A interface
The A interface is the interface between the BSC and the MSC. At the physical layer, it uses a 2-Mbps PCM30 link. One or more 64-Kbps timeslots are used to carry signaling information. Typically, more than one 2-Mbps link is required to handle the traffic between the BSC and the MSC.
The Base Station System Application Part (BSSAP) is a GSM-specific protocol designed for signaling over the A interface. The BSSAP uses CCS7 MTP and SCCP transport and addressing services for the signaling message transfer. The BSSAP supports both connectionless and connection-oriented services provided by the SCCP. The connectionless services are used to support global procedure such as PAGING (for MS) and RESET (a circuit). The connection-oriented services are used for dedicated procedures such as handover and assignment procedures. The BSSAP supports messages sent between the MSC and the BSS, as well as transparent message transfer between the MSC and the MS. To enable this functionality, the BSSAP is divided into two parts, i.e., the Base Station Subsystem Management Application Part (BSSMAP) and Direct Transfer Application Part (DTAP).
Figure 3-17 Transparent Layer 3 message over Abis.
TABLE 3-10 Abis Layer 3 Messages
|
RLM |
CCM |
TRXM |
DCM |
|
DATAREQuest |
BCCH |
RF REsource |
CHANnel ACTivation |
|
INFOrmation |
INDication |
||
|
DATAINDication |
CCCH LOAD |
SACCH |
CHANnel ACTivation |
|
INDication |
FILling |
ACKnowledge |
|
|
ERROR |
CHANnel |
OVERLOAD |
CHANnel ACTivation |
|
INDication |
REQuired |
Negative ACKnowledge |
|
|
ESTablish |
DELETE |
ERROR |
CONNection FAILure |
|
REQuest |
INDication |
REPORT |
INDication |
|
ESTablish |
PAGING |
DEACTivate SACCH |
|
|
CONFirm |
CoMmand |
||
|
ESTablish |
IMMEDIATE |
ENCRyption CoMmand |
|
|
INDication |
ASSign CoMmand |
||
|
RELease |
SMS Broadcast |
HANDOver DETection |
|
|
REQuest |
REQuest |
||
|
RELease |
MEASurement RESult |
||
|
CONFirm |
|||
|
RELease |
MODE MODIFY |
||
|
INDication |
REQuest |
||
|
UNIT DATA |
MODE MODIFY |
||
|
REQuest |
ACKnowledge |
||
|
UNIT DATA |
PHYsical CONTEXT |
||
|
INDication |
REQuest |
||
|
PHYsical CONTEXT |
|||
|
CONFirm |
|||
|
RF CHANnel RELease |
|||
|
MS POWER |
|||
|
CONTROL |
|||
|
BS POWER |
|||
|
CONTROL |
|||
|
PREPROCess |
|||
|
CONFIGure |
|||
|
PREPROCessed |
|||
|
MEASurement RESult |
|||
|
RF CHANnel RELease |
|||
|
ACKnowledge |
Figure 3-18 illustrates the protocol stack on the MSC and the BSC side. The MM and CM sublayer signaling information from the MS is routed to the BSS transparently over the signaling channels (FACCH, DACCH, SACCH). From the BSS, this information is relayed to the MSC by DTAP. The DTAP uses SCCP logical connection to transfer the information to the peer MM or CC entity in the MSC.
BSS management application part. The BSSMAP data are part of Layer 3 and carry messages related to radio resource and the BSC management. The BSSMAP process within the BSC controls the radio resources in response to the instructions given by the MSC. Examples of BSSMAP messages are paging, handover request, reset, and block. Table 3-11 lists the BSSMAP messages.
Figure 3-18 A-interface signaling protocol.
Direct transfer application part. The DTAP data is user information and carries messages related to call control and mobility management between two users, i.e., MS and any subsystem of NSS such as the MSC. The messages are transparent to BSS except for a few exceptions. These exceptions are location update request, CM service request, and IMSI detach indication. These messages are partially processed by the BSC to add necessary information required for other entities to process these requests.
