13.5 Call Waiting (CW)

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

The procedures specified in 3GPP TS 23.083 [13] for the Call Waiting supplementary service shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network. The following clauses assume subscriber A to be the served subscriber with the call waiting supplementary service, subscriber B to be the one who is engaged in a call with user A, subscriber C to be the one who has originated a call to subscriber A which causes the call waiting supplementary service to be invoked.

Call confirmation to the waiting call

The MSC server shall, on reception of the call confirmation, select the MGW that will be used for the waiting call. The MSC server should select the MGW which is already in use for the active call. If out-of-band transcoder control is applied for the waiting speech call, it shall be performed in accordance with 3GPP TS 23.153 [3].

Existing call on hold

The clause "Hold request" in clause 13.6 applies.

Existing call released

If the active call is disconnected while another call is waiting, the bearer termination towards the waiting party (C) as well as to the called party (A) is not removed.

Acceptance of waiting call

If the mobile subscriber decides to accept the waiting call, it handles (according to 3GPP TS 23.083 [13]) the existing call as described in clause 13.5 (i.e. it either puts the call on hold or the call is released). When the MSC server receives the connect indication from subscriber A, if required the MSC server shall modify the access bearer as described in clause 13.18.1. Finally, the MSC server shall connect the access side bearer termination to the previously created bearer termination of the remote party in the waiting call and modify the waiting call’s bearer termination so that it is both-way through-connected.

If a different MGW is selected for the incoming call, then a bearer from the new MGW (MGW2) shall be connected towards the old MGW (MGW1) before offering the call to the subscriber A.

If out-of-band transcoder control is applied for the waiting speech call, it shall be performed in accordance with 3GPP TS 23.153[3].

Waiting call released by calling subscriber (subscriber C)

The respective resources already allocated at the selected MGW for the waiting call shall be released.

Example

Figure 13.13 shows the network model for a waiting call at the serving MSC server/MGW. The "thick, squared" line represents the call control signalling for the existing call and, on the Iu interface, the already existing control plane toward the serving RNC. The "thin, squared" line represents the call control signalling for the waiting call. The "thick, dotted" line represents the bearer control signalling and the bearer for the existing call, whereas the "thin, dotted" line represents the ones for the waiting call. Note that for a TDM access there is no separation of call and bearer control signalling.

NOTE: There shall be only one instance of bearer resource/bearer control signalling on the radio side.

If the CW condition applies, the MSC server seizes a new context with one bearer termination, T_C, in the MGW. T_A and T_B are the terminations of the already existing call.

Figure 13.13: Call Wait (network model)

13.5.1 Call Confirmation of the waiting call

Figure 13.14 shows the sequence chart for the actions necessary within the bearer independent CS core network during call confirmation of the waiting call. Call and bearer establishment shall be as described for the mobile terminating call. When the MSC server receives the Alerting indication from the called subscriber (subscriber A), it shall apply the ringing tone to the waiting termination (T_C).

Figure 13.14: Call Confirmation of the Waiting Call.

13.5.2 Acceptance of the Waiting Call

Figure 13.15 shows the sequence chart for the actions necessary within the bearer independent CS core network for the acceptance of a waiting call. When the MSC server receives the Connect indication from the UE (subscriber A) (bullet 1 in figure 13.15), it shall request the MGW to disconnect subscriber C from the applied ringing tone (bullet 2 in figure 13.15) and move T_A to the context of the waiting call (bullet 3 in figure 13.15). The MSC server then requests the MGW to change the through-connection of the termination T_C so that it will be both way through-connected (bullet 4 in figure 13.15).

Figure 13.15: Acceptance of the Waiting Call.