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M900/M1800 Base Station Subsystem

Signaling Analysis Manual Contents

Issue 02 (2007-06-30) Huawei Technologies Proprietary i

Contents

1 Signaling Fundamentals...........................................................................................................1-1

1.1 Overview of Interfaces..................................................................................................................................1-2

1.1.1 A Interface............................................................................................................................................1-2

1.1.2 Abis Interface.......................................................................................................................................1-3

1.1.3 Um Interface ................................................................ .................................................................. ......1-3

1.2 A Interface................................................................ ..................................................................... ................1-3

1.2.1 Overview of the A Interface.................................................................................................................1-3

1.2.2 Layers of the A Interface......................................................................................................................1-4

1.3 Abis Interface .......................................................... ....................................................................... .............1-10

1.3.1 Overview of the Abis Interface ................................................................. .........................................1-10

1.3.2 Layers of the Abis Interface...............................................................................................................1-13

1.4 Um Interface................................................................................................................................................1-19

1.4.1 Overview of the Um Interface............................................................................................................1-19

1.4.2 Physical Layer....................................................................................................................................1-20

1.4.3 Data Link Layer ............................................................ ..................................................................... 1-20

1.4.4 Layer 3 ............................................................ ....................................................................... ............1-22


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Figures


Signaling Analysis Manual

ii Huawei Technologies Proprietary Issue 02 (2007-06-30)

Figures

Figure 1-1 Signaling model of the GSM .................................................................... ........................................1-2

Figure 1-2 Protocol reference model of the A interface...................................................................................... 1-3

Figure 1-3 Signaling message processing part ........................................................................ ...........................1-5

Figure 1-4 Protocol reference model of the Abis interface...............................................................................1-11Figure 1-5 Structure of the Abis interface ........................................................................... .............................1-12

Figure 1-6 L2 logical links on the Abis interface ................................................................... ..........................1-13

Figure 1-7 Basic structure of the managed objects...........................................................................................1-17

Figure 1-8 Three layers of the Um interface ............................................................ ........................................1-19

Figure 1-9 Interfaces of L1...............................................................................................................................1-20

Figure 1-10 L3 protocol model of the Um interface.........................................................................................1-24

Figure 1-11 Communications between RR sub-layers ............................................................ .........................1-25

Figure 1-12 Communications between MM sub-layers .......................................................... .........................1-25


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Signaling Analysis Manual Tables

Issue 02 (2007-06-30) Huawei Technologies Proprietary iii

Tables

Table 1-1 Major functions of the BSSAP................................................................. ..........................................1-8

Table 1-2 Administrative state ................................................................. ......................................................... 1-17

Table 1-3 Operational state...............................................................................................................................1-18

Table 1-4 Availability status .................................................................. ........................................................... 1-18


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Signaling Analysis Manual 1 Signaling Fundamentals

Issue 02 (2007-06-30) Huawei Technologies Proprietary 1-1

1 Signaling FundamentalsAbout This Chapter

The following table lists the contents of this chapter.

Section Describes

1.1 Overview of Interfaces The functions of the BSS interfaces.

1.2 A Interface The protocol model on the A interface and each layer of the Ainterface.

1.3 Abis Interface The protocol model on the Abis interface and each layer of

the Abis interface.1.4 Um Interface The protocol model on the Um interface and each layer of the

Um interface.


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1 Signaling Fundamentals



1-2 Huawei Technologies Proprietary Issue 02 (2007-06-30)

1.1 Overview of Interfaces

The Base Station Subsystem (BSS) use standard external interfaces: Um interface between aMobile Station (MS) and the BSS and the A interface between the BSS and the Mobile

Switching service Center (MSC).

The interface protocols and interface procedures strictly follow the EuropeanTelecommunications Standards Institute (ETSI) specifications.

The Abis interface between the Base Transceiver Station (BTS) and the Base Station Controller

(BSC) and the interface between the BSS and the Operation and Maintenance Center (OMC)are internal interfaces. These interfaces are related to the specific equipment from differentmanufacturers. There are many regulations drafted by the ETSI for the Abis interface between

the BTS and the BSC, but the regulations are incomplete.

Figure 1-1 shows the signaling model of the Global System for Mobile communications

(GSM).

Figure 1-1Signaling model of the GSM

CM

MM

RR

LAPDm

Sign.

Layer1

L3

L2

L1

BTSM

MS

Um

SCCP

MTP

BTSM

RR BSSMAP

Abis

BTS

BSC

MSC

A

BSC

Sign.

Layer1

Sign.

Layer1

Sign.

Layer1

RR

LAPDLAPDm LAPD

CM

MM

BSSMAP

SCCP

MTP

MS: Mobile Station BTS: Base Transceiver Station

BSC: Base Station Controller MSC: Mobile services Switching Center

CM: Connection Management MM: Mobility Management

RR: Radio Resource Management MTP: Message Transfer Part

SCCP: Signaling Connection Control Part LAPD: Link Access Procedure on the D channel

BTSM: Base Transceiver Station Management LAPDm: Link Access Procedure on the Dm channel

BSSMAP: Base Station Subsystem Management Application Part

1.1.1 A Interface

The A interface is a standard interface between the BSS and the Network Subsystem (NSS),that is, between the BSC and the MSC. The physical links on the A interface belong to standard

2.048 Mbit/s Pulse Code Modulation (PCM) digital links. The A interface is used to transmit

the information about MS management, BTS management, mobility management, connectionmanagement, and to perform service flow control.


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1.1.2 Abis Interface

The Abis interface defines the standard for the communications between the BSC and the BTSin the BSS. The links on the Abis interface belong to standard 2.048 Mbit/s PCM digital links.

The Abis interface provides all MS-oriented services, controls BTS radio equipment, andmanages the allocation of radio frequencies.

