8.1.1 Intra-MSC SRNS/SBSS Relocation

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

The procedures specified in 3GPP TS 23.009 [8] for "Intra-3G_MSC SRNS Relocation" shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

8.1.1.1 Relocation Required

When the Relocation Required message is received, the MSC server requests the MGW to provide a binding reference and a bearer address, using the Prepare Bearer procedure. For speech calls, the MSC server shall provide the MGW with the speech coding information for the bearer. For non-speech calls the MSC server also provides the MGW with the same PLMN Bearer Capability [4] as was provided at the last access bearer assignment. The MSC server uses the Change Flow Direction Procedure to request the MGW to set the Handover Device to initial state. The MSC server sends the Relocation Request message, containing the bearer address and the binding reference, to RNC-B (bullet 1 in figure 8.2/1).

For Relocation towards GERAN Iu mode the GERAN capabilities of the target cell will be indicated to the MSC-Server within the RANAP RELOCATION REQUIRED message if the target cell provides different capabilities than the current cell. If no information about the GERAN capabilities of the target cell are received within this message, the MSC-Server shall assume that the GERAN target cell will provide the same capabilities as the current cell (for details see [29]). The MSC server shall indicate to GERAN the selected services within the RANAP RELOCATION REQUEST message. The MSC server shall not set codec information in the NAS Synchronisation Indicator (see [4]). Instead it shall set codec information in the GERAN BSC container.

8.1.1.2 Relocation Command/Relocation Detect

When the MSC server sends the Relocation Command message or alternatively if it receives the Relocation Detect message, the MSC server uses the Change Flow Direction procedure to request the MGW to set the Handover Device to intermediate state (bullet 2 in figure 8.2/1).

8.1.1.3 Relocation Complete

When the MSC server receives the Relocation Complete message, it requests RNC-A to release the IU. The MSC server also requests the MGW to set the Handover Device to its final state by removing the bearer termination towards RNC‑A, using the Release Termination procedure (bullet 3 in figure 8.2/2).

8.1.1.4 Interworking function

The interworking function used by the MGW before relocation will also be used after relocation.

8.1.1.5 Codec handling

The MGW may include a speech transcoder based upon the speech coding information provided to each bearer termination.

8.1.1.6 Voice Processing function

After relocation, the MGW may continue or modify voice-processing function(s) provided to each bearer termination.

8.1.1.7 Handling of multiple bearers (multicall)

If the UE is engaged with multiple bearers all procedures related to the handling of bearers and terminations described for the relocation of a single bearer shall be repeated for each bearer.

8.1.1.8 Failure Handling in MSC server

When a procedure between the MSC server and the MGW fails the MSC server shall handle the failure as an internal error in accordance with 3GPP TS 23.009 [8] and 3GPP TS 29.010 [23]. If MGW resources have been already seized at the target access side then the resources shall be released using the Release Termination procedure. If the call is to be cleared, then it shall be handled as described in clause 7.3.

8.1.1.9 Example

Figure 8.1 shows the network model for the Intra-MSC SRNS Relocation. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling and the bearer. The bearer termination T1 is used for the bearer towards RNC-A, bearer termination T3 is used for the bearer towards RNC-B and the bearer termination T2 is used for the bearer towards the succeeding/preceding MGW.

Before Relocation:

During Relocation:

After Relocation:

Figure 8.1: Intra-MSC SRNS Relocation (network model)

Figure 8.2 shows the message sequence example for the Intra-MSC SRNS Relocation.
It is assumed that the Handover Device is located in the MGW, which has been selected for the call establishment by the MSC server. The MSC server controls the call and the mobility management. It is also assumed that only one bearer has been established towards RNC-A.
In the example the MSC server requests seizure of RNC-B side bearer termination with specific flow directions. The MSC server orders the establishment of the bearer by sending Relocation Request towards RNC-B. When the relocation is detected in RNC-B the MSC server requests to change the flow directions between the terminations within the context. When the MSC server receives a Relocation Complete indication from RNC-B it orders RNC-A to release the IU. This action causes release of the bearer between the RNC and the MGW. Finally the MSC server requests the MGW to release RNC-A side bearer termination.

Figure 8.2/1: Intra-MSC SRNS Relocation (message sequence chart)

Figure 8.2/2: Intra-MSC SRNS Relocation (message sequence chart)