9 Compatibility Issues
23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS
A Release 4 (or later) node, according to 3GPP TS 23.205, is backward compatible with a Release 99 (or earlier) node.
9.1 Interworking with GERAN (A i/f)
9.1.1 Introduction
The A-interface signalling terminates in the MSC server and the user plane terminates in the MGW. TDM and/or IP transports may be supported on the A-interface. The MSC server uses the Mc interface to remotely control the TDM or IP resources in the MGW.
For intra-MSC handover, the target A-interface is handled as described in clause 8. If the target A-interface user plane terminates in a different MGW from the MGW that terminates the serving A-interface user plane, a bearer has to be established between the two MGWs using Prepare Bearer and Establish Bearer procedures. Because the same MSC server controls both MGWs, no external call control signalling is involved.
It is important to note that the separation between the user and control planes remains the same before and after interaction with services in the 3G BICC CSCN.
9.1.2 A interface over TDM
Only one MSC server may control the TDM circuits connected to one GERAN node.
For each TDM circuit a physical termination is provisioned in the MGW. The TDM circuit is identified by the termination Id in the Mc interface. Since TDM circuits are also grouped together, the physical termination Ids are structured in accordance with the grouping of TDM circuits. The MSC server also knows the termination Ids and the grouping of termination Ids. The physical termination exists as long as the TDM circuit(s) exists in the MGW.
Figure 9.1 shows the network model for the A-interface. The "squared" line represents the call control signalling and the "dotted" line represents the TDM circuits. The terminations T1-Tn represent the TDM circuits in the MGW. The MSC server has a mapping table between circuits CIC1-CICn and the terminations T1-Tn.
Figure 9.1 TDM circuits used for A-interface (network model)
For call-independent transactions the general (G)MSC server-MGW procedures, as described in clause 10, apply to the physical terminations in the same way as to any other terminations.
For call related transactions the handling as described in the clauses 6, 7 and 8 apply to physical terminations in the same way as any other terminations. All call related procedures, except Prepare Bearer, Establish Bearer, Release Bearer and Tunnel Information Up/Down, as described in clause 16, apply to the physical terminations in the same way as any other terminations.