1.1.3 Um Interface

The Um interface lies between an MS and the BTS. It is used for the interworking between the

MS and the fixed part of the GSM system. The links on the Um interface belong to radio links.The Um interface is used to transmit the information about radio resource management,mobility management, and connection management.

1.2 A Interface

1.2.1 Overview of the A Interface

The A interface lies between the BSC and the MSC. It is a standard interface in GSMspecifications. It may interconnect with the equipment from different manufactures. SS7 is used

on the A interface.

Physically, the A interface is the trunk circuit and trunk interface between the BSC and the MSC.Figure 1-2 shows the signaling protocol reference model of the A interface.

Figure 1-2Protocol reference model of the A interface

DTAP: Direct Transfer Application Part MTP: Message Transfer Part

SCCP: Signaling Connection Control Part BSSAP: Base Station Subsystem Application Part

BSSMAP: Base Station Subsystem Management Application Part


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1.2.2 Layers of the A Interface

Physical Layer

The physical layer of the A interface is a 75-ohm coaxial cable or 120-ohm twisted pair. Thetransmission rate of this layer is 2 Mbit/s. The physical layer of the A interface has thefollowing features:

z The 2 Mbit/s transmission rate complies with G.703.

z The frame structure, synchronization, and timing comply with G.705.

z The fault management complies with G.732.

z CRC4 complies with G.704.

MTP

The Message Transfer Part (MTP) provides reliable signaling message transfer in the signalingnetwork. It takes measures to avoid or reduce the loss of messages, repeated messages, and

out-of-sequence packetsin case of system failures and signaling network failures.

The MTP protocols are defined in ITU-T Q.701Q.710 recommendations.

The MTP comprises three function layers: signaling data link function layer, signaling link

function layer, and signaling network function layer.

z Signaling data link function layer

The signaling data link function layer (layer 1) defines physical, electrical, and functionalfeatures of signal data. It specifies how to connect with data links.

A signaling data link is used to transmit signaling in both directions. It comprises two data

paths with the same data rate (64 kbit/s) but in two opposite directions. Generally, thesignaling data link occupies timeslot 16 of a trunk. The specific timeslot is determinedthrough the negotiation between the BSC and the MSC. The timeslot can be used toestablish a semi-permanent connection.

A signaling data link serves as an information bearer of SS7. One of the important features

of the signaling data link is that the information transmitted on the link is transparent, thatis, the data transferred on the link cannot be changed. Therefore, equipment such as echocanceller, digital attenuator, and A/u rate converter, cannot be connected to this link.

z Signaling link function layer

Signaling link function layer (layer 2) specifies the functions and procedures for sendingsignaling to data links. It, together with layer 1, provides reliable signaling message

transfer between two directly connected signaling points.Due to long-distance transmissions, a certain rate of bit errors may be caused on the data

link between adjacent signaling points. However, no error is allowed in SS7 signalingmessage codes. The function of layer 2 is to guarantee error-free transmissions of message

codes in the case that there are bit errors on layer 1. Layer 2 performs the followingfunctions: signaling unit delimitation, signaling unit alignment, error detection, error

correction, initial alignment, processor fault detection, flow control, and signaling linkerror rate monitoring.

z Signaling network function layer

By controlling the route and performance of the signaling network, signaling network

function layer (layer 3) guarantees reliable transmission of signaling information to theuser part, no matter whether the signaling network is in normal state or not. The signaling


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network is functionally classified into signaling message processing part and signalingnetwork management part.

z Signaling message processing part

The signaling message processing part sends signaling messages from the user part of a

signaling point to the target signaling links or user parts. The user part in the BSS onlyrefers to the SCCP. The signaling message processing part comprises three smaller parts:message routing (MRT), message discrimination (MDC), and message distribution(MDT), as shown in Figure 1-3.

Figure 1-3Signaling message processing partMTP user part

MDC MRT

MDT

Messages to

the local office

Messages to other

signaling points

Signaling message

processing at MTP layer 3

Signaling message

processing at MTP layer 2

MRT

The MRT selects message routes. In other words, by using the information contained in

the route mark, destination signaling point code (DPC) and signaling link selectioncode (SLS), the MRT selects a signaling link for signaling messages, so that themessages can be transmitted to a destination signaling point.

MDC

The MDC receives the messages from layer 2 to ascertain whether the destination ofthe messages is the local signaling point.If the destination is the local signaling point,

the MDC sends the messages to the MDT. If the destination is not the local signalingpoint, the MDC part sends the messages to the MRT.

MDT

The MDT allocates the messages from the MDC to the user part, the signaling networkmanagement part, and the test & maintenance part.

z Signaling network management part

The signaling network management part reconstructs the signaling network, and keepsand recovers the normal transmission of signaling units when the signaling network fails.

It consists of three smaller parts: signaling traffic management (STM), signaling linkmanagement (SLM), and signaling route management (SRM).

STM


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The STM is used to transmit the signaling data from one link or route to another or tomultiple available links or routes when the signaling network fails. It is also used totemporarily reduce signaling traffic in case of congestion at a signaling point.

SLM

The SLM is used to recover, enable, or disconnect the signaling links in the signalingnetwork. It is used to ensure the provisioning of certain pre-determined link groups.The connections between signaling data links and signaling terminals are normally

established through man-machine commands. The operations in the signaling systemcannot automatically change the previous connection relations.

SRM

The SRM is used to ensure the reliable exchange of signaling route availabilityinformation between signaling points so that signaling routes can be blocked orunblocked if necessary. The SRMmainly comprises procedures such as prohibited

transfer, allowed transfer, controlled transfer, and restricted transfer, as well assignaling route group test and signaling route group congestion test.

SCCP

The SCCP, with the help of MTP layer 3, is designed to provide sound network layer functions

and reliable services for information exchange in any form.

The network layer services provided by the SCCP are classified into connectionless service andconnection-oriented service.

