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1 BSS Signaling FundamentalAbout This Chapter

The external BSS interfaces, which are the Um interface between the BSS and the MS, and the

A interface between the BSS and the MSC, are standard interfaces. The Abis interface between

the BSC and the BTS is an internal interface.

1.1 A Interface

This topic describes the A interface protocol model that consists of the physical layer, MTP

layer, SCCP layer, and BSSAP layer.

1.2 Abis Interface

The Abis interface lies between the BTS and the BSC. It complies with GSM Rec.08.5X series.

The Abis interface is an internal interface of the BSS. The interworking between the BSC and

BTS equipment from different manufactures is not available. The terrestrial traffic channels on

the Abis interface map the radio traffic channels on the Um interface.

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 are radio links. The

Um interface transmits the information about radio resource management, mobility

management, and connection management.

HUAWEI BSC6000 Base Station Subsystem

BSS Signaling Analysis Guide 1 BSS Signaling Fundamental

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1.1 A Interface

This topic describes the A interface protocol model that consists of the physical layer, MTP

layer, SCCP layer, and BSSAP layer.

1.1.1 A Interface Protocol Model

The A interface is defined as a standard communication interface between the NSS and the BSS.

1.1.2 Physical Layer on the A Interface

The physical layer on the A interface is a 2 Mbit/s 75-ohm coaxial cable or 120-ohm twisted

pair.

1.1.3 MTP Layer on the A Interface

The MTP layer on the A interface provides reliable signaling message transfer in the signaling

network. In case of system failure and signaling network failure, it takes measures to avoid or

reduce the message loss, repetition, and out of sequence.

1.1.4 SCCP Layer on the A Interface

The network layer services provided by the SCCP are classified into connectionless service and

connection-oriented service.

1.1.5 BSSAP Layer on the A Interface

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

messages, BSSMAP messages and DTAP messages.

1.1.1 A Interface Protocol Model

The A interface is defined as a standard communication interface between the NSS and the BSS.It is between the BSC and the MSC. The physical links on the A interface are standard 2.048

Mbit/s Pulse Code Modulation (PCM) digital links. The A interface transmits the information

about MS management, mobility management, connection management, and service flow

control.

The A interface connects the BSC and the MSC from different manufactures. The GSM system

uses the SS7 on the A interface.

Physically, the A interface is the trunk circuit interface between the BSC and the MSC. Figure

1-1 shows the A interface signaling protocol model.

1 BSS Signaling Fundamental


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Figure 1-1 A interface signaling protocol model

DTAP BSSMAP

Distribution function

BSSAP

SCCP

MTP

DTAP BSSMAP

Distribution function

BSSAP

SCCP

MTP

Physical layer

A

BSS MSC

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

1.1.2 Physical Layer on the A Interface

The physical layer on the A interface is a 2 Mbit/s 75-ohm coaxial cable or 120-ohm twisted

pair.

The features of the physical layer on the A interface are as follows:

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

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

l The fault management complies with the G.732.

l CRC4 complies with the G.704.

1.1.3 MTP Layer on the A Interface

The MTP layer on the A interface provides reliable signaling message transfer in the signaling

network. In case of system failure and signaling network failure, it takes measures to avoid or

reduce the message loss, repetition, and out of sequence.

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

comprises three sublayers, the signaling data link sublayer, signaling link sublayer, and signaling

network sublayer.

Signaling Data Link Sublayer

The signaling data link function layer (L1) defines the physical, electrical, and functional features

of signal data. It specifies the way to connect with data links. A signaling data link transmits

signaling in both directions. It comprises two data paths of 64 kbit/s and of opposite directions.Generally, a signaling data link occupies timeslot 16 of a trunk. The specific timeslot is





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determined through the negotiation between the BSC and the MSC. The timeslot can be used to

establish 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 transferred on the link is transparent, that is, the

data transferred on the link cannot be changed. Therefore, equipment such as echo canceler,digital attenuator, and A/u rate converter, cannot be connected to this link.

Signaling Link Function Layer

Signaling link function layer (L2) specifies the functions and procedures for sending signaling

to data links. Together with L1, it 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 CCS7 signaling message

codes. L2 guarantees error-free transmission of message codes when there are bit errors on L1.

L2 performs signaling unit delimitation, signaling unit alignment, error detection, errorcorrection, initial alignment, processor fault detection, flow control, and signaling link error rate

monitoring.

Signaling Network Function Layer

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

layer (L3) guarantees reliable transmission of signaling information to the user part, no matter

whether the signaling network is functional or not. The signaling network is functionally

classified into the signaling message processing part and the signaling network management

part.

l

Signaling message processing partThe 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 refers to

the SCCP only. The signaling message processing part comprises three smaller parts:

message routing (MRT), message discrimination (MDC), and message distribution (MDT),

as shown in Figure 1-2.






