4 Principles for the use of BSSAP on the E‑interface

09.083GPPApplication of the Base Station System Application Part (BSSAP) on the E interfaceRelease 1999TS

4.1 General

The mechanisms for the transfer of the BSSAP messages on the E‑interface is defined in 3GPP TS 09.02. The operation of the handover procedures between MSCs and the use of the BSSMAP messages for those procedures is described in 3GPP TS 03.09 and 3GPP TS 09.10.

In the same way as a SCCP signalling connection is used for the messages relating to one MS on the MSC‑BSS interface a TCAP dialogue is used on the E‑interface for messages relating to one MS. As no correspondence to the connectionless service on the MSC‑BSS interface is used on the E‑interface none of the global procedures (see 3GPP TS 08.08 subclause 3.1) are applicable.

The management of the terrestrial circuits between the MSCs is outside the scope of the E‑interface (see 3GPP TS 03.09), therefore all procedures, messages and information elements relating to terrestrial circuits are also excluded from the BSSMAP procedures and messages used on the E‑interface.

4.2 Transfer of DTAP and BSSMAP layer 3 messages on the E‑interface

The BSSAP data which on the MSC‑BSS interface is contained in the user data field of the exchanged SCCP frames (see 3GPP TS 08.06) is on the E‑interface transferred as the contents of the signalling info in a BSS‑APDU parameter as described in 3GPP TS 09.02.

The BSSAP data consists of a BSSAP header and a DTAP or BSSMAP layer 3 message. The BSSAP header contains, as specified in 3GPP TS 08.06 (subclauses 6.3.1, 6.3.2 and 6.3.3), of a discrimination parameter, possibly a Data Link Connection Identification (DLCI) parameter, and a length indicator.

4.3 Roles of MSC‑A, MSC‑I and MSC‑T

For the description in this ETS, the MSC’s functionality related to the handover between MSCs has been split into three logical parts, MSC‑A, MSC‑T and MSC‑I. The different roles need not necessarily be performed by different MSCs.

MSC‑A is the call/connection controlling part of the MSC where the call/connection was originally established and the switching point for handover between MSCs. (This corresponds to MSC‑A as defined in 3GPP TS 03.09 and 09.02). The MSC that is the MSC‑A will not be changed during the duration of a call/connection.

MSC‑T is the part relating to the transitory state during the handover for the MSC to which the MS is handed over when Basic handover or Subsequent handover (see 3GPP TS 03.09) take place. (This corresponds, depending on the type of handover to MSC‑A, MSC‑B or MSC‑B’ in 3GPP TS 03.09 and 09.02).

MSC‑I is the part of an MSC through which the MSC‑A, via an E‑interface (or an internal interface) is in contact with the MS. (This corresponds, depending on the type of handover to MSC‑A, MSC‑B or MSC‑B’ in 3GPP TS 03.09 and 09.02).

The MSC that is the MSC‑A can also have the role of either the MSC‑I or the MSC‑T during a period of the call/connection.

The following is applicable for the involved MSCs concerning the exchange of BSSAP data on an E‑interface before and after a successful inter MSC handover:

1) At Basic handover, two MSCs are involved, one MSC being MSC‑A and one being MSC‑T. When this handover has been performed, the two MSCs interworking on the E‑interface have the roles of MSC‑A and MSC‑I respectively, i.e. the MSC that is the MSC‑T during the handover is now the MSC‑I.

2) At Subsequent handover back to MSC‑A, two MSCs are involved. The MSC having the role of MSC‑A has also the role of MSC‑T. The other MSC involved has the role of MSC‑I. When this handover has been completed, there is no exchange of BSS data on the E‑interface, i.e. the MSC being the MSC‑I before and during the handover is now no longer taking part.

3) At subsequent handover to an MSC not being MSC‑A, three MSCs are involved. The roles of these MSCs are MSC‑A, MSC‑I, and MSC‑T respectively. When this handover has been performed, the two MSCs interworking on an E‑interface have the roles of MSC‑A and MSC‑I respectively, i.e. the MSC that is the MSC‑T during the handover is now the MSC‑I and the MSC being MSC‑I during the handover is now no longer taking part.