The connectionless service indicates that an MS does not establish a signaling connection inadvance, but uses the routing function of the SCCP and of the MTP to directly transmit datainformation in the signaling network. It is applicable to the transmission of a small number of

data.

The connection-oriented service indicates that an MS establishes a signaling connection inadvance and directly transmits data on the signaling connection, instead of using the routeselection function of the SCCP. The connection-oriented service is applicable to the

transmission of a large number of data, and effectively shortens the transmission delay of batch

data.

The SCCP can also perform routing and network management functions.

The SCCP performs addressing based on address information such as the DPC, subsystem

number (SSN), and global title (GT). The DPC and SSN are used to identify different SCCP

users, such as the ISUP users, MAP users, TCAP users, and BSSAP users in the same node, soas to compensate the insufficiency of MTP users and to enlarge the addressing scope. The BSSdoes not use the GT addressing mode, which is not introduced here.

The SCCP performs the following network management functions:

z Managing signaling point states and subsystem states

z Performing active/standby subsystem switchover

z Broadcasting status information

z Testing subsystem states

The SCCP management (SCMG) is used to maintain the network functions by reselecting a

route or adjusting the traffic volume in case of network failures or congestion.

The SCCP protocols are defined in ITU-T Q.711Q.716 recommendations.


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BSSAP

1. Overview of BSSAP

The BSSAP protocol, which serves as the A interface specification, describes two types of

messages: BSSMAP messages and DTAP message.For DTAP messages, the A interface is merely equivalent to a transport channel. On the BSSside, DTAP messages are directly transmitted to radio channels. On the NSS side, DTAPmessages are transmitted to the specific functional processing unit.

The BSSAP protocol is defined in GSM Rec. 08.08 and 04.08.

2. Typical message contents

z DTAP messages

The DTAP messages can be classified into Mobile Management (MM) messages and Call

Control (CC) messages based on the functional units used to process DTAP messages onthe NSS side.

The MM messages consist of authentication-related messages, ConfigurationManagement (CM) service request, identification request, IMSI detach, location update,MM state, TMSI reallocation, and so on.

The CC messages consist of the messages related to alerting, call proceeding, connection,setup, modification, release, disconnection, notification, state query, DTMF startup, and

so on.

z BSSMAP messages

The BSSMAP messages can be classified into connectionless messages andconnection-oriented messages.

The connectionless messages include Block/Unblock, Circuit Group Block/Unblock,Handover, Resource, Reset, and Paging.

The Block/Unblock messages consist of Block, Block ACK, Unblock, and UnblockACK.

The Circuit Group Block/Unblock messages include Circuit Group Block, CircuitGroup Block ACK, Circuit Group Unblock, and Circuit Group Unblock.

Handover messages include Handover Candidate Enquire and Handover CandidateEnquire Response.

The resources messages include Resource Request and Resource Indication.

The Reset messages include Reset and Reset ACK.

The connection-oriented messages include Assignment, Handover, Clear, and Ciphermessages.

The Assignment messages include Assignment Request, Assignment Complete, andAssignment Error.

The Handover messages include Handover Request, Handover Request ACK,Handover Command, Handover Complete, and Handover Failure.

The Clear messages include Clear Request and Clear Complete.

The Cipher messages include Cipher Mode Command and Cipher Mode Complete.

3. BSSAP protocol functionality

The BSSAP protocol can function in connection-oriented mode and connectionless mode.

When an MS needs to exchange service-related messages with the NSS over radio channelswhen there is no MS-related SCCP connection between the BSS and the NSS, a new connection

shall be established. A new connection shall also be set up for external handovers.


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There are two types of connection setup:

z When an MS sends an Access Request message on the RACH, the BSS assigns a

dedicated channel (SDCCH or TCH) to the MS. After a layer 2 connection is set up on theassigned SDCCH or FACCH, the BSS starts a connection setup.

z When the MSC decides to carry out an external handover (the target BSS might be theoriginal BSS), it must reserve a new DCCH or TCH from the target BSS. In this scenario,the MSC starts a connection setup.

By using the connection-oriented and connectionless messages, the BSSAP protocolimplements the functional procedures as shown in Table 1-1.

Table 1-1Major functions of the BSSAPNumber Function Description

1 Assignment Assignment is to ensure that dedicated radio resources areproperly allocated or re-allocated to an MS. The BSS

automatically processes the initial random access andimmediate assignment of an MS to a DCCH, without thecontrol of the MSC.

2 Block/unblock circuit

During an assignment procedure, the MSC needs to select

available terrestrial circuits. If the BSS regards that someterrestrial circuit can no longer be used, it must notify the

MSC by initiating a Block/Unblock procedure.

3 Resourceindication

Resource indication serves to notify the MSC of thefollowing:

zNumber of radio resources that can be used as TCHs in theBSS

zNumber of all usable radio resources that can provideservices or have been assigned

It is not easy to obtain the previous information directly from

the MSC-controlled services. The information must be takeninto consideration when the MSC decides an externalhandover.

4 Reset Reset is to initialize the BSS or the MSC. For instance, if the

BSS goes faulty or loses all the reference messages aboutprocessing, it sends a Reset message to the MSC. On receipt

of the Reset message, the MSC releases the affected calls,deletes the affected reference messages, and sets all the

circuits related to the BSS to idle.If the MSC or BSS is only partially faulty, the affected partscan be cleared through the Clear procedure.


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Number Function Description

5 Handover

request

The BSS may send a Handover Request to the MSC,

requesting the MSC to hand over an MS, to which dedicatedresources have been allocated. The handover reasons as are asfollows:

z The BSS detects a radio cause for a handover.

z The MSC starts a Handover Candidate Enquirer procedureand the MS is waiting for a handover.

z Due to congestion, the serving cell needs to be changedduring the call setup such as directed retry.

