6.2.2 Backward bearer establishment

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

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

The basic mobile terminating call shall be established in accordance with 3GPP TS 23.108 [4]. 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].

6.2.2.1 GMSC server

6.2.2.1.1 MGW selection

The GMSC server shall select an MGW for the bearer connection before it performs the incoming side bearer establishment or the outgoing side bearer establishment. This happens before sending the IAM. If the GMSC server received an MGW-id from the preceding node, it may use this for the MGW selection (bullet 1 in figure 6.8).

NOTE 1: As an implementation option, if there is no need for the GMSC server to manipulate the bearer, the GMSC server may perform call control signalling without any associated MGW. In that case the bearer related information shall be provided transparently through the GMSC server.

6.2.2.1.2 Outgoing side bearer establishment

The GMSC server shall select bearer characteristics for the outgoing side bearer connection before it sends the IAM. The GMSC server requests the MGW to prepare for the outgoing side bearer establishment using the Prepare Bearer procedure. The GMSC 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 6.8). After the MGW has replied with the bearer address and the binding reference, the GMSC server sends the IAM to the succeeding node.

6.2.2.1.3 Initial addressing

The GMSC server shall indicate in the IAM that backward bearer establishment is to be used. The GMSC server shall also indicate in the IAM that the Continuity message will follow 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 GMSC server selected an MGW, but a notification of successful bearer establishment on the incoming side has not been received from the MGW.

The GMSC server shall provide the bearer characteristics to the succeeding node in the IAM. The MGW-id may also be provided in the IAM (bullet 4 in figure 6.8).

6.2.2.1.4 Incoming side bearer establishment

The GMSC server requests the MGW to establish a bearer to the given destination MGW and to notify when the bearer is established using the Establish Bearer procedure. The GMSC server provides the MGW with the bearer address, the binding reference and the bearer characteristics that were received from the preceding node in the IAM (bullet 1 in figure 6.8).

NOTE 2: The incoming side bearer establishment may take place either before or after HLR interrogation.

6.2.2.1.5 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]. The notification of bearer establishment shall not be sent until the Iu/Nb UP has been initialised

6.2.2.1.6 Through-Connection

During the Prepare Bearer and Establish Bearer procedures, the GMSC server will use the Change Through-Connection procedure to request the MGW to both-way through-connect the bearer termination (bullet 1 and bullet 3 in figure 6.8).

6.2.2.1.7 Confirmation of bearer establishment

If the IAM which was sent to the succeeding node indicated that the 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 from the preceding node, or

b. The incoming IAM did not indicate that the Continuity message will follow;

2. Either:

a. The GMSC server has selected an MGW, and a notification of successful bearer establishment in the incoming side has been received from the MGW (bullet 2 in figure 6.8), or

b. MGW selection is not required for this call.

6.2.2.1.8 Voice Processing function

A voice processing function located on the MGW may be used to achieve desired acoustic quality on the bearer terminations. The (G)MSC server shall request the activation of voice processing functions in the bearer terminations. For non-speech calls, the GMSC server has the ability to instruct the MGW to disable the voice processing functions (bullet 12 in figure 6.8). The voice activation request from the GMSC server to MGWa may be issued as soon as bullet 4 in figure 6.8, and may be issued as late as bullet 12 in figure 6.8 as illustrated.

6.2.2.1.9 Failure handling in GMSC server

If any procedure between the MSC server and the MGW is not completed successfully or the GMSC 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 GMSC 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.

6.2.2.2 MSC server

6.2.2.2.1 Paging

If the network side bearer establishment is delayed whilst the paging procedure is completed, the MSC server starts the Start_Bearer_Establishment timer when the paging procedure is started. The Start_Bearer_Establishment timer is stopped when the paging procedure is completed, or optionally when the Call Confirmed message is received in accordance with 3GPP TS 23.153 [3]. If the Start_Bearer_Establishment timer expires, the MSC server starts the network side bearer establishment.