The DTAP messages are identical to the transparent MM and CM messages listed in the previous section.
Inter-MSC signaling
For call control, an MSC may have signaling interfaces to other MSC, GMSC, PLMN, and PSTN. Figure 3-19 shows the protocol stack used on these interfaces. ITU-T ISUP or TUP protocol is used for call setup and supervision. TUP is an old protocol and may not be used in newer implementations. ISUP is used for both speech and data call setup. ISUP relies on the MTP protocol for transportation, addressing, and routing of call control messages.
The MSC also has interfaces with the VLR, HLR, EIR ,GMSC, and interworking MSCs for non-circuit-related call control. Figure 3-20 shows this interface and protocol stack. The GSM Mobile Application Part (MAP) has been specifically designed for transfer of non-circuit-related signaling information between MSCs and between MSCs and databases. MAP relies on TCAP capabilities to establish non-circuit-related communication between two entities in the signaling network to exchange data and control information.
TABLE 3-11 BSSMAP Messages
|
Message type |
ID (hex) |
Direction |
Remarks |
|
|
Assignment messages |
||
|
Assignment request |
01 |
 |
ASS REQ is sent from the BSC to the MSC to request a traffic channel on A and air interface. |
|
Assignment complete |
02 |
 |
ASS COM is a positive response from the BSC. |
|
Assignment failure |
03 |
 |
ASS FAIL is a negative response from the BSC. |
|
Handover messages |
|||
|
Handover request |
10 |
 |
The MSC sends HND REQ to the new BSC that is the target for handover. |
|
Handover required |
11 |
 |
The BSC sends HND RQD to the MSC in case of inter-BSC or inter-MSC handover. |
|
Handover request acknowledge |
12 |
 |
HND REQ ACK is the acknowledge for the previously received HND REQ. |
|
Handover command |
13 |
 |
HND CMD provides the information to the BSC related to the new radio channel resource to which the MS should switch. |
|
Handover complete |
14 |
 |
HND CMP indicates successful handover |
|
Handover failure |
16 |
 |
HND FAIL indicates unsuccessful handover. |
|
Handover performed |
17 |
 |
HND PERF indicates that the BSC has performed the intra-BSC handover. |
|
Handover candidate enquiry |
18 |
 |
HND CND ENQ is sent by the MSC to get the list of MSs in a particular cell that could be handed over to another cell to reduce the load on a target cell. |
|
Handover candidate response |
19 |
 |
HND CND RES is a response to the HND CND ENQ message. |
|
Handover required reject |
1A |
 |
HND RQD REJ indicates an unsuccessful response to HND_RQD message. |
|
Handover detect |
1B |
 |
The BSC sends HND DET to the MSC updating changes in the radio resource on receiving the HND DET message from the BTS. |
|
Release messages |
|||
|
Clear command |
20 |
 |
CLR CMD is used to release a traffic channel allocated to a specific MS. |
TABLE 3-11 BSSMAP Messages
|
Message type |
ID (hex) |
Direction |
Remarks |
|
Release messages |
|||
|
Clear complete |
21 |
 |
CLR CMP is the confirmation of resource release in response to CLR CMD message. |
|
Clear request |
22 |
 |
The BSC sends CLR REQ to the MSC on detecting any severe problem with an existing connection to an MS. |
|
SAPI "n" reject |
25 |
 |
The BSC sends SAPI REJ message to the MSC on receiving a message with SAPI not equal to zero but for which no Layer 2 connection exists. |
|
Confusion |
26 |
 |
This message is sent to in response of a message which can not be treated correctly by the receiving entity and for which another failure message can not substitute. |
|
General messages |
|||
|
Reset |
30 |
 |
Send by the MSC to the BSC (or vice versa) if the sending entity detects any fatal error in communication data. |
|
Reset acknowledge |
31 |
 |
RES ACK is sent to confirm that the RESET message was received. |
|
Overload |
32 |
 |
The BSC sends overload message to indicate the overload situation in a BTS or whole BSS. The MSC sends overload message to the BSC to indicate processor overload within the MSC. |