The BSS shall resend the Handover Request message at acertain interval till one of the following situations occurs:The BSS receives a Handover Command message from the

MSC.

z

The BSS receives a Reset message.z All the communications with MSs are disrupted and the

processing is stopped.

z The processing is complete, for example, the call is cleared.

6 Handover

resourceallocation

Handover resources allocation enables the MSC to request for

resources from the target BSS based on the handover request.The target BSS will reserve resources and wait for an MS toaccess the reserved resources (channel).

7 Handoverprocedure

Handover procedure is a procedure in which the MSC

instructs an MS to access the radio resources of a target cell.

When the handover is carried out, the original dedicated radioresources and terrestrial resources are maintained all the time

until the MSC sends a Clear Command message or theresources are reset.

8 Radio

resource

release andterritorialresourcerelease

When a processing is complete, the MSC sends a Clear

Command message to the BSS to release radio resources.

Upon receipt of the message, the BSS starts the Clearprocedure on the radio interface, sets the configured terrestrial

circuit to idle, and responds with a Clear Complete message tothe MSC.

Upon receipt of the Clear Complete message, the MSC

releases the terrestrial resources.If the BSS needs to release resources, it sends a Clear Request

message to the MSC, requesting the MSC to initiate a releaseprocedure to release the specific terrestrial and radioresources.

9 Paging The paging to an MS is transmitted through the BSSMAP

over the SCCP connectionless service. When the BSSreceives a Paging Response message on the Um interface, itestablishes an SCCP connection to the MSC. The paging

response message, which is carried in the Complete L3Information, is transmitted to the MSC through this SCCPconnection.


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Number Function Description

10 Flow control A flow control can prevent a network entity from receiving

too much traffic. The flow control on the A interface isimplemented through the control the traffic at the traffic

source. Five levels of flow control are available. Flow controlcan be carried based on subscriber classes.

11 Classmarkupdate

Classmark Update serves to notify a receiving entity of theclassmark messages from an MS. Generally, the BSS notifies

the MSC upon receipt of the classmark messages from an MS.It is also likely that when a handover is complete, the MSC

sends the new BSS the classmark messages of a specific MSthrough the A interface.

12 Cipher modecontrol

The Cipher Mode Control procedure allows the MSC to sendthe Cipher Mode Control message to the BSS and to start the

subscriber equipment and signaling cipher equipment using acorrect ciphering key (Kc).

13 Queueindication

The Queue Indication procedure serves to notify the MSC thatthe BSS wants to delay the allocation of necessary radio

resources. The procedure is valid only when the queuingfunction is used for traffic channel assignment and trafficchannel handover in the BSS.

14 Loadindication

Load Indication serves to notify all neighbor BSS entities of

the traffic state of a cell so that the handover services in anMSC can be controlled in a centralized manner. In a certainvalid period, the neighbor BSS will take the traffic states of

neighbor cells into account during a handover.

1.3 Abis Interface

1.3.1 Overview of the Abis Interface

The Abis interfaces lies between the BTS and the BSC. It complies with the specifications

defined by GSM Rec. 08.5X series. The Abis interface is merely an internal interface of the

BSS. The interworking between the BSC and BTS equipment from different manufactures isnot available temporarily.

The terrestrial traffic channels on the Abis interface map with the radio traffic channels on theUm interface in one-to-one manner.

Protocol Reference Model of the Abis Interface

Figure 1-4 shows the protocol reference model of the Abis interface.


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Figure 1-4Protocol reference model of the Abis interface

LAPDm

Sign.

Layer 1

LAPD

Layer 1 Layer 1

SCCP

MTP

BTSM

RR BSSAP

Abis interface

BTS BSC

RR BTSM

LAPD

BTSM: BTS Management BSSAP: Base Station Subsystem Application Part

RR: Radio Resource management SCCP: Signaling Connection Control Part

LAPD: Link Access Procedure on the D channel MTP: Message Transfer Part

LAPDm: Link Access Procedure on the Dm channel

z Layer 1 of the Abis interface is a physical link. It receives and transmits data based on thebottom-layer driver of the hardware.

z The layer 2 protocol of the Abis interface runs based on the LAPD. The LAPD addresses aTransceiver (TRX) or Base Control Function (BCF) through the Terminal EquipmentIdentifier (TEI). It uses different logical links for message transmissions:

Radio signaling links (RSL) used to transmit service management messages

Operation and maintenance links (OML) used to transmit network management

messages

Layer 2 management links (L2ML) used to transmit L2 management messages

z RR messages are mapped onto the BSSAP in the BSC. Most RR messages are treated astransparent messages in the BTS, but some of them have to be interpreted and executed by

the BTS. For example, ciphering, random access, paging, and assignment messages areprocessed by the BTS Management (BTSM) entities in the BSC and in the BTS.

z Neither the BSC or the BTS interprets Connection Management (CM) messages andMobility Management (MM) messages. These messages are transmitted on the A interface

by the Direct Transfer Application Part (DTAP). On the Abis interface, DTAP messagesare transmitted as transparent messages.

Structure of the Abis Interface

The Abis interface supports three types of internal BTS configuration:

z A single TRX

z Multiple TRXs connected with the BSC through one physical link

z Multiple TRXs connected with the BSC through different physical links


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Figure 1-5Structure of the Abis interface

Abis

BTS3

BTS2

BTS1

A

Abis

MSCBSC

TRX

BCF

TRX

BCF

Abis

BSS

TRX

TRX

TRX

TRX

TRX

TRX

BCF

z Transceiver (TRX) is a functional entity defined in the Public Land Mobile Network

(PLMN). It supports eight physical channels belonging to the one TDMA frame.z BCF is the functional entity that performs common control functions including BTS

initialization, software loading, channel configuration, and operation and maintenance.

