13.4.2 Call Forwarding on mobile subscriber Busy (CFB)

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The procedures specified in 3GPP TS 23.082 [12] for the Call Forwarding on Busy supplementary service shall be followed. The following clauses describe the additional requirements for the bearer independent CS core network.

13.4.2.1 Network Determined User Busy (NDUB)

If the mobile is Network Determined User Busy the incoming call for the specified basic service(s) shall be forwarded without being offered to the served mobile subscriber.

13.4.2.1.1 MGW selection

The MSC server shall select an MGW for the bearer connection 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, then it may use one of them for the 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 there is no need for the MSC server to manipulate the bearer, the MSC server may only perform call control signalling without any associated MGW. In that case the bearer related information shall be provided transparently through the MSC server.

13.4.2.1.2 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 NDUB condition.

13.4.2.1.3 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.

13.4.2.1.4 Initial addressing

The call towards the forwarded-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 [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.

13.4.2.1.5 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.

13.4.2.1.5 Confirmation of bearer establishment

If the outgoing IAM indicated that the Continuity message will follow, the Continuity message is 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.

13.4.2.1.6 Failure handling in MSC server

The failures are handled as described for the basic mobile originating call.

13.4.2.1.7 Example

Figure 13.5 shows the network model for call forwarding busy (network determined user busy). 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 T_A is used for the bearer towards the preceding MGW (calling subscriber) and the bearer termination T_C is used for the bearer towards the succeeding MGW (forwarded-to subscriber).

Figure 13.5: CFB; NDUB (Network model)

Figure 13.6 shows the message sequence example for the call forwarding busy (network determined user busy) using 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 13.6: CFB (NDUB)

13.4.2.2 User Determined User Busy (UDUB)

13.4.2.2.1 MGW selection

The MGW selected for the mobile terminating call is used, if already selected by the mobile terminating call procedures.

The MSC server selects an MGW for the bearer either before sending the IAM of after receiving the Bearer Information message. If the MSC server received an MGW-id from the preceding node and/or from the succeeding node, then it may use one of them for the MGW selection.

13.4.2.2.2 Incoming side bearer establishment

For bearer establishment, the sending of bearer information is handled in the MSC server as described for the basic mobile terminating call indicating either forward or backward bearer establishment. The incoming side bearer establishment can take place either before or after the detection of UDUB condition.

13.4.2.2.3 IU/A-interface release

If the mobile is not Network Determined User Busy (NDUB as defined in GSM TS 02.01) the incoming call is offered (as a normal or waiting call) to the served mobile subscriber. If the mobile indicating "User Busy" subsequently releases the call, the call towards the served mobile subscriber is released as described in the clause for call clearing.

NOTE: The MSC server orders the MGW to remove the bearer termination towards the served mobile subscriber only in the case where the radio resources had already been allocated in the MGW (bullet 1 in figure 13.8).

13.4.2.2.4 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.

13.4.2.2.5 Initial addressing

The call towards the forwarded-to subscriber is established as 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.

13.4.2.2.6 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.

13.4.2.2.7 Confirmation of bearer establishment

If the outgoing IAM indicated that the Continuity message will follow, the Continuity message is 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 from the preceding node (bullet 8 in figure 6.6), 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 of successful bearer establishment in the incoming side has been received from the MGW (bullet 7 in figure 6.6), or

b. MGW selection is not required for this call.

13.4.2.2.8 Failure handling in MSC server

The failures are handled as described for the basic mobile originating call.

13.4.2.2.9 Example

Figure 13.7 shows the network model for call forwarding busy (user determined user busy). 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 signalling. The MSC server replaces the bearer termination for the served mobile subscriber (T_B) with the bearer termination for the forwarded-to subscriber (T_C) in an existing context in the MGW. The bearer termination T_A is used for the bearer towards the preceding MGW (calling subscriber).

Figure 13.7: CFB; UDUB (Network model)

Figure 13.8 shows the message sequence example for the call forwarding busy (user determined user busy) with a possible notification to the calling party using an announcement. In the example, after the call and the bearer towards the access have been released the MSC server requests the MGW to remove the bearer termination for the served mobile subscriber, optionally requests the MGW to play an announcement and to notify the announcement completion. After the possible announcement has been completed the MSC server requests the establishment of the call towards the forward-to subscriber.

Figure 13.8: CFB (UDUB)