8.3.2 Basic Inter-MSC GSM to UMTS Handover

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The following handling shall be applied for a call that started as UMTS call. The procedures specified in 3GPP TS 23.009 [8] for "Basic Handover Procedure Requiring a Circuit Connection between MSC-A and 3G_MSC‑B" shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

8.3.2.1 MSC-A

8.3.2.1.1 Handover Required

For Handover towards GERAN Iu mode the GERAN capabilities of the target cell will be indicated to the MSC-A server within the Handover 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-A server shall assume that the GERAN target cell will provide the same capabilities as the current cell (for details see [29]).

The MSC-A server shall indicate to the MSC-B server the GERAN capabilities of the target cell, if available, with the MAP Prepare Handover request. For speech calls, the MSC-A server shall additionally indicate to the MSC-B server the selected codec, the list of supported codecs, and the currently used codec.

8.3.2.1.2 Bearer establishment between MGW-A and MGW-B

The handling of the bearer establishment between MGW-A and MGW-B is as described for a Basic Mobile Originating Call, using either forward or backward bearer establishment. For speech calls, the MSC-A server shall provide the MGW-A with the speech coding information for the bearer. For non-speech calls the MSC-A server shall provide MGW-A with the same PLMN Bearer Capabilities [4] as were provided at the last access bearer assignment. The MSC‑A server also uses the Change Flow Direction procedure to request MGW-A to set the Handover Device to initial state (bullet 3 in figure 8.16/1).

8.3.2.1.3 Handover Command/Handover Detect

When the MSC-A server sends the Handover Command message or alternatively if it receives the Handover Detect message, the MSC-A server uses the Change Flow Direction procedure to requests MGW-A to set the Handover Device to intermediate state (bullet 4 in figure 8.16/2).

8.3.2.1.4 Handover Complete

When the MSC-A server receives the Handover Complete message, it releases the A-interface line towards BSC-A and requests MGW-A to set the Handover Device to its final state by removing the bearer termination towards BSC-A, using Release Termination procedure (bullet 5 in figure 8.16/2).

8.3.2.1.5 Interworking function

The interworking function used by MGW-A before handover will also be used after handover.

8.3.2.1.6 Codec handling

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

8.3.2.1.7 Voice Processing function

Voice processing function(s) provided by MGW-A before handover, may be modified or disabled by MGW-A after handover.

8.3.2.1.8 Failure Handling in MSC server

When a procedure between the MSC-A server and MGW-A fails the MSC-A server shall handle the failure as an internal error in accordance with 3GPP TS 23.009 [8] and 3GPP TS 29.010 [23]. If the call establishment towards MSC-B has already started then the call towards MSC-B server shall be cleared as described in clause 7.3. If the original call is to be cleared, then it shall be handled as described in clause 7.3.

8.3.2.2 MSC-B

8.3.2.2.1 MGW selection

The MSC-B server selects an MGW when it receives Prepare Handover Request message (bullet 1 in figure 8.16).

8.3.2.2.2 Bearer establishment towards RNC-B

When the MSC-B server has selected MGW-B it requests MGW-B 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. The MSC-B server sends the Relocation Request message to the RNC-B containing the bearer address and binding reference (bullet 2 in figure 8.16).

For Handover towards GERAN Iu-mode, the MSC-B Server shall select a service according to the Channel Type received with the Handover Request message and the capabilities of the GERAN target cell, if the GERAN classmark was received. For speech calls, the MSC-B server shall additionally take into account the selected codec, the list of supported codecs and the currently used codec received with MAP Prepare Handover request. The list of permitted speech versions received with the Channel Type in the Handover Request message is applicable to GERAN A/Gb mode only.

The MSC-B server shall indicate to the GERAN the selected service within the RANAP RELOCATION REQUEST message and shall set the RAB parameters within the RANAP RELOCATION REQUEST message according to the selected service. 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..

For speech calls, the MSC-B server shall report the chosen codec and codec modes back to the MSC-A server with MAP Prepare Handover response.

8.3.2.2.3 Bearer establishment between MGW-A and MGW-B

The handling of the bearer establishment is as described at Basic Mobile Terminating Call, using either forward or backward bearer establishment.

8.3.2.2.4 Codec handling

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

8.3.2.2.5 Voice Processing function

Voice processing function(s) provided by MGW-A before handover, may be continued or modified by MGW-B after handover.

8.3.2.2.6 Failure Handling in MSC server

When a procedure between the MSC-B server and MGW-B fails the MSC-B 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-B resources have already been seized at the target access side then the resources shall be released using the Release Termination procedure. The call from MSC-A server shall be released as described at clause 7.1.

8.3.2.2.7 Example

Figure 8.15 shows the network model for the Basic Inter-MSC GSM to UMTS Handover. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling (not applicable in case of GSM access) and the bearer. In MGW-A the bearer termination T1 is used for the bearer towards BSC-A, bearer termination T3 is used for the bearer towards MGW-B, and the bearer termination T2 is used for the bearer towards the succeeding/preceding MGW. In MGW-B the bearer termination T4 is used for the bearer towards RNC-B, bearer termination T5 is used for the bearer towards MGW-A.

Before Handover

During Handover

After Handover

Figure 8.15: Basic Inter-MSC GSM to UMTS Handover (network model)

Figure 8.16 shows the message sequence example for the Basic Inter-MSC GSM to UMTS Handover.
It is assumed that the Handover Device is located in the MGW (MGW-A) selected for the call establishment by the MSC server (MSC-A server) which controls the call and the mobility management.

In the example the MSC-B server requests MGW-B to seize RNC-B side bearer termination. The call is established between MSC-A server and MSC-B server, and the bearer is established between MGW-A and MGW-B. When the relocation is detected in RNC-B the MSC-A server requests to change the flow directions between the terminations within the context in MGW-A. When MSC-A server receives Handover Complete indication from MSC-B server it releases the A-interface line towards the BSC-A. Finally MSC-A server requests MGW-A to remove BSC-A side bearer termination.

Figure 8.16/1: Basic Inter-MSC GSM to UMTS Handover (message sequence chart)

Figure 8.16/2: Basic Inter-MSC GSM to UMTS Handover (message sequence chart)