There are two types of channels on the Abis interface. They are described as follows:

z Traffic channels, which have transmission rates of 8 kbit/s, 16 kbit/s, and 64kbit/s, andcarry speech or data from radio channels.

z Signaling channels, which have transmission rates of 16 kbit/s, 32 kbit/s, and 64kbit/s, andcarry signaling between the BSC and an MS, and between the BSC and the BTS

A TEI is assigned to obtain the unique address of a TRX. Each BCF has a unique TEI. Three

different logical links are defined with a TEI, as shown in Figure 1-6.

z RSL: used to support traffic management procedures, one for each TRX

z OML: used to support network management procedures, one for each BCF

z L2ML: used to transmit L2 management messages


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Figure 1-6L2 logical links on the Abis interface

TEI 1L2ML SAPI=63

RSL SAPI=0

OML SAPI=62

Layer 2

TEI

Management

TRX

TRX

TRX

BCF

L2ML SAPI=63

RSL SAPI=0

OML SAPI=62

L2ML SAPI=63

RSL SAPI=0

OML SAPI=62

BCF

BCF

BCF

BCF

L2ML SAPI=63

OML SAPI=62

BSC BTS

TEI 2

TEI 3

TEI 4

1.3.2 Layers of the Abis Interface

Physical Layer

The physical layer of the Abis interface uses PCM links with the rate of 2048 kbit/s. It provides32 channels with the rate of 64 kbit/s. The electrical parameter of the physical layer conforms to

the ITU-T G.703 recommendations.

The BSS is the connection point of radio channels and terrestrial channels. Each type ofchannels has its own transfer patterns and coding rates. The coding rate of the radio channels inthe BSS is 16 kbit/s while the rate of the channels on the Abis interface is 64 kbit/s. Therefore,

transcoding and rate adaptation functions are needed. Different multiplexing ratio modes, forexample, 10:1, 12:1, and 15:1, are used on the Abis interface to save transmission cost more

efficiently.

Data Link Layerz Overview

SAP is a data link connection endpoint is identified by a data link connection endpointidentifier as seen from layer 3 and by a data link connection identifier (DLCI) as seen fromthe data link layer.

The data link layer of the Abis interface uses the LAPD protocol. It uses the data

transmission services on the physical layer and provides connection-oriented orconnectionless services for layer 3. The data link layer provides services at the ServiceAccept Point (SAP), which is identified by a Service Access Point Identifier (SAPI).Seen

from layer 3, a data link connection endpoint is identified by a data link connection


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endpoint identifier. Seen from the data link layer, a data link connection endpoint isidentified by a data link connection identifier (DLCI). For the information exchangebetween two or more layer 3 entities, a connection must be established between the layer 3entities on the data link layer using a data link layer protocol.

The co-operation between data link layer entities is controlled by a peer-to-peer protocolspecific to the layer. The message units on the data link layer are transmitted between layer2 entities through the physical layer. Inter-layer service requests are processed throughservice primitives.

z Functions

The LAPD is used to transfer reliable end-to-end information between layer 3 entities byusing the D channel. Specifically, the LAPD supports:

Multiple terminal devices between MSs and physical interfaces

Multiple layer 3 entities

The LAPD performs the following functions:

Establishing one or several data links on the D channel

Delimiting, locating, and transmitting transparently frames so that a string of bitstransmitted on the D channel in the form of frames can be identified

Controlling and keeping the sequence of frames

Checking for transmission errors, format errors, and operation errors on data linkconnections

Correcting detected transmission errors, format errors, and operation errors

Notifying the management layer entities of unrecoverable errors

Performing flow control

The data link layer provides the means for information transfer between multiple combinations

of data link connection points. The information may be transferred through point-to-point datalink connections or broadcast data link connections.

Traffic Management Part of Layer 3

The traffic management part of the layer 3 on the Abis interface is described in GSM Rec. 08.58.

The procedures defined in these specifications have two major functions:

z Performing the interworking on the Um interface between an MS and the BSS or NSS

z Performing part of the radio resource management functions under the control of the BSC

Traffic management messages are classified into transparent and non-transparent messages.

z The transparent messages refer to the messages directly forwarded without interpretation

or being processed by the BTS.

z The non-transparent messages refer to the messages that are transmitted between the BSCand the BTS and that must be processed and structured by the BTS.

Traffic management messages can also be classified into four groups in terms of functions. The

four groups are as follows:

z Radio link layer management messages used to manage the data link layer on radiochannels

z Dedicated channel management messages used to manage dedicated channels such as theSDCCH and TCH


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z Common control channel management messages used to manage common controlchannels

z TRX management messages used to manage TRXs

The transparency and group of traffic management messages are determined by the message

discriminator at the header of the messages.

z Radio link management procedures

Radio link management procedures consist of the following:

Link establishment indication procedure: used by the BTS to indicate to the BSC that

an MS-originated multi-frame-mode link setup is successful. On receipt of theindication from the BTS, the BSC establishes an SCCP link to the MSC.

Link establishment request procedure: used by the BSC to request the establishment ofa multi-frame link on a radio channel

Link release indication procedure: used by the BTS to indicate to the BSC that anMS-initiated radio link release is complete

Link release request procedure: used by the BSC to request the release of a radio link

Transmission of a transparent RIL3 message on the Um interface in acknowledgedmode: used by the BSC to request the transmission of a transparent L3 message to anMS on the Um interface in acknowledged mode

Reception of a transparent RIL3 message on the Um interface in acknowledged mode:

used by the BTS to indicate to the BSC that a transparent L3 message is received on theUm interface in acknowledged mode

Transmission of a transparent RIL3 message on the Um interface in unacknowledgedmode: used by the BSC to request the transmission of a transparent L3 message to anMS on the Um interface in unacknowledged mode

Reception of a transparent RIL3 message on the Um interface in unacknowledged

mode: used by the BTS to indicate to the BSC that a transparent L3 message is receivedon the Um interface in unacknowledged mode

Link error indication procedure: used by the BTS to indicate to the BSC in case of anyerrors in the radio link layer.

z Dedicated channel management procedures

The dedicated channel management procedures consist of the following:

Channel activation procedure: used by the BSC to request the BTS to activate adedicated channel for an MS. The BSC later will assign the activated channel to the MSthrough an Immediate Assignment, Assignment Command, Additional Assignment, ora Handover Command message.