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Figure 1-2 L3 signaling message processing procedure

MTP user part

Message

distribution

Message

discriminationMessage routing

Messages to the local

signaling point

Messages to the other

signaling points

MTP2 signaling link

Message Routing (MRT)

The MRT selects message routes. By using the information contained in the route mark,

destination signaling point code (DPC), and signaling link selection code (SLS), the

MRT selects a signaling link that transfers the signaling messages to a destination

signaling point.

Message Discrimination (MDC)

The MDC receives the messages from L2 to ascertain whether the destination of the

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 signaling point,

the MDC sends the messages to the MRT.

Message Distribution (MDT)

The MDT allocates the messages from the MDC to the user part, the signaling network

management part, and the test & maintenance part.

l Signaling network management part

The signaling network management part reconstructs the signaling network, and keeps and

recovers the normal transmission of signaling units when the signaling network fails. Itconsists of three smaller parts: signaling traffic management (STM), signaling link

management (SLM), and signaling route management (SRM).

Signaling Traffic Management (STM)

The STM part transmits the signaling data from one link or route to another or to multiple

available links or routes when the signaling network fails. It also temporarily reduces

signaling traffic in case of congestion at a signaling point.

Signaling Link Management (SLM)

The SLM part recovers, enables, or disconnects the signaling links in the signaling

network. It ensures the provisioning of certain pre-determined link groups. The

connections between signaling data links and signaling terminals are normallyestablished through man-machine commands. The operations in the signaling system





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cannot automatically change the previous connection relations. The SLM comprises

link test, link prohibition and unprohibion, link switchover and switchback, as well as

link activation and deactivation.

Signaling Route Management (SRM)

The SRM ensures reliable exchange of signaling route availability information betweensignaling points so that signaling routes can be blocked or unblocked. It comprises

prohibited transfer, allowed transfer, controlled transfer, and restricted transfer, as well

as signaling route group test and signaling route group congestion test.

1.1.4 SCCP Layer on the A Interface


connection-oriented service.

The SCCP, with the help of MTP L3, provides complete network layer functions and reliable

services for information exchange in any form.


connection-oriented service. The connectionless service indicates that an MS does not establish

a signaling or connection in advance, but uses the routing function of the SCCP and of the MTP

to directly transmit data in the signaling network. The connectionless service is applicable to the

transmission of a small quantity of data. The connection-oriented service indicates that an MS

establishes a signaling connection in advance and directly transfers data on the signaling

connection, instead of using the route selection function of the SCCP. The connection-oriented

service is applicable to the transmission of a large quantity of data, and effectively shortens the

delay of batch data transmission.

The SCCP also performs routing and network management functions. It performs addressing

based on the address information such as the DPC, subsystem number (SSN), and global title(GT). The DPC is the destination singling point code used by the MTP. The SSN is the subsystem

number. The DPC and the SSN are used to identify different SCCP users, such as the ISUP users,

MAP users, TCAP users, and BSSAP users in the same node. They help to compensate the

insufficiency of MTP users and to enlarge the addressing scope. The BSS does not use the GT

addressing mode, which is not described here.

The SCCP performs signaling point state and subsystem state management, active/standby

subsystem switchover, status information broadcast, and subsystem state test. The SCCP

management (SCMG) maintains the network functions by reselecting a route or adjusting the

traffic volume in case of network failure or congestion.

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

1.1.5 BSSAP Layer on the A Interface


messages, BSSMAP messages and DTAP messages.

Overview of the BSSAP Protocol


messages, BSSMAP messages and DTAP messages. For DTAP messages, the A interface is

merely equivalent to a transport channel. On the BSS side, DTAP messages are directly

transmitted to radio channels. On the NSS side, DTAP messages are transmitted to the specificfunctional processing units.






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

Typical Messages

The BSSAP protocol, which serves as the A interface specification, describes two types ofmessages, BSSMAP messages and DTAP messages.

l DTAP messages

Based on the functional units that process DTAP messages on the NSS side, the DTAP

messages are classified into Mobile Management (MM) messages and Call Control (CC)

messages.

The MM messages include messages related to authentication, Configuration

Management (CM) service request, identification request, IMSI detach, location update,

MM state, and TMSI reallocation.

The CC messages include messages related to alerting, call proceeding, connection,

establishment, modification, release, disconnection, notification, state query, and

DTMF startup.

l BSSMAP messages

The BSSMAP messages are classified into connectionless messages and connection-

oriented messages.

Connectionless messages

The connectionless messages include block, unblock, handover, resource, reset, and

paging messages.

The block and unblock messages consist of block, block acknowledge, unblock, and

unblock acknowledge messages.

The group block and unblock messages consist of group block, block acknowledge,

unblock, and unblock acknowledge messages. The handover messages include

handover candidate request messages and handover candidate response messages.