6.2.2.2.2 Call setup

The MSC server indicates to the UE in the SETUP message that early access bearer assignment is used in order to establish the bearer end-to-end before the UE starts alerting. The MSC server indicates to the UE in the SETUP message that early access bearer assignment is used, if and only if, either of the following conditions are satisfied before sending the SETUP message (bullet 5 in figure 6.8):

1. If the IAM indicated that the Continuity message will follow, but no Continuity message has been received.

2. A notification of successful bearer establishment in the network side has not been received from the MGW.

6.2.2.2.3 MGW selection

The MSC server shall select an MGW for the bearer connection before it performs the network side bearer establishment or the access bearer assignment. This happens at latest after the UE has sent the Call Confirmed message. If the MSC server received an MGW-id from the preceding node, it may use this for the MGW selection (bullet 6 in figure 6.8). For GSM, if performing Service based handover (see 3GPP TS 48.008 [27]) the MSC Server may omit MGW selection at this time.

6.2.2.2.3 Network side bearer establishment

The MSC server requests the MGW to establish a bearer to the given destination MGW and to notify when the bearer is established using the Establish Bearer procedure. The MSC server provides the MGW with the bearer address, the binding reference and the bearer characteristics that were received from the preceding node in the IAM (bullet 6 in figure 6.8).

6.2.2.2.4 Access bearer assignment

The access bearer assignment shall be started only 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, or

b. The incoming IAM did not indicate that the Continuity message will follow;

2. A notification of successful bearer establishment in the network side has been received from the MGW (bullet 7 in figure 6.8).

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

For the access bearer assignment in UTRAN 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 the access bearer assignment using the provided bearer address and the binding reference (bullet 8 in figure 6.8) 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 9 in figure 6.8) 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.

6.2.2.2.5 Framing protocol initialisation

6.2.2.2.6 Called party alerting

For a speech call, when the MSC server receives an Alerting message, it requests the MGW to provide a ringing tone to the calling party using the Send Tone procedure (bullet 10 in figure 6.8).

NOTE: Other kind of tones may be provided to the calling party at an earlier stage of the call establishment.

6.2.2.2.7 Called party answer

For a speech call, when the MSC server receives a Connect message, it requests the MGW to stop providing the ringing tone to the calling party using the Stop Tone procedure (bullet 11 in figure 6.8).

6.2.2.2.8 Through-Connection

During any one of the Prepare Bearer, Reserve Circuit and Establish Bearer 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 not through-connected (bullet 6, and bullet 8 or 9 in figure 6.8).

When the MSC server receives the Connect message, it requests the MGW to both-way through-connect the bearer using the Change Through-Connection procedure (bullet 11 in figure 6.8).

6.2.2.2.9 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 Connect message using the Activate Interworking Function procedure (bullet 11 in figure 6.8).

6.2.2.2.10 Codec handling

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

6.2.2.2.11 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 11 in figure 6.8).

6.2.2.2.12 Failure handling in MSC server

If any procedure between the MSC server and the MGW is 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.

6.2.2.2.13 Example

Figure 6.7 shows the network model for the basic mobile terminating 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 MGWb. 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 GMSC server selected MGWa. The GMSC server seizes one context with two bearer terminations in MGWa. The bearer termination T3 is used for the bearer towards the MSC server selected MGWb and the bearer termination T4 is used for the bearer towards the preceding MGW.

Figure 6.7: Basic Mobile Terminating Call, Backward Bearer Establishment
(network model)

Figure 6.8 shows the message sequence example for the basic mobile terminating call. In the example the GMSC server requests seizure of the incoming side bearer termination and establishment of the bearer first. After a notification of incoming side bearer establishment has been received from the MGW, the GMSC server requests seizure of the outgoing side bearer termination. The MSC server requests seizure of the network side bearer termination and establishment of the bearer when the Call Confirmed message is received from the UE. After a notification of the network side bearer establishment has been received from the MGW the MSC server requests seizure of the access side bearer termination. For a speech call, When the MSC server receives an alerting message, it requests MGW to provide a ringing tone to the calling party. When the MSC server receives an answer indication, it requests MGW to both-way through-connect the bearer. For a speech, when the MSC server receives an answer indication, it requests MGW to stop the ringing tone to the calling party and requests the possible activation of the interworking function in both bearer terminations. The (G)MSC server shall request the possible activation of the voice processing functions for the bearer terminations.

Figure 6.8/1: Basic Mobile Terminating Call, Backward Bearer Establishment
(message sequence chart)

Figure 6.8/2: Basic Mobile Terminating Call, Backward Bearer Establishment
(message sequence chart continue)