1. HUAWEI BSC6000 Base Station Subsystem
Signaling Analysis Guide 1 BSS Signaling Fundamental
1 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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Signaling Analysis Guide 1 BSS Signaling Fundamental
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 SCCP: Signaling Connection
Part Control Part
BSSAP: Base Station Subsystem BSSMAP: Base Station Subsystem
Application Part 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.
Generally, a signaling data link occupies timeslot 16 of a trunk. The specific timeslot is
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1 BSS Signaling Fundamental Signaling Analysis Guide
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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Signaling Analysis Guide 1 BSS Signaling Fundamental
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
– 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. It
consists 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 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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Signaling Analysis Guide 1 BSS Signaling Fundamental
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.
– 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 be
established.
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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 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 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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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:
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 Resource Through handover resource allocation, the
Allocation 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 When a processing is complete, the MSC
Resource Release 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.
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 BSSAP
BTSM
RR BTSM SCCP
LAPDm LAPD LAPD
Sign. MTP
Layer 1 Layer1
Layer 1
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:
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
BSS
Abis
TRX
BTS1
BCF
TRX
TRX BTS2
A Abis
MSC 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

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
L2ML SAP1=63 TEI2
TEI
BCF
RSL SAP1=0
OML SAP1=62
TRX
MANA L2ML SAP1=63 TEI3
GEMENT
BCF
OML SAP1=62
L2ML SAP1=63 BCF TEI4
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
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 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 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 control
channels
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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 to
a 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 downlink
message 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 Chann Chanel
…
el 0 el 1 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 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.
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
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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
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 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 Other functional units
MPH primitive 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.
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 is
identified by a data link connection identifier (DLCI).
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
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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 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.
l 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:
l Establishing, operating, and releasing dedicated radio channels (RR)
l Performing location update, authentication, and TMSI reallocation (MM)
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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 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
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 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 SM
CC
S S
MMSS-SAP
MMCC-SAP
MMSMS-SAP
MMREG -SAP
MM MM CC SS SMS
signaling
Layer 3
RR-SAP
RR RR
PD
RR
SAPI 0 SAPI 3
BCCH
AGCH+PCH
SDCCH
SDCCH
SACCH
RACCH
SACCH
FACCH
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
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:
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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, and
transmit 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
RR-
primitive
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 Network side
CC SS SMS CC SS SMS
Protocol of the peer layer
Mobile of the MM sublayer Mobile
management management
sublayer sublayer
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