The resource messages include resource request messages and resource indication

messages. The reset messages include reset and reset acknowledge messages.

Connection-oriented messages

The connection-oriented messages include messages related to assignment, handover,

clear, and ciphering.

The Assignment messages include the assignment request message, assignment

complete message, and assignment failure message.

The handover messages include the Handover Request, Handover Request

Ackowledge, Handover Command, Handover Complete, and Handover Failure

messages.

The clear messages include the Clear Request and Clear Complete messages.

The ciphering messages include the Cipher Mode Command and the Cipher Mode

Complete messages.

BSSAP Protocol Functionality

The BSSAP protocol functions in connection-oriented mode or connectionless mode. When an

MS needs to exchange service-related messages with the NSS on radio channels and there is no

MS-related SCCP connection between the BSS and the NSS, a new connection must beestablished.





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

channel (SDCCH or TCH) to the MS. After an L2 connection is established on the assigned

SDCCH or FACCH, the BSS starts a connection establishment.

l When the MSC decides to perform an external handover (the target BSS might be the

serving BSS), it must reserve a new DCCH or TCH from the target BSS. Then the MSCstarts a connection establishment.

Using the connection and connectionless messages, the BSSAP protocol implements the

functionalality described in Table 1-1.

Table 1-1 BSSAP protocol functionality

Number Function Description

1 Assignment Assignment ensures that dedicated radio

resources are properly allocated or re-

allocated to an MS. The BSS automatically

processes the initial random access and

immediate assignment of an MS to a DCCH,

without the control of the MSC.

2 Block / Unblock Circuit During an assignment procedure, the MSC

needs to select available terrestrial circuits.

If the BSS considers that some terrestrial

circuits become unavailable or available, it

notifies the MSC by initiating a Block/

Unblock procedure.

3 Resource Indication Resource indication serves to notify the

MSC of the following:l Number of radio resources that can be

used as TCHs in the BSS

l Number of available and allocated radio

resources

l The MSC does not easily obtain the

previous information directly from the

MSC-controlled services. The MSC must

take the information into consideration

when the it decides to perform an external

handover.

4 Reset The purpose of reset is to initialize the BSS

or the MSC. For example, if the BSS is

faulty and loses all the reference messages

about processing, it sends a Reset message

to the MSC. Upon receiving 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 parts can be cleared through the

Clear procedure.






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5 Handover Required The BSS may send the MSC a Handover

Required message, requesting the MSC to

hand over an MS that are allocated dedicated

resources. The handover reasons as are as

follows:

The BSS detects a radio cause for a

handover.

The MSC starts a handover candidate

enquiry procedure, and the MS is waiting for

a handover.

Due to congestion, the serving cell must be

changed during call establishment, for

example, during directed retry.

The Handover Required message is resent ata certain interval till one of the following

situations occurs:

l A Handover Command message is

received from the MSC.

l A Reset message is received.

l All the communications with MSs are

disrupted and the processing is stopped.

l The processing is complete, for example,

the call is cleared.

6 Handover ResourceAllocation

Through handover resource allocation, theMSC requests resources from the target BSS

based on the handover request, and the target

BSS reserves resources and waits for an MS

to access the reserved resources (channel).

7 Handover Procedure Handover procedure is a procedure in which

the MSC instructs an MS to access the radio

resources in a target cell. When handover is

performed, the original dedicated radio

resources and terrestrial resources are

maintained until the MSC sends a Clear

Command message or until the resources arereset.





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8 Radio and Terresterial

Resource Release

When a processing is complete, the MSC

sends the BSS a Clear Command message,

requesting the BSS to release radio

resources. Upon receiving the Clear

Command message, the BSS starts a clear

procedure on the Um interface, sets the

configured terrestrial circuits to idle, and

responds the MSC with a Clear Complete

message. Upon receiving the Clear

Complete message, the MSC releases the

terrestrial resources.

If the BSS needs to release resources, it

sends the MSC a Clear Request message.

Then the MSC initiates a release procedure

to release the specific terrestrial and radioresources.

9 Paging The paging to an MS is transmitted through

the SCCP connectionless service over the

BSSMAP. When the BSS receives a Paging

Response message on the Um interface, it

establishes an SCCP connection to the MSC.

The paging response message, which is

carried in the Complete L3 Information, is

transmitted to the MSC through this SCCP

connection.

10 Flow Control Flow control ensures stable working state of

an entity by preventing the entity from

receiving too much traffic. Flow control on

the A interface is performed through traffic

control at the traffic source. It comprises five

levels. It is performed based on subscriber

classes.

11 Classmark Update Classmark update serves to notify a

receiving entity of the classmark messages

from an MS. Generally, the BSS notifies the

MSC upon receiving the classmark

messages from an MS. When a handover is

complete, the MSC sends the new BSS the

classmark messages from the relevant MS

on the A interface.

