13.12 Explicit Call Transfer (ECT)

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

The procedures specified in 3GPP TS 23.091 [15] for the Explicit Call Transfer supplementary service shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

Party A is the subscriber controlling the Explicit Call Transfer Call (served mobile subscriber). Party B is the first remote called party (held party). Party C is the second remote called party.

13.12.1 Connection of remote parties

If the result of the ECT checks is successful the MSC server will order the MGW to connect the bearer termination of the C-party to the bearer termination of the B-party (bullet 1 in figure 13.24 or in figure 13.21). As a result of this action the held party will be retrieved.

If the call towards the C-party has not been answered, the MSC server requests the MGW to both-way through-connect the bearer termination towards the C-party.

13.12.2 IU/A-interface release

The served party is disconnected after a successful transfer request. The call towards the served mobile subscriber shall be released as described in the clause for call clearing.

13.12.3 Failure handling in MSC server

If the bearer terminations for the remote parties can not be connected successfully, the MSC server shall reject the ECT request.

13.12.4 Example

Figure 13.23 shows the network model for call explicit call transfer. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling and the bearer. Note that for a TDM access case there is no separation of call and bearer control signalling. The MSC server moves the bearer terminations of the remote parties in the same context and removes the bearer termination for the served mobile subscriber. The bearer termination T_A is used for the bearer towards the served mobile subscriber, the bearer termination T_B is used for the bearer towards the B-party and the bearer termination T_C is used for the bearer towards the C-party.

Figure 13.23: ECT (Network model)

Figure 13.24 shows the message sequence example for the explicit call transfer when both calls have been answered. In the example the MSC server requests the MGW to move the bearer termination for the C-party in the active call to the same context which contains the bearer termination for the B-party. The held party is informed about the retrieval of the held call. Both the remote parties are informed about the call transfer. After the move the MSC server releases the call and the bearer connection towards the served mobile subscriber and requests the MGW to remove the bearer termination for the served mobile subscriber.

Figure 13.24: Explicit call transfer; both calls answered (message sequence chart)

Figure 13.25 shows the message sequence example for the explicit call transfer when one call is answered and the other call has been delivered. In the example the MSC server requests the MGW to move the bearer termination for the C-party in the active call to the same context which contains the bearer termination for the B-party. The held party is informed about the retrieval of the held call. Both the remote parties are informed about the call transfer. After the move the MSC server releases the call and the bearer connection towards the served mobile subscriber and requests the MGW to remove the bearer termination for the served mobile subscriber. The B-party is informed about the active call when the C-party sends the Answer indication.

Figure 13.25: Explicit call transfer; other call delivered (message sequence chart)