Channel mode modification procedure: used by the BSC to request the BTS to changethe mode of an activated channel

Handover detection procedure: used to detect the access of an MS being handed overon the Abis interface between the target BTS and the target BSC

Ciphering mode command procedure: used to start the ciphering procedure defined inGSM Rec. 04.08

Measurement report procedure: consists of the mandatory basic measurement reportprocedure and optional measurement report preprocessing procedure. The BTS reportsall the parameters related to handover decisions to the BSC through this procedure.

SACCH deactivation procedure: used by the BSC to deactivate the SACCH related to aTRX according to the Channel Release procedure defined in GSM Red. 04.08


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Radio channel release procedure: used by the BSC to release a radio channel that is nolonger needed

MS power control procedure: used by the BSS to control the transmit power of an MS

for which a channel is already activated. The power control decision must be

performed in the BSC. It can also be performed in the BTS. BTS transmit power control procedure: used by the BSS to control the transmit power

of a TRX with an activated channel. The BTS transmit power control decision must beperformed in the BSC. It can also be performed in the BTS.

Connection failure procedure: used by the BTS to indicate to the BSC that an activateddedicated channel is disconnected

Physical context request procedure: used by the BSC to obtain the information aboutthe "physical context" of a radio channel prior to a channel change. This is an opticalprocedure.

SACCH information modification procedure: used by the BSC to command the BTS tochange the information (system information) filled in on a specific SACCH

z Common channel management proceduresThe common channel management procedures consist of the following:

MS-initiated channel request procedure: triggered when a TRX detects a randomaccess from an MS (Channel Request message from the MS)

Paging procedure: used to page an MS on a specific paging sub-channel. Thisprocedure is used in an MS-terminated call procedure. It is initiated by the BSC after

receiving a Paging Command message from the MSC. The BSC determines the paginggroup to be used according to the IMSI of the called MS. The BSC sends the paginggroup number together with the identity of the MS to the BTS.

Immediate assignment procedure: used by the BSC to assign a dedicated channel for anMS immediately when the MS accesses the BTS

Delete indication procedure: used by the BTS to indicate to the BSC that an Immediate

Assign Command has been deleted due to AGCH overload

CCCH load indication procedure: used by the BTS to indicate to the BSC the load on aspecified CCCH if the load exceeds the preset threshold on the OMC. The indicationperiod is also set on the OMC.

Broadcast information modification procedure: used by the BSC to indicate to the BTSthe new system information to be broadcast on the BCCH

Short message cell broadcast procedure: used by the BSC to request short messageservice cell broadcast messages from the BTS

z TRX management procedures

The TRX management procedures consist of the following: SACCH filling information modify procedure: used by the BSC to indicate to the BTS

the new system information to be used as filling information on all downlink SACCHs

Radio resources indication procedure: used by the BTS to indicate to the BSC theinterference levels on the idle channels of a TRX

Flow control procedure: used by the Frame Unit Controller (FUC) in a TRX to informthe BSC of the TRX overload due to CCCH overload, AGCH overload, or TRXprocessor overload

Error reporting procedure: used by the BTS to inform the BSC of the detecteddownlink message errors, which cannot be reported through any other procedure


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OM Part of Layer 3z OM information model

Managed objects

There are four types of management objects: site, cell, carrier (TRX), and channel.Figure 1-7shows the basic structure of the managed objects.

Figure 1-7Basic structure of the managed objectsSite

Cell 0 Cell 1 Cell n

BTS TRX

Channel 7Channel 0 Channel 1

TRX 0 TRX 1 TRX m

Addressing of managed object

Network management messages are addressed through the classes and instances ofmanaged object. For each object instance in the BTS there is a complete L2 connection

description. The first established connection is assigned a semi-permanent orpermanent default TEI. The subsequent connections are assigned the TEIs provided by

the TEI setup procedure. Object instances can also use layer 3 addresses. The mixeduse of layer 2 addressing and layer 3 addressing enables one BTS site to have one ormultiple physical links.

States of managed objects

A managed object has three types of states: administrative state, operational state, andavailability status. SeeTable 1-2, Table 1-3, and Table 1-4for details.For a managedobject, the usage state further describes the operational state, and only the BSCcontrols the administrative state.

Table 1-2Administrative stateState Description

Locked The BSC has disconnected all the calls passing this managed object, and nonew services can be connected to this object.

Shut down No new services can be connected to this managed object, but ongoing callswill be maintained.

Unlocked New services can be connected to this managed object.


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Table 1-3Operational stateState Description

Disabled Resources are totally inoperable, and can no longer provide services for MSs.

Enabled Resources are partially or fully operable.

Table 1-4Availability statusState Description

ln test The resource is undergoing a test procedure. The operational state is

disabled.

Failed The resource has an internal fault that prevents it from operating The

operational state is disabled.

Power off The resource requires power to be applied and is not powered on. Theoperational state is disabled.

Off line The resource requires a routine operation to be performed to place it onlineand make it available for use. The operation may be manual or automatic,

or both. The operational state is disabled.

Dependency The resource cannot operate because some other resources on which it

depends is unavailable. The operational state is disabled.

Degraded The service available from the resource is degraded in some respect, such

as in speed or operating capacity.The operational state is enabled.