12 Cipher Mode Control The cipher mode control procedure allows

the MSC to send the Cipher Mode Control

message to the BSS and to start the

subscriber equipment and the signaling

cipher equipment using a correct ciphering

key (Kc).






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13 Queue Indication The queue indication procedure serves to

notify the MSC that the BSS will delay the

allocation of some radio resources. The

procedure takes effect only when the

queuing function is used for traffic channel

assignment and traffic channel handover in

the BSS.

14 Load Indication Load indication serves to notify all neighbor

BSSs of the traffic state of a cell so that the

handover services in an MSC can be

controlled as a whole. In a certain period, the

neighbor BSSs take the traffic states of

neighbor cells into account during a

handover.

1.2 Abis Interface

The Abis interface lies between the BTS and the BSC. It complies with GSM Rec.08.5X series.

The Abis interface is an internal interface of the BSS. The interworking between the BSC and

BTS equipment from different manufactures is not available. The terrestrial traffic channels on

the Abis interface map the radio traffic channels on the Um interface.

1.2.1 Abis Interface Protocol Model

This topic describes the Abis interface protocol model.

1.2.2 Abis Interface Structure

The Abis interface supports three types of internal BTS configurations.

1.2.3 Physical Layer on the Abis Interface

The physical layer on the Abis interface are 2 Mbit/s PCM links. It provides thirty-two 64 kbit/

s channels.

1.2.4 LAPD Layer on the Abis Interface

This topic describes the functions of the LAPD layer on the Abis interface.

1.2.5 L3 Traffic Management Messages on the Abis Interface

L3 traffic management messages on the Abis interface enables the MS to exchange informationwith the BSS or NSS on the Um interface and to perform some radio resource management

functions under the control of the BSC.

1.2.6 L3 OM Messages on the Abis Interface

This topic describes the L3 OM messages on the Abis interface.

1.2.1 Abis Interface Protocol Model

This topic describes the Abis interface protocol model.

Figure 1-3 shows the Abis interface protocol model.





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Figure 1-3 Abis interface protocol model

BTSMSCCP

MTP

BTSM

RR BSSAP

Abis interface

BTS BSC

Sign.Layer 1

Layer 1 Layer1

RR

LAPDLAPDm LAPD

BTSM: BTS Management BSSAP: Base Station Subsystem Application Part

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

LAPD: Link Access Procedure on the Dm Channel RR: Radio Resource Management

MTP: Message Transfer Part

The following describes the Abis interface protocol model:

l Layer 1 on the Abis interface is a bottom-layer driver based on the hardware. It transfers

data to the physical link.

l The layer 2 protocol on the Abis interface is based on the LAPD. The LAPD addresses a

Transceiver (TRX) or Base Control Function (BCF) through the Terminal Equipment

Identifier (TEI). The LAPD uses different logical links for message transmissions. Radio

signaling links (RSL) transmit service management messages. Operation and maintenance

links (OML) transmit network management messages. Layer 2 management links (L2ML)

transmit L2 management messages.

l RR messages are mapped onto the BSSAP by the BSC. Most RR messages are transparently

transmitted by the BTS, except for some messages that must be interpreted and executed.

For example, ciphering, random access, paging, and assignment messages are processed

by the BTS Management (BTSM) entities in the BSC and in the BTS.

l Neither the BSC nor the BTS interprets Connection Management (CM) messages and

Mobility Management (MM) messages. These messages are transmitted on the A interface

by the Direct Transfer Application Part (DTAP). On the Abis interface, DTAP messages

are transmitted as transparent messages.

1.2.2 Abis Interface StructureThe Abis interface supports three types of internal BTS configurations.

Figure 1-4 shows the Abis interface structure. The three types BTS configurations on the Abis

interface are as follows:

l A single TRX

l Multiple TRXs connected with the BSC through one physical link

l Multiple TRXs connected with the BSC through different physical links






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Figure 1-4 Abis interface structure

Abis

BTS3

BTS2

BTS1

A

Abis

MSCBSC

TRX

BCF

TRX

BCF

Abis

BSS

TRX

TRX

TRX

TRX

TRX

TRX

BCF

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

(PLMN). It supports eight physical channels of one TDMA frame.

l The Base Control Function (BCF) is a functional entity that performs common controlfunctions including BTS initialization, software loading, channel configuration, and

operation and maintenance.

The following two types of channels are on the Abis interface:

l Traffic channels of 8 kbit/s, 16 kbit/s, and 64 kbit/s, which carry speech or data from radio

channels

l Signaling channels of 16 kbit/s, 32 kbit/s, and 64 kbit/s, which carry 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 1-5.

