Protocols on the A-Interface

I. Physical layer

The physical layer of the A-interface is 120-ohm symmetrical twisted pair or 75-ohm coaxial cable whose rate is 2 Mbit/s. The physical layer of A-interface has the following features:

The 2 Mbit/s transfer rate complies with G.703. Frame structure, synchronization and timing comply with G.705. Fault management complies with G.732. CRC4 complies with G.704.

II. Message Transfer Part (MTP)

The main function of MTP is to ensure reliable signaling message transfer in the signaling network. In case of system and signaling network faults, it takes measures to avoid or reduce the loss of messages, repeated messages and out-of-sequence packets.

MTP protocols are defined in ITU-T Q.701~710 Recommendations.

MTP comprises three functional levels: signaling data link function, signaling link function and signaling network function.

1) Signaling data link function

Signaling data link (layer 1) is the channel used for signaling transmission. It comprises two data channels of the same data rate but two opposite working directions. The data rate is 64kbit/s. Generally, the signaling data link occupies timeslot 16 of a trunk cable. The specific timeslot is to be determined by negotiation between BSC and MSC. By data configuration, the timeslot can be used to establish a semi-permanent connection.

The signaling data link is the information bearer of SS7. One of its important features is that the signaling link is transparent, i.e. the data transferred on it cannot be changed. Therefore, equipment such as echo canceller, digital attenuator, A/u rate converter, cannot be connected to this link.

2) Signaling link function

Signaling link function (layer 2) regulates the functions and procedures to send the signaling to the data link, and together with layer 1, it implements reliable signaling message transfer between two directly-connected signaling points. Due to long-distance transmission, a certain rate of bit errors may be caused on the data link between two adjacent signaling points. However, no error is allowed in CCS7 signaling message codes. The purpose of layer 2 is to guarantee error-free transmission of message codes in the case that there exist bit errors on layer 1. Functions of layer 2 include: signaling unit delimitation, signaling unit alignment, error detection, error correction, initial alignment, processor fault, level-2 flow control, and signaling link error rate monitoring.

3) Signaling network function

By controlling the route and performance of the signaling network, signaling network function (level 3) guarantees that signaling information can be reliably transferred to the user part, whether the signaling network is in normal state or not.. Signaling network functions include signaling message processing and signaling network management.

a) Signaling message processing

Signaling message processing function sends signaling messages to the corresponding signaling links or user parts. The user part in BSS only contains SCCP. Signaling message processing functions comprise three parts: message routing (MRT), message discrimination (MDC) and message distribution (MDT), as shown in Figure 1-3.

Figure 1-3 Signaling message processing procedure

Message Routing

MRT (Message Routing) function is used at each signaling point to determine the signaling link group and the signaling link to destination signaling point. The MRT part implements the selection of message routes. In other words, by using the information (DPC and SLS) contained in the route mark, it selects a signaling link for signaling messages, so that the messages can be transferred to the DPC.

Message Discrimination (MDC)

Message Discrimination (MDC) part is designed to receive the messages from Layer 2 to ascertain whether the destination of the messages is the local signaling point. If the destination is the local signaling point, the MDC part will send the messages to the Message Distribution (MDT) part. If the destination is not the local signaling point, the MDC part will send the messages to the Message Routing (MRT) part.

Message Distribution (MDT)

Message Distribution (MDT) part is designed to allocate the messages from the MDC part to the user part and the signaling network management and test & maintenance part accordingly.

b) Signaling network management

Signaling network management is to re-construct the signaling network and to keep and recover the normal transfer ability of the signaling unit when the signaling network fails. Signaling network management includes three parts: signaling traffic management, signaling link management and signaling route management.

Signaling Traffic Management (STM)

Signaling Traffic Management (STM) is to transfer the signaling data from one link/route to another or multiple available links/routes when the signaling network fails. It is also used to temporarily reduce signaling traffic in case of congestion at the signaling point.

Signaling link management

Signaling link management (SLM) is to recover or enable the signaling link in the signaling network or to disconnect the signaling link. It ensures the provision of certain pre-determined link groups. The connection between the signaling data link and the signaling terminal is normally

established by the man-machine commands. Operations in the signaling system can not automatically change the above connection relationship.