Not Installed The resource represented by the managed object is not present, or isincomplete. The operational state is disabled.

z Basic procedures

All procedures are based on formatted OM messages. Most formatted OM messagesinitiated by the BSC or BTS require the peer layer 3 to respond with a response or an

acknowledgment in the form of formatted OM messages. This pair of formatted OMmessages or a single formatted OM message that does not require a response is called abasic procedure.

All formatted OM messages are sent on layer 2 in the form of I frames. A group ofmessages, also called structured procedures, are based on the combination of some basicprocedures.

For a specific object instance, if a certain basic procedure is not complete, the system willnot start subsequent basic procedures. When there is no response to a formatted OM

message from the peer layer 3 upon L3 timer expiry, the basic procedure is regarded asnot completed. When there is no response (ACK or NACK) in the previous basicprocedure upon L3 timer expiry, then no subsequent basic procedure is sent to this objectinstance. The default value for L3 timer expiry is 10 seconds.

If part of an original message is not understood or supported, the entire message isdiscarded. An ACK message from an object instance indicates an affirm response. It isused to notify the message sender that the command has been executed or will be executed.

A NACK message from an object instance indicates a disaffirm response. It is used to


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notify the message sender of the unsuccessfully execution of the command and of thefailure cause.

The basic procedures are classified as follows:

Software loading management procedure

Abis interface management procedure

Transmission management procedure

Um interface management procedure

Test management procedure

State management and event reporting procedure

Equipment state management procedure

Other procedures

1.4 Um Interface1.4.1 Overview of the Um Interface

In a GSM network, an MS is connected through radio channels to a fixed part in the network sothat the MS can use communication services. For the purpose of interconnection between anMS and the BSS, it is necessary to standardize the transmission of the signals on the radio

channel. The specifications concerning the signal transmission on radio channels are calledradio interface, or Um interface.

The Um interface is specified by the following features:

z Channel structure and access capability

z MS-BSS protocols

z Operation and maintenance characteristics

z Performance characteristics

z Service characteristics

The Um interface is classified into three layers, as shown in Figure 1-8.

Figure 1-8Three layers of the Um interface

Layer 2 (L2)

Layer 1 (L1)

Layer 3 (L3)

The physical layer (L1) is the bottom layer. It defines the radio access capabilities of the GSM.It provides basic radio channels for information transfer on higher layers.The data link layer(L2) uses the LAPDm protocol. It defines various data transmission structures for controlling

data transmission.


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Layer 3 (shorted as L3 hereinafter in this document) is the highest layer. It processes variousmessages and procedures and controls services. L3 consists of three sub-layers: Radio Resourcemanagement (RR), Mobility Management (MM), and Connection Management (CM).

1.4.2 Physical LayerThe physical layer (L1) is the bottom layer of the Um interface. It provides physical links fortransmitting bit streams. It also provides for higher layers with various logical channels,

including traffic channels and signaling channels. Each logical channel has its own logicalaccess point.

Figure 1-9 shows the interfaces between L1 and L2, L1 and the radio resource management

sublayer (RR) of L3, L1 and other functional unit.

Figure 1-9Interfaces of L1Radio resource

management (L3)

L2

MPH

primitivePH

primitive

L1

TCH

Other functional units

L1 provides the following services:

z

Access capabilityL1 provides a series of limited logical channels for transmission service. Logical channelsare multiplexed on physical channels. Each TRX has eight physical channels. Through

data configuration, logical channels are mapped to physical channels.

z Error code detection

L1 provides error protection transmission, including error detection and correction.

z Ciphering

The BSS encrypts transmitted bit sequences based on a selected ciphering algorithm.

1.4.3 Data Link Layer

The purpose of the data link layer (L2) is to establish reliable dedicated data links between anMS and the BTS. The GSM system uses the LAPDm protocol on the L2 of the Um interface.This protocol evolves from the LAPD protocol.

L2 receives the services from L1 and provides services to L3 at service access points (SAPs).An SAP is identified by an SAPI. Each SAP is associated with one or multiple Data Link

Connection End Points (DLCEPs). Currently, two SAPI values, 0 (main signaling) and 3 (shortmessages), are defined in the LAPDm protocol.

The LAPDm used on the L2 of the Um interface is described here.

z Function


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The LAPDm transfers information between L3 entities through the Dm channel on the Uminterface. LAPDm supports multiple L3 entities and L1 entities, and signaling on BCCH,PCH, AGCH, and DCCH.

The Dm channel is a generic term for all the signaling channels on the Um interface in the GSM system.For instance, the Dm channel can be PCH or BCCH.

The LAPDm performs the following functions:

Providing one or more data link connections (DLCs) on the Dm channel. Each DLC isidentified by a data link connection identifier (DLCI).

Allowing frame type identification

Allowing L3 message units to be transparently transmitted between L3 entities

Performing sequence control to maintain the order of the frames transmitted a DLC

Detecting for format errors and operation errors on data links

Performing flow control

Establishing a contention resolution on a data link after an access request is detected on

the RACH

z Operation types

L2 transmits L3 information in unacknowledged and acknowledged modes. The twomodes can applied to one Dm channel.

Unacknowledged mode

In unacknowledged mode, L2 transmits L3 information in Unnumbered Information

(UI) frames. It does not acknowledge the transmitted UI frames or perform flowcontrol or error correction. The unacknowledged mode is applicable to different typesof control channels except the RACH.

Acknowledged mode

In acknowledged mode, L2 transmits L3 information in numbered Information (I)frames. It acknowledges the transmitted I frames. It performs error correction byresending unacknowledged frames. When L2 fails to correct errors, it informs the

specific L3 entity of the correction failure. Flow control procedures are also defined.The acknowledged mode is applicable to the DCCH.

z Information transmission mode

Information is transmitted in different modes on different channels.

Information transfer on the BCCH: The BCCH directional and used by the BTS tobroadcast system information to MSs. Only the acknowledged mode can be used on theBCCH.