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

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

l L2ML: used to transmit L2 management messages





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Figure 1-5 Logical links on the Abis interface

BSC BTS

LAYER 2

TEI

MANA

GEMENT

RSL SAP1=0

OML SAP1=62

L2ML SAP1=63TEI1

TRX

BCF

RSL SAP1=0

OML SAP1=62

L2ML SAP1=63

RSL SAP1=0

OML SAP1=62

L2ML SAP1=63

TRX

BCF

TEI2

TRX

BCF

TEI3

OML SAP1=62

L2ML SAP1=63 BCF TEI4

BCF

1.2.3 Physical Layer on the Abis InterfaceThe physical layer on the Abis interface are 2 Mbit/s PCM links. It provides thirty-two 64 kbit/

s channels.

The electrical parameters of the physical layer conform to the ITU-T G.703 recommendations.

The BSS is the connection point between radio channels and terrestrial channels. The coding

schemes and rates of the two types of channels are different. The coding rate of the radio channels

in the BSS is 16 kbit/s, and the rate of the channels on the Abis interface is 64 kbit/s. To save

the transmission cost, different multiplexing modes, for example, 10:1, 12:1, and 15:1, are used

on the Abis interface.

1.2.4 LAPD Layer on the Abis Interface

This topic describes the functions of the LAPD layer on the Abis interface.

Overview

The data link layer (L2) on the Abis interface uses the LAPD protocol. It receives data from the

physical layer (L1) and provides connection-oriented or connectionless services for L3. The

Service Access Point (SAP) of L2 is the connection point for providing services for L3. It is

identified by an SAPI. A data link connection endpoint is identified by a data link connection

endpoint identifier or a data link connection identifier (DLCI) from the perspective of L2 or L2,respectively.






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For the information exchange between two or more L3 entities, data links must be established

between L3 entities.

The co-operation between L2 entities is controlled by the protocol of the peer layer. The message

units at L2 are transmitted between L2 entities through L1. Inter-layer service requests are

processed through service primitives.

Functions

The LAPD reliably transfers end-to-end information between L3 entities through the D channel.

Specifically, the LAPD supports:

l Multiple terminal devices between MSs and physical interfaces

l Multiple L3 entities

The functions of the LAPD layer on the Abis interface are as follows:

l Providing one or multiple data links on the D channel

l Delimiting, locating, and transparently transmitting frames so that a string of bits

transmitted in the form of frames on the D channel can be identified

l Controlling and keeping the sequence of frames

l Checking for transmission errors, format errors, and operation errors on data link

connections

l Making recoveries based on the detected transmission errors, format errors, and operation

errors

l Notifying the management layer entities of unrecoverable errors

l Performing flow control

The LAPD layer on the Abis interface provides the means for information transfer betweenmultiple combinations of data link connection points. The information may be transferred

through point-to-point data link connections or broadcast data link connections.

1.2.5 L3 Traffic Management Messages on the Abis Interface

L3 traffic management messages on the Abis interface enables the MS to exchange information

with the BSS or NSS on the Um interface and to perform some radio resource management

functions under the control of the BSC.

In terms of processing, traffic management messages are classified into transparent and non-

transparent messages.

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

or processing by the BTS.

l The non-transparent messages refer to the messages that are transmitted between the BSC

and the BTS and that must be processed and structured by the BTS.

In terms of functions, traffic management messages are classified into the following:

l Radio link layer management messages that are used to manage the data link layer on radio

channels

l Dedicated channel management messages that used to manage dedicated channels such as

the SDCCH and TCH

l

Common control channel management messages that are used to manage common controlchannels





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l TRX management messages that are used to manage TRXs

NOTE

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

at the header of the messages.

l 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 establishment is successful. Upon receiving

the indication from the BTS, the BSC establishes an SCCP link to the MSC.

Link establishment request procedure: used by the BSC to request the establishment of

a multi-frame link on a radio channel.

Link release indication procedure: used by the BTS to indicate to the BSC that an MS-

initiated radio link release is complete.

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

Transmission of transparent L3 messages on the Um interface in acknowledged mode:

used by the BSC to request the transmission of transparent L3 messages to an MS on

the Um interface in acknowledged mode

Reception of transparent L3 messages on the Um interface in acknowledged mode: used

by the BTS to notify the BSC that transparent L3 messages are received on the Um

interface in acknowledged mode

Transmission of transparent RIL3 messages on the Um interface in unacknowledged

mode: used by the BSC to request the transmission of transparent L3 messages to an

MS on the Um interface in unacknowledged mode

Reception of transparent RIL3 messages on the Um interface in unacknowledged mode:used by the BTS to notify the BSC that transparent L3 messages are received on the

Um interface in unacknowledged mode

Link error indication procedure: used by the BTS to notify the BSC in case of errors at

the radio link layer

l 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 a

dedicated channel for an MS. Then the BSC assigns the activated channel to the MS

through an Immediate Assignment, Assignment Command, Additional Assignment, or

Handover Command message.

