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HUAWEI BSC6000 Base Station Subsystem Signaling Analysis Guide

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1 BSS Signaling Fundamental

BSS Signaling Fundamental

About 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.

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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.

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Figure 1-1 A interface signaling protocol model BSS

MSC

BSSAP

BSSAP

DTAP

BSSMAP

DTAP

BSSMAP

Distribution function

Distribution function

SCCP

SCCP

MTP

MTP

Physical layer

A

DTAP: Direct Transfer Application MTP: Message Transfer Part Part BSSAP: Base Station Subsystem Application Part

SCCP: Signaling Connection Control 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.701–Q.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.

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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, error correction, 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 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 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 Messages to the local signaling point Messages to the other signaling points Message Message routing discrimination

MTP2 signaling link

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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.

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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.

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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.

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

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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.

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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 normally established 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 between signaling 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 The network layer services provided by the SCCP are classified into connectionless service and 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.

The network layer services provided by the SCCP are classified into connectionless service and 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.711–Q.716 recommendations.

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.

Overview of the BSSAP Protocol The BSSAP protocol, which serves as the A interface specification, describes two types of 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 specific functional 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 of messages, 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.

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The MM messages include messages related to authentication, Configuration Management (CM) service request, identification request, IMSI detach, location update, MM state, and TMSI reallocation.

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The CC messages include messages related to alerting, call proceeding, connection, establishment, modification, release, disconnection, notification, state query, and DTMF startup.

BSSMAP messages The BSSMAP messages are classified into connectionless messages and connectionoriented messages.

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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.

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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 be established.

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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 MSC starts 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 reallocated 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:

4

Reset

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.

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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Number

Function

Description

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 at a certain interval till one of the following situations occurs:

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A Handover Command message is received from the MSC.

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A Reset message is received.

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All the communications with MSs are disrupted and the processing is stopped.

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

6

Handover Resource Allocation

Through handover resource allocation, the MSC 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 are reset.

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Number

Function

Description

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 radio resources.

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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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Number

Function

Description

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 information with 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 BTS

BSC

RR RR BTSM LAPDm LAPD Sign.

Layer 1 Layer 1

BTSM LAPD Layer1

BSSAP

SCCP MTP

Abis interface

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 Structure The 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:

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A single TRX

l

Multiple TRXs connected with the BSC through one physical link

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Multiple TRXs connected with the BSC through different physical links

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Figure 1-4 Abis interface structure BSS Abis TRX

BTS1

BCF TRX TRX

A MSC

Abis

BTS2

TRX

BSC

BCF Abis TRX TRX

BTS3

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 control functions 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

ď Ź 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 RSL SAP1=0

TRX

OML SAP1=62

TEI1

L2ML SAP1=63 BCF RSL SAP1=0 LAYER 2

OML SAP1=62

TRX TEI2

L2ML SAP1=63

TEI

BCF RSL SAP1=0 OML SAP1=62

MANA GEMENT

TRX

TEI3

L2ML SAP1=63 BCF OML SAP1=62 L2ML SAP1=63

TEI4

BCF

BCF

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.

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

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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 between multiple 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 nontransparent messages. ď Ź ď Ź 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 control channels

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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:

l

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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.

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

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Link release indication procedure: used by the BTS to indicate to the BSC that an MSinitiated radio link release is complete.

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

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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

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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

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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

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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

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

Dedicated channel management procedures The dedicated channel management procedures consist of the following:

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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.

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

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Handover detection procedure: used between the target BTS and the target BSC to detect the access of an MS being handed over.

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

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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.

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

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1 BSS Signaling Fundamental

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

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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.

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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.

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Connection failure procedure: used by the BTS to notify the BSC that an activated dedicated channel is unavailable.

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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.

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SACCH information modification procedure: used by the BSC to request the BTS to change the filling system information on a specific SACCH.

Common channel management procedures The common channel management procedures consist of the following:

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

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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.

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

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Delete indication procedure: used by the BTS to notify the BSC that an Immediate Assign Command message is deleted due to AGCH overload.

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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.

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Broadcast information modification procedure: used by the BSC to notify the BTS of the new system information to be broadcast on the BCCH.

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

TRX management procedures The TRX management procedures consist of the following:

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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

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Radio resource indication procedure: used by the BTS to notify the BSC of the interference levels on the idle channels of a TRX

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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

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Error reporting procedure: used by the BTS to notify the BSC of the detected downlink message errors, which cannot be reported through any other procedure Huawei Technologies Proprietary

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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

TRX0

TRX1

BTS

Chann el 0

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Chann el 1

CELL n

…

…

TRXm

TRX

…

Chanel 7

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

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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 management object.

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 operational state is disabled.

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 operational state is disabled.

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.

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.

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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

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Abis interface management procedure

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Transmission management procedure

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Abis interface management procedure

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Test management procedure

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State management and event report procedure

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Equipment state management procedure

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Other procedures

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.

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 higher layers.

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.

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Figure 1-7 Interfaces of L1 on the Um interface Radio resource management (3) Data link layer MPH primitive

Other functional units

PH primitive

TCH

Physical layer

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.

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

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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 is identified by a data link connection identifier (DLCI).

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Allowing frame type identification

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Allowing L3 message units to be transparently transmitted between L3 entities

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

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Detecting format errors and operation errors on data links

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Performing flow control

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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.

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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.

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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.

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Information transfer on the DCCH: Either the acknowledged or the unacknowledged mode can be used. The transfer mode is determined by L3.

Data Link Release L2 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.

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Local release No frames are exchanged. Generally used in abnormal cases.

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.

Overview 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.

L3 performs the following functions:

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Establishing, operating, and releasing dedicated radio channels (RR)

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Performing location update, authentication, and TMSI reallocation (MM) Huawei Technologies Proprietary

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

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Supporting supplementary services (SS)

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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 adjacent sublayers 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

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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.

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Call Control (CC) Performing all necessary functions to establish or release CS connections

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Supplementary Service (SS) Performing all necessary functions to support GSM supplementary services

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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 tobe-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 routing function according to the PD. If a message belongs to the RR sublayer, this message is transmitted 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.

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Figure 1-8 L3 signaling message processing procedure Mobile network services

MNCC-SAP

MNSS-SAP

MNSMS-SAP

S S

CC

SM S MMSS-SAP

MMCC-SAP

MMSMS-SAP

MMREG -SAP MM CC

Layer 3 signaling

MM

SS

SMS

RR-SAP RR

RR PD RR

SDCCH SACCH

SAPI 3 RACCH BCCH AGCH+PCH SDCCH SACCH FACCH

SAPI 0

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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Call control services, including normal establishment of MS originating calls, emergency establishment of MS originating calls, call hold, call termination, and support for callrelated supplementary services

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Support for call independent supplementary services

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Support for short messages service

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

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Support for call independent supplementary services

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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, and transmit control data.

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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 RRprimitive RR SAP Protocol of the peer layer of the RR sublayer

Radio resource management sublayer

Figure 1-10 Services provided by the MM sublayer MS side CC

SS

Network side SMS

Mobile management sublayer

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Protocol of the peer layer of the MM sublayer

SS

SMS

Mobile management sublayer

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