8.4.5 BSS Internal Handover

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

8.4.5.1 Introduction

The procedures specified in 3GPP TS 23.009 [8] for "BSS Internal Handover" shall be followed. The protocol messages are defined in 3GPP TS 48.008 [27]. The following clauses describe the additional requirements for the bearer independent CS core network due to BSS internal handover for A-Interface User Plane over IP (AoIP).

A BSS Internal Handover may be initiated by the BSS on its own, or triggered by the MSC server with the transmission of an Internal Handover Enquiry message to the BSS.

8.4.5.2 Internal Handover Required

When the MSC server receives an Internal Handover Required message, it requests the MGW to seize a TDM circuit using the Reserve Circuit procedure or an RTP termination using the Reserve and Configure RTP Connection Point procedure. In the latter case, the MSC server sends the BSC IP address and UDP port and the determined A-Interface Codec Type/Configuration to the MGW and requests the MGW to reserve a local IP address and UDP port.

NOTE: During a BSS Internal handover procedure involving an IP-based target A-Interface Type before and after the handover, the BSC IP addresses and UDP ports used before and after the handover may be identical or different. This is transparent to the MSC Server. In the former case, the BSC can receive on its BSC IP address and UDP port identical RTP flows received from the source and target access terminations reserved in the MGW.

For non-speech calls the MSC server shall provide the MGW with the GSM Channel coding properties and the same PLMN Bearer Capability 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 MGW reserves the TDM circuit or RTP termination and indicates in the latter case its own IP address and UDP port to the MSC server. The MSC server then sends the Internal Handover Command message to the BSC containing the speech codec type/configuration chosen and either the CIC or the MGW IP address and UDP port.

If the resources requested by the BSS in the Internal Handover Required message (e.g. codec type or codec configuration, interface type, etc.) are not available, an Internal Handover Required Reject message shall be returned to the BSS with the appropriate failure cause, and the procedure shall be terminated.

8.4.5.3 Internal Handover Command/Handover Detect

When the MSC server sends the Internal Handover Command message or alternatively if it receives the Handover Detect message, the MSC server uses the Change Flow Direction procedure to requests the MGW to set the Handover Device to intermediate state (bullet 2 in figure 8.4.5.8.2).

8.4.5.4 Handover Complete

When the MSC server receives the Handover Complete message, it requests the MGW to set the Handover Device to its final state by removing the bearer termination towards the source BSC IP address and UDP port number, using the Release Termination procedure (bullet 3 in figure 8.4.5.8.2).

8.4.5.5 Interworking function

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

8.4.5.6 Voice Processing function

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

8.4.5.7 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 already been seized for BSS internal handover 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.

If the MSC server receives the Handover Failure message during BSS internal handover procedure, it shall release MGW resources if they have been seized for BSS internal handover using the Release Termination procedure.

8.4.5.8 Example for a successful BSS Internal Handover

Figure 8.4.5.8.1 shows the network model for the BSS Internal Handover, where IP transport is used on the A interface before and after the BSS Internal Handover procedure. The "squared" line represents the call control signalling. The "dotted" line represents the bearer. The bearer termination T1 is used for the bearer towards the source BSC IP address and UDP port number Ta, bearer termination T3 is used for the bearer towards the target BSC IP address and UDP port number Tb and the bearer termination T2 is used for the bearer towards the succeeding/preceding MGW.

Before BSS Internal Handover

During BSS Internal Handover

After BSS Internal Handover

Figure 8.4.5.8.1: Intra-MSC BSS Internal Handover (network model)

Figure 8.4.5.8.2 shows the message sequence example for the Intra-MSC BSS Internal Handover.

In the example, the MSC server receives the Internal Handover Required message and requests the MGW to reserve an RTP bearer termination (T3) using the Reserve and Configure RTP Connection Point procedure with specific flow directions.

The MSC server starts the Internal Handover execution by sending the Internal Handover Command towards the BSC in which it provides the IP address and UDP port of the target MGW RTP termination (T3). When the handover is detected in the BSC, the MSC server requests the MGW to change the flow directions between the terminations within the context. When the MSC server receives the Handover Complete indication from the BSC it requests the MGW to release the bearer termination T1 towards the source BSC IP address and UDP port number Ta that was used before the BSS Internal Handover was initiated. The sequence is shown in Figure 8.4.5.8.2.

Figure 8.4.5.8.2: Intra-MSC BSS Internal Handover for AoIP (message sequence chart)