Channel mode modification procedure: used by the BSC to request the BTS to change

the mode of an activated channel.

Handover detection procedure: used between the target BTS and the target BSC to detect

the access of an MS being handed over.

Ciphering mode command procedure: used to start the ciphering procedure defined in

GSM Rec. 04.08.

Measurement report procedure: consists of the mandatory basic measurement report

procedure and optional measurement report preprocessing procedure. The BTS reports

all the parameters related to handover decisions to the BSC through this procedure.

SACCH deactivation procedure: used by the BSC to deactivate the SACCH related toa TRX according to the Channel Release procedure defined in GSM Rec. 04.08.






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Radio channel release procedure: used by the BSC to release a radio channel that is no

longer 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 be

performed in the BSC. It can also be performed in the BTS.

Connection failure procedure: used by the BTS to notify the BSC that an activated

dedicated channel is unavailable.

Physical context request procedure: used by the BSC to obtain the information about

the physical context of a radio channel prior to a channel change. It is an optional

procedure.

SACCH information modification procedure: used by the BSC to request the BTS to

change the filling system information on a specific SACCH.

l Common channel management procedures

The common channel management procedures consist of the following:

MS-initiated channel request procedure: triggered when a TRX detects a Channel

Request message from an MS

Paging procedure: used to page an MS on a specific paging sub-channel This procedure

is used in an MS terminating call establishment procedure. It is initiated by the MSC

through the BSC. Based on the IMSI of the called MS, the BSC determines the paging

group to be used. Then it sends to the BTS the paging group number together with the

identity of the MS.

Immediate assignment procedure: used by the BSC to immediately assign a dedicated

channel to an MS when the MS accesses the BTS.

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

Assign Command message is deleted due to AGCH overload.

CCCH load indication procedure: used by the BTS to notify the BSC of the load on a

specified CCCH if the load exceeds the preset threshold on the OMC. The indication

period is also set on the OMC.

Broadcast information modification procedure: used by the BSC to notify the BTS of

the new system information to be broadcast on the BCCH.

Short message service cell broadcast procedure: used by the BSC to request short

message service cell broadcast messages from the BTS.

l TRX management procedures

The TRX management procedures consist of the following:

SACCH filling information modify procedure: used by the BSC to notify the BTS of

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

Radio resource indication procedure: used by the BTS to notify the BSC of the

interference levels on the idle channels of a TRX

Flow control procedure: used by the Frame Unit Controller (FUC) in a TRX to notify

the BSC of the TRX overload due to CCCH overload, AGCH overload, or TRX

processor overload

Error reporting procedure: used by the BTS to notify the BSC of the detected downlinkmessage errors, which cannot be reported through any other procedure





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1.2.6 L3 OM Messages on the Abis Interface

This topic describes the L3 OM messages on the Abis interface.

OM Information Model

The OM information model consists of the following:

l Management objects

The management objects are site, cell, carrier (TRX), and channel. Figure 1-6 shows the

management objects.

Figure 1-6 Management objects

SITE

CELL 0 CELL 1 CELL n

TRX0 TRX1 TRXm

BTS TRX

Chann

el 0

Chann

el 1

Chanel

7

l Addressing of management objects

Network management messages are addressed through the classes and instances of the

management objects. Each object instance in the BTS has a complete L2 connection

description. The first established connection is assigned a semi-permanent or permanent

default TEI. The subsequent connections are assigned the TEIs provided during the

establishment of TEI procedures. Object instances can also use L3 addresses. The mixed

use of L2 addressing and L3 addressing enables one site to have one or multiple physical

links.

l Management object state

A management object can be in three states, the administrative state, operational state, and

availability state. For details, see Table 1-2, Table 1-3, and Table 1-4. The available state

further describes the operational state, and only the BSC controls the administrative state.

Table 1-2 Administrative State

Status Description

Locked The BSC has disconnected all the calls passing this

management object, and no new services can be

connected to this object.






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Status Description

Shut Down No new services can be connected to this management

object, and ongoing calls are maintained.

Unlocked New services can be connected to this managementobject.

Table 1-3 Operational State

Status Description

Disabled Resources are totally inoperable and can no longer

provide services for MSs.

Enabled Resources are partially or fully operable.

Table 1-4 Available State

Status Description

In 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 and is not powered on. The


Off Line The resource requires automatic or manual operations to

make it available for use. The operational state is

disabled.

Dependency The resource cannot operate because some other

resources on which it depends are unavailable. The


Degraded The service is degraded due to some reasons such as

speed or capacity. The operational state is enabled.

Not Installed The hardware or software of the management object is

not installed. The operational state is disabled.

Basic Procedures

All procedures are based on formatted OM messages. Most formatted OM messages initiated

by the BSC or the BTS require the peer L3 to respond with formatted OM messages. This pair

of formatted OM messages or a formatted OM message that does not require a response is called

a basic procedure.