|
Reset circuit |
34 |
 |
RES CIRC is used to initialize a single circuit between the BSC and the MSC. |
|
Reset circuit acknowledge |
35 |
 |
Response to RES CIRC on a successful reset of a circuit. |
|
MSC invoke trace |
36 |
 |
Request to start a trace of a single connection. MSC INV TRC will enable tracing of messages in the direction from MSC to BSC. |
|
BSS invoke trace |
37 |
 |
Request to start a trace of a single connection. BSS INV TRC will enable tracing of messages in the direction from BSC to MSC. |
|
Terrestrial resource messages |
|||
|
Block |
40 |
 |
The BSC requests the MSC to block a single channel, using BLO message. |
|
Blocking acknowledge |
41 |
 |
The MSC acknowledges the blocking of a channel by sending BLO ACK message to the BSC. |
|
Unblock |
42 |
 |
UNBLO message is used to cancel the blocking. |
TABLE 3-11 BSSMAP Messages
|
Message type |
ID (hex) |
Direction |
Remarks |
|
|
Terrestrial resource messages |
||
|
Unblocking acknowledge |
43 |
 |
Acknowledgment of BLO message. |
|
Circuit group block |
44 |
 |
The BSC requests the MSC to block the multiple channels or complete PCM link, using CIRC GRP BLO message. |
|
Circuit group blocking acknowledge |
45 |
 |
The MSC acknowledges the blocking of the multiple channels/complete link by sending CIRC GRP BLO ACK message to the BSC. |
|
Circuit group unblock |
46 |
 |
CIRC GRP UNBLO message is used to cancel the blocking of multiple channels or complete PCM link. |
|
Circuit group unblocking acknowledge |
47 |
 |
Acknowledgment of CIRC GRP UNBLO message. |
|
Unequipped circuit |
48 |
 |
This message is sent by an entity to its peer entity that it is using one or more circuit identity code which are unknown. |
|
Radio resource messages |
|||
|
Resource request |
50 |
 |
The MSC requests the BSC to provide information on available radio resources. |
|
Resource indication Paging |
51 52 |
 |
Response to RES REQ. The MSC sends this message to locate the MS in case of MTC. |
|
Cipher mode command |
53 |
 |
The MSC sends a CIPHER MOD CMD to the BSC to instruct to start ciphering on air interface. |
|
Classmark update |
54 |
 |
The MS sends CLS MRK UPD to the MSC via BSC to update the classmark changes, if any. |
|
Cipher mode complete |
55 |
 |
The BSC sends this message to the MSC in response to the previously received CIPH MOD CMD to indicate that ciphering has successfully initiated on air interface. |
|
Queuing indication |
56 |
 |
Indicates the delay in assignment of traffic channels because of no availability of resources in response to previously received ASS REQ or HND REQ. |
|
Complete Layer 3 information |
57 |
 |
The CL3I message contains the initial message received from the MS for which SCCP connection at A interface can be set up. The typical example of such an initial message is CM SERV REQ. |
|
Classmark request |
58 |
 |
This requests MS to send its classmark information. |
|
Cipher mode reject |
59 |
 |
An unsuccessful response to the previously received CIPH MOD CMD. |
An unsuccessful response to the previously received CIPH MOD CMD.
Figure 3-19 Circuit-switched call—protocol stack.
Figure 3-20 Noncircuit call control protocol stack.
TCAP in turn relies on SCCP addressing and MTP transport capabilities.
MAP is also used between the VLR and the HLR. The B interface between the MSC and the VLR is an internal interface in most of the implementations.
The communication between applications and the MAP is done via MAP services or local operation codes defined in the GSM MAP specification. Table 3-12 lists local operation codes and their functions.
The local operation codes defined for the MAP operation on the B interface (MSC-VLR) are not listed in Table 3-12. The B interface is an internal interface and is not visible in most implementations. The GSM recommendations do not encourage the use of this interface and have not updated specifications for this interface since Release 99.