Signaling route management

Signaling route management (SRM) is used to ensure the reliable exchange of signaling route availability information between signaling points so as to block or unblock signaling routes when necessary. It mainly comprises such procedures as transfer prohibited, transfer allowed, controlled transfer and restricted transfer, signaling route group test, and signaling route group congestion test.

III. Signaling Connection & Control Part (SCCP)

The purpose of SCCP is to provide complete network layer functions with the help of MTP. Network layer provides connectionless services and connection-oriented services.

The network layer services provided by SCCP can be classified into connectionless services and connection-oriented services. The connectionless service means that MS does not establish a signaling connection in advance, and uses the routing functions of SCCP and MTP to directly transfer data information in the signaling network. It is applicable to the transfer of a small quantity of data. The connection-oriented service means that a signaling connection is established in advance, and data are directly transferred on the signaling link, instead of using the route selection function of SCCP. It is applicable to the transfer of large quantities of data, and effectively shortens the transmission delay of batch data.

SCCP has routing and network management functions. The routing function of SCCP is to perform addressing as per the address information such as DPC, SSN, GT, etc. DPC refers to the destination signaling point code adopted by MTP, and SSN refers to the subsystem No., which is used to identify the different users (such as ISUP, MAP, TCAP and BSSAP) of SCCP in the same node, so as to compensate the insufficiency of users of MTP and to enlarge the addressing scope. GT addressing mode is not introduced as BSS does not adopt this addressing mode.

The network management function of SCCP is to implement management of signaling point state and subsystem state, switchover of active/standby subsystem, broadcasting of status messages and testing of subsystem state. SCMG (SCCP management) is to maintain the network functions by reselecting a route or adjusting the traffic volume when network fault or congestion occurs. MTP protocols are defined in ITUT Q.711~716 Recommendations

IV. BSSAP

1) Protocol overview

The BSSAP protocol, which serves as A-interface specification, describes two kinds of messages, BSSMAP and DTAP message. BSSMAP messages are used for traffic flow control, and are to be processed by the internal functional module of the corresponding A interface. For DTAP messages, A interface is merely equivalent to a transport channel, On BSS side, DTAP messages are directly transferred to radio channels. On MSC subsystem side, DTAP messages are transferred to the corresponding functional processing unit.

BSSAP protocols are defined in ETSI GSM 08.08 and ETSI GSM 04.08 specifications.

2) Typical message contents

a) DTAP messages

The DTAP messages can be divided into Mobile Management (MM) messages and Call Control (CC) messages.

The MM messages consist of messages related to authentication, CM service request, identification request, IMSI detach, location update, MM state, TMSI re-allocation, etc.

The call control messages consist of alerting, call proceeding, connection, setup, modification, release, disconnection, notification, state query, DTMF startup messages, etc.

b) BSSMAP messages

The BSSMAP messages can be divided into connectionless and connection-oriented messages.

The connectionless messages consist of Block/Unblock, Handover, Resource, Reset, Paging messages, etc.

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

Handover messages include Handover Candidate Enquire and Handover Candidate Enquire Response.

The resources messages include Resources Request and Resource Indication messages.

The Reset messages include Reset and Reset ACK messages.

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

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

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

The Clear messages include Clear Request and Clear Complete messages.

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

c) BSSAP protocol functionality

The BSSAP protocol can deliver its own functions in connection-oriented mode and connectionless mode of SCCP. When MS needs to exchange service-related messages over radio resources with the network side while there is no MS-related SCCP connection between MSC and BSS, a new connection will be established. A new connection shall also be set up for external handover. There are two kinds of connection setup:

While MS sends the Access Request message on the RACH, BSS allocates a dedicated radio resource (DCCH or TCH) to MS. After the L2 connection is set up on the SDCCH (or FACCH) where resources are allocated, BSS starts the connection setup. When MSC decides to execute an external handover (the target BSS might be the original BSS), it must reserve a new DCCH or TCH from the target BSS. In this scenario, MSC starts the connection setup.