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All formatted OM messages are sent on L2 in the form of I frames. A group of messages, also

called structured procedures, are based on the combination of some basic procedures.

For a specific object instance, if a certain basic procedure is not complete, the system does not

start the subsequent basic procedures. When there is no response to a formatted OM message

from the peer L3 upon L3 timer expiry, the basic procedure is considered as not "completed."When there is no response (ACK or NACK) in the previous basic procedure upon L3 timeout,

no subsequent basic procedure is sent to this object instance. The default value for L3 timeout

is 10 seconds. If part of an original message is not understood or supported, the entire message

is discarded. An ACK message from an object instance indicates an affirm response. It is used

to notify the sender that the command is executed or will be executed. An NACK message from

an object instance indicates a disaffirm response. It is used to notify the sender of the unsuccessful

execution of the command and of the failure cause.

The basic procedures are classified into the following:

l Software loading management procedure

l Abis interface management procedure

l Transmission management procedure

l Abis interface management procedure

l Test management procedure

l State management and event report procedure

l Equipment state management procedure

l Other procedures

1.3 Um InterfaceThe 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 are radio links. The

Um interface transmits the information about radio resource management, mobility

management, and connection management.

1.3.1 Physical Layer on the Um Interface

The physical layer (L1) is the bottom layer on the Um interface. It defines the radio access

capabilities of the GSM and provides basic radio channels for information transfer at higher

layers.

1.3.2 LAPD Layer on the Um Interface

The data link layer (L2) is the middle layer on the Um interface. It uses the LAPDm protocol.

It defines various data transmission structures for controlling data transmission.

1.3.3 L3 Entity on the Um Interface

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

message units between all L3 entities and between L3 and its adjacent layers.

1.3.1 Physical Layer on the Um Interface

The physical layer (L1) is the bottom layer on the Um interface. It defines the radio access

capabilities of the GSM and provides basic radio channels for information transfer at higherlayers.






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L1 is the bottom layer on the Um interface. It provides physical links for transmitting bit streams.

It also provides higher layers with various logical channels, including traffic channels and

signaling channels. Each logical channel has its own logical access point.

Figure 1-7 shows the interfaces between L1 and the data link layer, the radio resource

management sublayer (RR) of L3, and other functional units.

Figure 1-7 Interfaces of L1 on the Um interface

Radio resource

management (3)

Data link layer

MPH primitive PH primitive

Physical layer

TCH

Other functional units

L1 provides the following services:

l Access capability

L1 provides a series of limited logical channels for transmission service. Logical channels

are multiplexed on physical channels. Each TRX has eight physical channels. Through data

configuration, logical channels are mapped to physical channels.

l Bit error detection

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

l Cyphering

Based on a selected ciphering algorithm, the BSS ciphers the bit sequence.

1.3.2 LAPD Layer on the Um Interface

The data link layer (L2) is the middle layer on the Um interface. It uses the LAPDm protocol.

It defines various data transmission structures for controlling data transmission.

L2 provides reliable dedicated data links between an MS and the BTS. It uses the LAPDm

protocol that evolves from the LAPD protocol. The SAP of L2 is the connection point for

providing services for L3. An SAP is identified by an SAPI. Each SAP is associated with one

or multiple DLCEPs. Currently, two SAPI values, 0 (main signaling) and 3 (short messages),

are defined in the LAPDm protocol.

Functions

LAPDm transfers information between L3 entities through the Dm channel on the Um interface.

LAPDm supports multiple L3 entities, L1 entities, and signaling on BCCH, PCH, AGCH, and

DCCH.

NOTE

The Dm channel is a generic term for all the signaling channels on the Um interface in the GSM system.

For example, the Dm channel can be PCH or BCCH.

LAPDm performs the following functions:

l

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





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l Allowing frame type identification

l Allowing L3 message units to be transparently transmitted between L3 entities

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

l Detecting format errors and operation errors on data links

l Performing flow control

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

RACH

Operation Type

L2 transmits L3 information in unacknowledged and acknowledged modes. One Dm channel

can be in both modes at the same time.

l Unacknowledged mode

In unacknowledged mode, L3 information is transferred in Unnumbered Information (UI)frames. L2 does not acknowledge the UI frames or perform flow control or error correction.

The unacknowledged mode is applicable to different types of control channels except the

RACH.

l Acknowledged mode

In acknowledged mode, L3 information is transferred in numbered Information (I) frames.

L2 acknowledges the I frames. It performs error correction by resending 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.

