2 Application to interface structures

08.083GPPMobile-services Switching Centre - Base Station system (MSC-BSS) Interface Layer 3 SpecificationRelease 1999TS

The underlying transport mechanism defined to carry signalling information between the BSS and the MSC is the Message Transfer Part (MTP), and the Signalling Connection Control Part (SCCP) of Signalling System No.7.

The MTP and SCCP are used to support communication between the MSC and two conceptual entities within the BSS, these are:

– the BSS Operation and Maintenance Application Part (BSSOMAP);

– the BSS Application Part (BSSAP).

The BSS Application Part is split into two sub application parts, these are:

– the BSS Management Application Part (BSSMAP);

– the Direct Transfer Application Part (DTAP).

Distribution of messages between the two sub application parts is described in 3GPP TS 08.06.

Figure 1 is a diagrammatical representation of these conceptual entities. It should be noted that this is not intended to imply a particular implementation and is only for the purposes of specifying the interface.

Differentiation between BSSAP and BSSOMAP is by addressing mechanisms within the SCCP, using the subsystem number (see 3GPP TS 08.06).

2.1 The BSS Operation and Maintenance Application Part

If operation and maintenance messages are transferred by means of this interface then they shall use SCCP messages. The application protocol for the Operation and Maintenance Application Part is defined in the 3GPP TS 12 series Technical Specifications. The routing and addressing is provided by the SCCP and allows the MSC and the O&M centre to be addressed directly by the BSS using, for example, two E164 numbers. The operator may also use an X.25 connection for the transfer of O&M messages between the BSS and the OMC. This option is not further discussed in this Technical Specification.

2.2 The Direct Transfer Application Part

The Direct Transfer Application Part (DTAP) is used to transfer call control and mobility management messages between the MSC and the MS. The DTAP information in these messages is not interpreted by the BSS. 3GPP TS 08.06 contains more detail relating to the handling of DTAP messages at the BSS, the multiplexing of the messages onto the relevant signalling channels of the radio interface, and the use of the SCCP services.

Messages received from the MS are identified as DTAP by the Protocol Discriminator Information Element as described in 3GPP TS 04.08, except for Initial Layer 3 messages (see sub-clause 3.1.16). The majority of radio interface messages are transferred across the BSS MSC interface by the DTAP, the exceptions being messages belonging to the Radio Resource (RR) management protocol.

2.3 The BSS Management Application Part

The BSSMAP supports all of the procedures between the MSC and the BSS that require interpretation and processing of information related to single calls, and resource management.

Some of the BSSMAP procedures result in, or are triggered by, Radio Resource (RR) management messages defined in 3GPP TS 04.08. The BSSMAP procedures are described in sub-clause 3.

2.4 Handling of abnormal events related to the BSSAP Header

The BSSAP header is specified in 3GPP TS 08.06. Several abnormal events may be detected by the receiver:

– use of a reserved value in the DLCI or discriminator;

– length octet with value zero;

– length octet with a value inconsistent with that indicated by the SCCP.

In these cases the receiver may send a BSSMAP CONFUSION message as specified in sub-clause 3.2.1. If so, depending on the error in the BSSAP header, the error pointer shall be set to one of the values reserved for the BSSAP header in sub-clause 3.2.2.32.

Spare bits in the BSSAP header shall not be checked by the receiving entity.