Information transfer on the PCH+AGCH: These channels are only used by the BTS to

transfer information to MSs. Only the unacknowledged mode is applicable to thePCH+AGCH.

Information transfer on the DCCH: Either the acknowledged or the unacknowledgedmode can be used. L3 can choose the specific mode at a certain time.

z Release of data links

The data links in multi-frame mode are released as follows:

Normal release: by exchange of commands/responses

Local end release, that is, without exchange of commands/responses, initiated andcontrolled by L3


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1.4.4 Layer 3

Overview of Layer 3

The layer 3 (L3) of the Um interface provides the functions to establish, maintain, and terminatecircuit-switched connections across a GSM PLMN and other networks to which the GSMPLMN is connected.

The L3 entity consists of many functional program blocks. These program blocks transmit

message units carrying various kinds of information between all L3 entities and between L3and its neighboring layers.

L3 consists of the following three sub-layers:

z Connection Management (CM)

z Mobility Management (MM)

z Radio Resource management (RR)

The CM sub-layer (the highest sub-layer) consists of three functional entities:

z Call Control (CC)

z Short Message Service (SMS)

z Supplementary Service (SS)

In total, the L3 on the Um interface has five functional entities, which performs the followingfunctions accordingly:

z Establishes, operates, and releases dedicated channels (RR)

z Performs location updating, authentication, and TMSI reallocation (MM)

z Establishes, maintains, and terminates circuit-switched calls (CC)z Supports supplementary services (SS)

z Supports short messages service (SMS)

The signaling L3 protocols between the MSs and the network (BSS) perform the functions of

L3. Here the functions of different entities in the BSS are not taken into consideration. Thefunctions of L3 and its supporting lower layers, therefore, provide the Mobile Network

Signaling (MNS) service to the upper layers.

The service interfaces between L3 and higher layers and the interactions between the neighbor

neighboring sub-layers in L3 can be described in primitives and parameters. The three

sub-layers of L3 perform the information exchange between peer entities.

Functions of Layer 3

The L3 functions through its five functional entities. The five functional entities perform the

following functions:

z Radio Resource management (RR)

Establishing, maintaining, and releasing physical channels and logical channels

Performing inter-cell connection upon the request from the CM sub-layer

z Mobility Management (MM)

Performing MS-specific functions and notifying the network when an MS is activated

and deactivated, or the when location area of an MS changes


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Responsible for the security of activated radio channels

z CC: performing all necessary functions to establish or release CS connections

z SS: performing all necessary functions to support GSM supplementary services

z SMS: performing all necessary functions to support point-to-point GSM short message

services

In addition to the previous functions, L3 performs functions related to the transmission ofmessages, for example, multiplexing and splitting. These functions are defined in the protocols

related to RR and MM.

The L3 routes messages according to the protocol discriminator (PD) and transaction identifier(TI), which are part of the message header.

The routing function of the MM enables the MM to route the messages of the CM entities andthe messages of the MM entity to the RR service access point (RR-SAP), and to multiplex the

messages in case of concurrent transactions.

The routing function of the RR distributes the to-be-sent messages according to their PD andthe actual channel configuration.

The routing function of the RR distributes the messages from different service access points of

L2 based on the PD. If a message belongs to the RR sub-layer, this message will be transmittedto the RR entity based on the TI.The other messages are sent to the MM sub-layer through theRR-SAP. If a message belongs to the MM sub-layer, the message will be transmitted to the MMentity based on the TI. The other messages are sent to the CM sub-layer through the MM-SAPs,

and then sent to the CM entities.

Figure 1-10 shows the L3 protocol model of the Um interface.


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Figure 1-10L3 protocol model of the Um interface

C

C

MNCC-SAP

S

S

MNSS-SAP

SM

S

MNSMS-SAP

MNS

MMREG-SAP

MMCC-SAP

MMSS-SAP

MMSMS-SAP

MM CC SS SMSMM

RR-SAP

..RR

RR

PD

RR

SAPI 0 SAPI 3

RACCH

SDCCH

SACCH

FACCH

BCCH

SDCCH

SACCH

AGCH+PCH

L3s

ignaling

The RR sub-layer at the bottom receives the services from L2 through various service accesspoints (that is, various types of channels) of L2, and provides services to the MM sub-layerthrough RR-SAP.

The MM sub-layer provides services to different entities through different SAPs:

z To the CC through MMCC-SAP

z To the SS through MMSS-SAP

z To the SMS through MMSMS-SAP

z To the high layer through MMREG-SAP

The three independent entities (CC, SS, and SMS) of the CM sub-layer provide services tohigher layers through MNCC-SAP, MNSS-SAP, and MNSMS-SAP respectively.

Service Characteristics of L3z Services provided by L3 on the MS side

Registration services, that is, IMSI attach and detach


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Call control services, including normal establishment of MS-originating calls,emergency establishment of MS-originating calls, call hold, call termination, andsupport for call-related supplementary services

Support for call independent supplementary services

Support for short messages service

z Services provided by L3 on the network side

Call control services, including call establishment, call hold, call termination, andsupport for call-related supplementary services

Support for call independent supplementary services

Support for short messages service

z Inter-layer services between an MS and the network

For the services provided by the RR, refer to Figure 1-11. These services are provided

to the MM through RR-SAP. They are used to set up control channel connections andtraffic channel connection, indicate ciphering mode, release control channel

connections, and transmit control data. For the services provided by the MM, refer to Figure 1-12. These services are used to

manage the three entities (CC, SS, and SMS) of the CM sub-layer.

Figure 1-11Communications between RR sub-layers

RR-SAP

MS side Network side

MM

RR

RR-primitive

Protocols between

RR sub-layers

Figure 1-12Communications between MM sub-layers

CC SMSSS

MS side

MM

CC SMSSS

Network side

MM

Protocols between

MM sub-layers

01-01 Signaling Fundmental

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