6.1.2 Backward bearer establishment

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

The basic mobile originating call shall be established in accordance with 3GPP TS 23.108 [17]. The following clauses describe the additional requirements for the bearer independent CS core network. If out-of-band transcoder control is applied for a speech call, it shall be performed in accordance with 3GPP TS 23.153 [3].

6.1.2.1 MGW selection

The MSC server shall select an MGW for the bearer connection before it performs the access bearer assignment or the network side bearer establishment. This happens before sending the IAM (bullet 1 or 2 in figure 6.4). For GSM, if performing Service based handover (see 3GPP TS 48.008 [27]) the MSC Server may omit MGW selection at this time.

6.1.2.2 Network side bearer establishment

The MSC server shall select bearer characteristics for the network side bearer connection before sending the IAM. The MSC server requests the MGW to prepare for the network side bearer establishment using the Prepare Bearer procedure. The MSC server requests the MGW to provide a bearer address and a binding reference, and provides the MGW with the bearer characteristics (bullet 3 in figure 6.4). After the MGW has replied with the bearer address and the binding reference, the MSC server sends the IAM to the succeeding node.

6.1.2.3 Initial addressing

The MSC server shall indicate in the IAM that backward bearer establishment is to be used. If access bearer assignment has not been completed, the MSC server shall indicate that the Continuity message will follow. However, if late access bearer assignment (assignment after alerting or answer) is used the MSC server shall not indicate that the Continuity message will follow. The MSC server provides the bearer characteristics, the bearer address and the binding reference to the succeeding node in the IAM. The MSC server may also provide the MGW-id in the IAM (bullet 4 in figure 6.4).

6.1.2.4 Access bearer assignment

The MSC server shall select bearer characteristics for the access bearer.

For UMTS, before the MSC server starts the access bearer assignment, the MSC server requests the MGW to prepare for the access bearer establishment using the Prepare Bearer procedure. The MSC server requests the MGW to provide a bearer address and a binding reference, provides the MGW with the bearer characteristics and requests notification that the bearer can be modified. For speech calls, the MSC server shall provide the MGW with the speech coding information and conditionally GTT related information in accordance with 3GPP TS 23.226 [28] for the bearer. For a non-speech call the MSC server also provides the MGW with a PLMN Bearer Capability [4]. After the MGW has replied with the bearer address and the binding reference the MSC server requests access bearer assignment using the provided bearer address and binding reference (bullet 1 in figure 6.4) in accordance with 3GPP TS 25.413 [26]. The MSC shall only be notified by the MGW using the Bearer Modification Support procedure if the existing link characteristics of the access bearer can be modified at a later stage, see clause 13.18.1.

For GERAN Iu mode the MSC Server receives the GERAN capabilities within the RANAP INITIAL UE MESSAGE, indicating the services (e.g. for CS speech services the supported codec types and, for an adaptive codec type, the supported codec modes (for definition see [27])), which will be available at the RAB establishment procedure. The MSC server shall take the indicated GERAN capabilities into account as well as the received MS capabilities when negotiating a service. Additionally, when requesting the access bearer assignment the MSC server shall indicate to the GERAN the selected service (e.g. selected codec type). The MSC server shall not set codec information in the NAS Synchronisation Indicator (see [4]). Instead it shall set codec information in the GERAN BSC container.

For GSM, before the MSC server starts the access bearer assignment, the MSC server uses the Reserve Circuit procedure to seize a TDM circuit. For a non-speech call the MSC server also provides the MGW with a PLMN Bearer Capability [4] and a GSM channel coding. After the MGW has replied the TDM circuit seizure the MSC server requests access bearer assignment (bullet 2 in figure 6.4) in accordance with 3GPP TS 48.008 [27]. If performing Service based handover (see 3GPP TS 48.008 [27]) the MSC Server may omit to perform Reserve Circuit procedure.

6.1.2.5 Framing protocol initialisation

In 3GPP CS CN speech and data shall be carried using the Iu/Nb User Plane Protocol. The specification for the Iu UP protocol is defined in [20] and the Nb UP Protocol in [7] and [21]. The Iu/Nb UP Protocol is established through the CN in a forward direction. This is established independently of the bearer establishment direction. The MGW derives the forward direction from information sent by the MSC server within the Establish Bearer and Prepare Bearer procedures [6].

6.1.2.6 Confirmation of bearer establishment

If the IAM was sent to the succeeding node indicating that the Continuity message will follow, the MSC server sends the Continuity message when the access bearer assignment has been completed.

6.1.2.7 Through-Connection

During the Prepare Bearer or Reserve Circuit procedures, the MSC server will use the Change Through-Connection procedure to request the MGW to through-connect the bearer terminations so that the bearer will be backward through‑connected (bullet 1 or 2, and bullet 3 in figure 6.4).

For a multimedia call, the MSC may request the MGW to both-way through-connect the bearer using the Change Through-Connection procedure to generate a multimedia CAT (see clause 14.10.1).

Otherwise when the MSC server receives the answer indication, it requests the MGW to both-way through-connect the bearer using the Change Through-Connection procedure (bullet 5 in figure 6.4).

6.1.2.8 Interworking function

The MGW may use an interworking function that is based on the PLMN Bearer Capability [4] of the bearer termination. The activation of the possible interworking function in both bearer terminations will be requested by the MSC server at reception of the answer indication using the Activate Interworking Function procedure (bullet 5 in figure 6.4).

6.1.2.9 Codec handling

The MGW may include a speech transcoder based upon the speech coding information provided to each bearer termination.

6.1.2.10 Voice Processing function

A voice processing function located on the MGW may be used to achieve desired acoustic quality on the bearer terminations. The MSC server shall request the activation of the voice processing functions in the bearer terminations. For non-speech calls, the MSC server has the ability to instruct the MGW to disable the voice processing functions (bullet 5 in figure 6.4).

6.1.2.11 Failure handling in MSC server

If any procedure between the MSC server and the MGW has not completed successfully or the MSC server receives a Bearer Released procedure from the MGW, the call shall be cleared as described in clause 7.3, (G)MSC server initiated call clearing or in clause 7.4, MGW initiated call clearing. Alternatively, the MSC server may only release the resources in the MGW that caused the failure, possibly select a new MGW for the bearer connection and continue the call establishment using new resources in the selected MGW.

6.1.2.12 Example

Figure 6.3 shows the network model for the mobile originating call. The "squared" line represents the call control signalling. The "dotted" line represents the bearer control signalling (not applicable in A/Gb mode for the A-interface) and the bearer. The MSC server seizes one context with two bearer terminations in the MGW. The bearer termination T1 is used for the bearer towards the RNC/BSC and the bearer termination T2 is used for the bearer towards the succeeding MGW.

Figure 6.3: Basic Mobile Originating Call, Backward Bearer Establishment
(network model)

Figure 6.4 shows the message sequence chart example for the mobile originating call. In the example the MSC server requests seizure of the access side bearer termination and network side bearer termination. As the access bearer assignment has been completed before the IAM, no Continuity message will be sent. When the MSC server receives an answer indication, it requests the MGW to both-way through-connect the bearer terminations. The MSC server, shall also request the possible activation of the interworking function in both bearer terminations. The MSC server shall request the possible activation of the voice processing functions for the bearer terminations.

Figure 6.4/1: Basic Mobile Originating Call, Backward Bearer Establishment
(message sequence chart)

Figure 6.4/2: Basic Mobile Originating Call, Backward Bearer Establishment
(message sequence chart continue)