAPDm) (LAPDm) (LAPD)Sig. layer 2

Sig. layer 1

(LAPD)

BTSMBTSMRR'

BTSM BTS Management

Um Interface Abis Interface A Interface Inter-MSCInterface

GSM Protocol Architecture

Content

GSM System Signaling Model GSM protocol message Basic Signaling Procedure

Um interface protocol stack

CM

MM

RR

LAPDm

Layer1 Layer1

LAPDm

RR

MSBTS

Um interface

LAPDm frame format

The address field contains the service access point identifier (SAPI). z SAPI = 0 represents the signaling linkz SAPI = 3 represents the short message link

In the control field, z N (S) represents the sending serial number z N (R) represents the receiving serial number

Address Control Information

SAPI N(S) N(R)

Operation mode

Acknowledged mode: Requires the confirmation from the receiver. This mode provides a whole set of control mechanism for error recovering and flow control, the establishment mechanism and release mechanism for multi-frame operations.

Unacknowledged mode: The receiver is not required to send a confirmation upon reception of a UI frame. This operation mode does not provide flow control or error recovering mechanism.

RR messagesType Message

Channel establishment message Immediate assignment

Immediate assignment reject

Ciphering message Ciphering mode command

Ciphering mode complete

Handover message

Handover command

Handover complete

Handover failure

Handover access

Channel release message Channel release

Paging message Paging request

Paging response

System info System info:18

MM messagesType Message

Registration messagesIMSI detach indicationLocation updating acceptLocation updating reject

Location updating request

Security messages

Authentication rejectAuthentication requestAuthentication responseIdentity requestIdentity responseTMSI reallocation commandTMSI reallocation complete

Connection-management messagesCM service acceptCM service rejectCM service request

CC messagesType Message

Call establishment messages

Alerting

Call confirmed

Call proceeding

Connect

Call clearing messages

Disconnect

Release

Release complete

Miscellaneous messagesCongestion control notify

Start DTMF

Stop DTMF

Abis Interface Protocol stack

Abis

BTS

BTSM

LAPD

Layer1

BSC

interface

Layer1

LAPD

BTSM

RR

LAPD Protocol

z SAPI = 0 represents the signaling link,

z SAPI = 62 represents the O&M link, and

z SAPI = 63 represents the management link of the LapD layer.

A interface protocol stack

BSC

MTP3

MTP2

Layer1

MSC

A interface

Layer1

MTP2

MTP3

RR

SCCP SCCP

BSSAP BSSAP

MM

CM

Relationship between SCCP and OSI model

MTP-1

MTP-2

MTP-3

SCCP

ISP

TCAP

INAP OMAP MAP BSSAP ISUP TUP

HLR VLR

Layer1

Layer2

Layer3

46

Layer 7

BSSAP

MTP

SCCP

Distribution

BSSMAP DTAP

BSSAP

RR

MM

CC

MTP

SCCP

Distribution

BSSMAP DTAP

BSSAP

L1

LAPDm

RR

CC

MM

RR

LAPDmL1

MSC BSS MS

DTAP

BSSMAP DTAP

SCCP message

CR Connection request CC Connection confirm CREF Connection refused RLSD Released RLC Release complete DT1 Data form 1 UDT Unit data UDTS Unit data service

BSSMAP Message -1 Assignment messages (setup of traffic channels)

z Assignment requestz Assignment completez Assignment failure

Handover messages z Handover request (to BSC: request for handover to that BSC)z Handover required (to MSC: inter BSC/MSC handover required)z Handover request ack. (to MSC: acknowledge of handover request)z Handover command (to BSC: contains new radio channel/BTS for the

MS)z Handover complete (to MSC: commanded handover successful)z Handover failure (to MSC: commanded handover unsuccessful)z Handover performed (to MSC: BSC has performed internal handover)z Handover candidate enquiry (to BSC: MSC requests list of MS that could be

handedz over to another cell)z Handover candidate response (to MSC: answer to handover candidate enquiry)z Handover required reject (to BSC: required handover unsuccessful)z Handover detect (to MSC: commanded handover successful)

BSSMAP Message -2 Release messages

z Clear command (release of traffic channel)z Clear completez Clear requestz SAPI n clear command (control of layer 2 SAPI n on the radio interface)z SAPI n clear completez SAPI n reject

General messagesz Reset (initialisation of BSS or MSC due to failure)z Reset acknowledgez Overload (processor or CCCH overload)z Trace invocation (start production of trace recordz Reset circuit (initialisation of single circuit due to failure)

Terrestrial resource messagesz Block (management of circuits/time slots between MSC and

BTS)z Blocking acknowledgez Unblockz Unblocking acknowledge

BSSMAP Message -2

Radio resource messagesz Resource request (available radio channels in

BSS cells)z Resource indicationz Paging (paging of MS)z Cipher mode command (start encryption)z Classmark update (change of MS power class)z Cipher mode completez Queuing indication (indicates delay in

assignment of tch)

Content

GSM System Signaling Model GSM protocol message Basic Signaling Procedure

Mobile Originating Call Establishment Procedure -1

Mobile Originating Call Establishment Procedure -2

Mobile Originating Call Establishment Procedure -3

Mobile Terminating Call Establishment Procedure -1

Mobile Terminating Call Establishment Procedure -2

Mobile Terminating Call Establishment Procedure -3

Location Update Procedure -1

Location Update Procedure -2

Intra Cell Handover Procedure

Intra BSC Handover Procedure

Inter BSC Handover Procedure

Call Re-establishment Procedure

Directed Retry Procedure

Short Message Procedure On SDCCH When MS Is Calling -1

Short Message Procedure On SDCCH When MS Is Calling -2

Short Message Procedure On SDCCH When MS Is Called -1

Short Message Procedure On SDCCH When MS Is Called -2

Short Message Procedure On SACCH When MS Is Calling

Short Message Procedure On SACCH When MS Is Called

SABM(3)

UA(4)EST CNF(SAPI3)(5)

CP DATA(8)

CP ACK(9)

CP DATA(6)

CP ACK(7)

ACTIVE CALL

ACTIVE CALL

CP DATA(1)EST REQ(SAPI3)(2)

GSM Signaling SystemObjectivesContentGSM InterfaceGSM System Signaling ModelPhysical LayerData Link LayerApplication LayerApplication LayerGSM protocolGSM Protocol ArchitectureContentUm interface protocol stackLAPDm frame formatOperation modeRR messagesMM messagesCC messagesAbis Interface Protocol stack LAPD ProtocolA interface protocol stackRelationship between SCCP and OSI modelBSSAPSCCP messageBSSMAP Message -1BSSMAP Message -2BSSMAP Message -2ContentMobile Originating Call Establishment Procedure -1Mobile Originating Call Establishment Procedure -2Mobile Originating Call Establishment Procedure -3Mobile Terminating Call Establishment Procedure -1Mobile Terminating Call Establishment Procedure -2Mobile Terminating Call Establishment Procedure -3Location Update Procedure -1Location Update Procedure -2Intra Cell Handover Procedure Intra BSC Handover Procedure Inter BSC Handover ProcedureCall Re-establishment ProcedureDirected Retry ProcedureShort Message Procedure On SDCCH When MS Is Calling -1Short Message Procedure On SDCCH When MS Is Calling -2Short Message Procedure On SDCCH When MS Is Called -1Short Message Procedure On SDCCH When MS Is Called -2Short Message Procedure On SACCH When MS Is CallingShort Message Procedure On SACCH When MS Is Called 48