23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS
The procedures specified in 3GPP TS 23.072  for the Call Deflection supplementary service shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.
13.2.1 MGW selection and incoming side bearer establishment
The MGW selected for the mobile terminating call is used. The incoming side bearer has already been established by the mobile terminating call procedures.
13.2.2 IU/A-interface release
If the call deflection request from a served subscriber is accepted the call towards the served mobile subscriber shall be released as described in the clause for call clearing.
13.2.3 Notification to the Calling Subscriber
If the served mobile subscriber has requested that the calling subscriber shall receive a notification about the call forwarding, a notification is sent to the calling party. If the notification is implemented using intermediate tones or announcements the MSC server requests the MGW to play an announcement/tone to the calling party, as described in clause 14.6, before establishing the call to the forwarded-to subscriber.
13.2.4 Initial addressing
The call towards the deflected-to subscriber is established as for basic call. If out-of-band transcoder control is applied for a speech call, it shall be performed in accordance with 3GPP TS 23.153 . After the possible generation of in-band information has been completed the MSC server shall indicate in the IAM that forward or backward bearer establishment is to be used. The MSC server shall indicate in the IAM that no Continuity message will follow since the incoming bearer has already been established.
The MGW-id can be provided to the succeeding node in the IAM.
13.2.5 Establishment of bearer towards the forwarded-to subscriber
The bearer establishment towards the forwarded-to subscriber is performed as described for the mobile originating call, network side bearer establishment, using either forward or backward bearer establishment. The MSC server also requests the MGW to both-way through-connect the bearer.
13.2.6 Failure handling in MSC server
The failures are handled as described for the basic mobile originating call.
Figure 13.1 shows the network model for call deflection. 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 replaces the bearer termination for the served mobile subscriber (TB) with the bearer termination for the deflected-to subscriber (TC) in an existing context in the MGW. The bearer termination TA is used for the bearer towards the preceding MGW (calling subscriber).
Figure 13.1: Call deflection (Network model)
Figure 13.2 shows the message sequence example for the call deflection with a possible notification to the calling party using an announcement. In the example, after the call and the bearer towards the access side have been released the MSC server requests the MGW to remove the bearer termination for the served mobile subscriber, and optionally requests the MGW to play an announcement and to notify the announcement completion. The MSC server shall request the establishment of the call towards the deflected-to subscriber after the possible announcement has completed.
Figure 13.2: Call deflection (message sequence chart)