The BSSAP protocol implements the functional flow as shown in Table below using the connection and connectionless messages.

Major functions of BSSAP

Serial number

Function Description

1 Assign “Assign” is to ensure the dedicated radio resources are allocated or re-allocated properly to the MS. The initial MS random access and “immediate assignment” to a DCCH is processed automatically by BSS but not controlled by MSC.

2 Block/Unblock During circuit assignment, MSC selects an available terrestrial channel. If this channel is no longer available then BSS notifies it to MSC. The Block/Unblock procedure can carry out this function.

3 Resource Indication

Resource indication serves to notify MSC:

Amount of the radio resource available for TCH in BSS,

Amount of all available radio resource (i.e. able to provide service or have been specified)

It is not easy to get this information from the MSC-controlled services. These must be considered when MSC decides an external handover.

4 Reset “Reset” is to initialize the BSS or MSC. For instance, if BSS goes faulty or loses all the reference messages about processing, BSS sends a Reset message to MSC, which releases the affected calls, deletes the affected reference messages and sets all the circuits related to the BSS to idle.

If MSC or BSS is only locally faulty, the affected parts can be cleared using the Clear procedure.

5 Handover Request BSS may send a “handover request” to MSC requesting to perform handover of the MS, to which dedicated resources have been allocated, for the reasons as listed below:

a) BSS detects a radio cause for handover.

b) MSC starts the Handover Candidate Enquirer procedure. The MS is waiting for the handover.

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

The Handover Request messages should be re-sent once in a while till one of the following situations occurs:

Receive the "Handover Command" message from MSC

Reset message is received

All communications with MS will be interrupted and the processing is aborted

Processing is over, such as call clearing.

6 Handover Resource Allocation

Handover Resources Allocation enables MSC to request for resources from BSS based on the handover request. The target BSS will reserve resources and wait for an MS to access this channel.

7 Handover Procedure

This is the procedure in which MSC instructs MS to access the radio resources of another cell. When handover is carried out, the original dedicated radio resources and terrestrial resources are maintained all the time until MSC sends a Clear Command message or Reset occurs.

8 Release of Radio Resources and Terrestrial Resources

When processing is done, MSC sends a “Clear Command” to BSS to release radio resources. On receiving the command, BSS starts the Clear procedure at the radio interface, then sets the configured terrestrial circuit to idle and returns a Clearing Complete message to MSC, which in turn releases the terrestrial resources of the local end.

If resources need to be released by BSS, BSS will send a “Clear Request” to notify MSC to start the release procedure to release the terrestrial and radio resources concerning MSC and BSS.

9 Paging The paging to MS is transported with the SCCP connectionless service via BSSMAP. If BSS receives the Paging Response message at the radio channel interface, it will establish an SCCP connection to MSC. The paging response message, which is loaded in the BSSMAP Full L3 Message, is transported on the signaling connection to MSC.

10 Flow Control Flow control can prevent the entities from receiving too much traffic. Flow control on the A-interface is implemented by controlling the traffic at the traffic source. Two levels of flow control are available. Flow control can be implemented based on subscriber classes.

11 Classmark Update Classmark Update serves to notify the class messages received from MS to the receiving entities. Generally, BSS notifies MSC after receiving the class messages from MS. It is also likely that when handover is complete, MSC sends the corresponding MS Classmark messages to the new BSS via the A-interface.

12 Cipher Mode Control

The Cipher Mode Control procedure allows MSC to transport the cipher mode control messages to BSS and start the subscriber equipment and signaling cipher equipment with a correct Kc.

13 Queuing Indication This procedure is designed to notify MSC that BSS wants to delay the allocation of necessary radio resources. This procedure is valid only when the queuing function is introduced for traffic channel assignment and traffic channel handover in the BSS.

14 Load Indication Load indication serves to notify the traffic state of a cell to all the adjacent BSSs so that an overall control over the handover services in an MSC can be exercised. In a certain valid period, the traffic state of the adjacent cells will be taken into account by the adjacent BSS during handover.