13.4.4 Call Forwarding on mobile subscriber Not Reachable (CFNRc)
23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS
The procedures specified in 3GPP TS 23.082 [12] for the Call Forwarding on Not Reachable supplementary service shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.
13.4.4.1 Rerouting by HLR
The same handling as for Call Forwarding Unconditional applies.
13.4.4.2 Rerouting by VLR
If the mobile is not reachable the incoming call for the specified basic service(s) will be forwarded without being offered to the served mobile subscriber.
13.4.4.2.1 MGW selection
If in-band information is to be provided to the calling subscriber the MSC server shall select the MGW before providing the in-band information. The MGW selection can be based on a possibly received MGW-Id from the preceding node.
NOTE: As an implementation option, if in-band information is not to be provided to the calling subscriber the MSC server may either perform call control without any associated MGW, or reserve resources from an MGW and request bearer establishment through that MGW. In the latter case the MSC server selects an MGW for the bearer either before sending the IAM or after receiving the Bearer Information message. If the MSC server received an MGW-Id from the preceding node and/or from the succeeding node, those can be used for the MGW selection.
13.4.4.2.2 Incoming side bearer establishment
The incoming side bearer establishment is handled in the MSC server as described for the mobile terminating call, using either forward or backward bearer establishment. The incoming side bearer establishment can take place either before or after the detection of the not reachable condition.
13.4.4.2.3 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 towards the forwarded-to party.
13.4.4.2.4 Initial addressing
The call towards the forwarded-to subscriber is established as a 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 [3]. In-band information may be played to the calling subscriber.
If no in-band information is to be played to the calling subscriber, in order to withhold the call completion until the establishment of the bearer is complete the MSC server shall indicate in the IAM that the Continuity message will follow from the preceding node, if either of the following conditions is satisfied before sending the IAM:
1. The incoming IAM indicated that the Continuity message will follow, but no Continuity message has been received, or
2. The incoming side bearer has not been established.
If the MGW is selected at an early stage the MGW-id can be provided to the succeeding node in the IAM.
13.4.4.2.5 Establishment of bearer towards the forwarded-to subscriber
The bearer establishment towards the forwarded-to subscriber is performed as described for 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.
13.4.4.2.6 Confirmation of bearer establishment
If the outgoing IAM indicated that a Continuity message will follow, the Continuity message shall be sent when both of the following conditions are satisfied:
1. Either:
a. The incoming IAM indicated that the Continuity message will follow, and a Continuity message has been received, or
b. The incoming IAM indicated that no Continuity message will follow.
2. Either:
a. The MSC server has selected an MGW, and a notification indicating successful completion of the incoming side bearer set-up has been received from the MGW using the Bearer Established procedure, or
b. MGW selection is not required for this call.
13.4.4.2.7 Failure handling in MSC server
The failures are handled as described for the basic mobile originating call.
13.4.4.2.8 Example
Figure 13.11 shows the network model for call forwarding on not reachable. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling and the bearer. The MSC server seizes one context with two bearer terminations in the MGW. The bearer termination TA is used for the bearer towards the preceding MGW (calling subscriber) and the bearer termination TC is used for the bearer towards the succeeding MGW (forwarded-to subscriber).
Figure 13.11: CFNRc; Rerouting by VLR (Network model)
Figure 13.12 shows the message sequence example for the call forwarding on not reachable with a possible announcement. In the example the MSC server optionally requests the MGW to play an announcement and to notify the announcement completion, after the bearer to the incoming side has been established. When the possible announcement has been completed the MSC server requests the establishment of the call towards the forward-to subscriber.
Figure 13.12: CFNRc (Rerouting by VLR) (message sequence chart)