Information Transfer Mode

Information is transferred in different modes on different channels.

l Information transfer on the BCCH: The BCCH transfers the broadcast messages from the

BTS to the MS. Only the acknowledged mode can be used on the BCCH.

l Information transfer on the PCH+AGCH: These channels transfer messages from the BTS

to the MS. Only the unacknowledged mode is applicable to the PCH+AGCH.

l Information transfer on the DCCH: Either the acknowledged or the unacknowledged mode

can be used. The transfer mode is determined by L3.

Data Link ReleaseL2 release is initiated by L3. The data links in frame mode are released in the following modes:

l Normal release

The BTS and the MS exchange DISC frames and UA frames or DM frames.

l Local release

No frames are exchanged. Generally used in abnormal cases.

1.3.3 L3 Entity on the Um Interface








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Overview



L3 performs the following functions:l Establishing, operating, and releasing dedicated radio channels (RR)

l Performing location update, authentication, and TMSI reallocation (MM)

l Establishing, maintaining, and terminating circuit-switched calls (CC)

l Supporting supplementary services (SS)

l Supporting short messages service (SMS)

L3 uses L3 signaling protocols between the MS and the network. Here the functions of different

entities in the BSS are not taken into consideration. L3 and its supported 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 adjacentsublayers in L3 are described in primitives and parameters. The three sublayers in L3 perform

information exchange between peer entities.

Structure and Functions

L3 consists of three sublayers. The CM sub-layer (the highest sub-layer) consists of three

functional entities: Call Control (CC), Short Message Service (SMS), and Supplementary

Service (SS). In total, L3 on the Um interface has five functional entities. The five functional

entities perform the following functions:

l Radio Resource Management (RR)

Establishing, maintaining, and releasing physical channels and logical channels, as well as

performing cross-cell connection upon the request from the CM sublayer

l Mobility Management (MM)

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

deactivated, or when the location area of an MS changes. It is also responsible for the

security of activated radio channels.

l Call Control (CC)

Performing all necessary functions to establish or release CS connections

l Supplementary Service (SS)

Performing all necessary functions to support GSM supplementary services

l

Short Messages Service (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 of

messages, for example, multiplexing and splitting. These functions are defined in the Radio

Resource Management and Mobility Management. They route 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 and

the messages of the MM entity to the RR service access point (RR-SAP), and multiplexes the

messages in case of concurrent transactions. The routing function of the RR distributes the to-

be-sent messages according to their PD and the actual channel configuration.

The messages provided at different service access points of layer 2 are split by the RR routingfunction according to the PD. If a message belongs to the RR sublayer, this message is transmitted





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to the RR entity based on the TI. The other messages are sent to the MM sublayer through the

RR-SAP. If a message belongs to the MM sublayer, the message is transmitted to the MM entity

based on the TI. The other messages are sent to the CM sublayer through the MM-SAPs, and

then to the CM entities.

Figure 1-8 shows the L3 signaling protocol model on the Um interface.

Figure 1-8 L3 signaling message processing procedure

CC

MNCC-SAP

S

S

MNSS-SAP

SM

S

MNSMS-SAP

Mobile

network

services

MMREG -SAPMMCC-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

Layer3

signaling

The RR sublayer at the bottom receives the services from L2 through various service access

points (that is, various types of channels) of L2, and provides services to the MM sublayer

through RR-SAP. The MM sublayer provides services to different entities through different

SAPs: to the CC through MMCC-SAP, to the SS through MMSS-SAP, to the SMS through

MMSMS-SAP, and to the high layer through MMREG-SAP. The three independent entities

(CC, SS, and SMS) of the CM sublayer provide services to higher layers through MNCC-SAP,

MNSS-SAP, and MNSMS-SAP respectively.

Service Feature

L3 on the MS side provides the following services:

l Registration services, that is, IMSI attach and detach






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l Call control services, including normal establishment of MS originating calls, emergency

establishment of MS originating calls, call hold, call termination, and support for call-

related supplementary services

l Support for call independent supplementary services

l Support for short messages service

L3 on the network side provides the following services:

l Call control services, including call establishment, call hold, call termination, and support

for call-related supplementary services

l Support for call independent supplementary services

l Support for short messages service

L3 provides the following services between the MS and the network:

l For the services provided by the RR, see Figure 1-9. These services are provided to the

MM through RR-SAP. They are used to set up control channel connections and traffic

channel connection, indicate ciphering mode, release control channel connections, andtransmit control data.

l For the services provided by the MM, see Figure 1-10. These services are used to manage

the three entities (CC, SS, and SMS) of the CM sublayer.

Figure 1-9 Services provided by the RR sublayer

MS side Network side

Mobile

management

sublayer

Radio resource

management sublayer

RR-primitive

Protocol of the peer layer of

the RR sublayer

SAPRR

Figure 1-10 Services provided by the MM sublayer

CC SMSSS

MS side

Mobile

management

sublayer

CC SMSSS

Network side

Mobile

management

sublayer

Protocol of the peer layer

of the MM sublayer





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bss signaling fundamental

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