Content for  TS 23.231  Word version:  18.0.0

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 shall happen before sending the initial INVITE message.

The GMSC server shall indicate in the initial INVITE message that the precondition has not been met if either of the following conditions is satisfied before sending the initial INVITE message:

1. The incoming INVITE(IAM) indicated that remote preconditions had not been met, and no new SDP has been received indicating that preconditions have been met.
2. The incoming side RTP connection point has not been successfully reserved and configured in the MGW.

The GMSC shall provide the supported SDP (e.g. core network side user plane IP transport address and port, codec(s), RTP telephony event) to the succeeding node in the initial INVITE message.

The GMSC server shall select the codec for the outgoing side bearer connection before it performs the Reserve RTP Connection Point procedure. If OoBTC is supported, the selected codec may be the preferred codec for this connection. After the succeeding node has provided a user plane IP transport address and port the GMSC server shall use the Configure RTP Connection Point procedure to request the MGW to configure the remote address and any additional negotiated payload types (e.g. RTP Telephony Event) of the bearer termination. If OoBTC is supported, the Configure RTP Connection Point procedure may amend the selected codec for this connection if different from the codec sent in the previous Reserve RTP Connection Point procedure. Both the Reserve RTP Connection Point procedure and the Configure RTP Connection Point procedure may indicate that the IP interface type is for Nb over IP with SIP-I based Nc.

The GMSC server shall request the MGW to prepare for the incoming side bearer establishment using the Reserve and Configure RTP Connection Point procedure. The GMSC server shall select the codec for the incoming side bearer connection before it performs the Reserve and Configure RTP Connection Point procedure. Within this procedure, the GMSC server shall request the MGW to provide a local user plane IP address and UDP port and may indicate that the IP interface type is for Nb over IP with SIP-I based Nc. The GMSC server shall also provide the MGW with the selected codec, the remote user plane IP address and port information that was received from the preceding node in the INVITE message and any additional payload types (e.g. RTP Telephony Event). The GMSC server shall include in the SDP answer to the preceding node: the user plane IP address and UDP port received from the MGW, the selected codec and any additional accepted payload types.

In combination with the Reserve RTP Connection Point or Configure RTP Connection Point procedures and with the Reserve and Configure RTP Connection Point procedure, the GMSC server should use the Change Through-Connection procedure to request the MGW to both-way through-connect the bearer termination.

The core network side bearer is established if the following conditions are satisfied:

1. Either:
   1. The incoming INVITE(IAM) indicated that remote preconditions had not been met, and new SDP has been received indicating that preconditions have been met, or
   2. The incoming INVITE(IAM) did not include any preconditions information or indicated that remote preconditions had been met;
   and
2. Either:
   1. The incoming side RTP connection point has been successfully reserved and configured in the MGW, or
   2. MGW selection is not required for this call.

If the initial INVITE message which was sent to the succeeding node indicated that the precondition has not been met, the GMSC server shall send an UPDATE message indicating that the preconditions are met when the core network side bearer is established (bullet 8 in Figure 6.2.1.3.14.2-1).

A voice processing function (i.e. echo cancellation) located on the MGW may be used to achieve desired acoustic quality on the bearer terminations. The GMSC server shall request the activation of the 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 14 in Figure 6.2.1.3.14.2-2).

If any procedure between the GMSC server and the MGW has not completed successfully or the GMSC server receives a Bearer Released procedure from the MGW, the call shall be cleared as described in clause 7.3.2, GMSC server initiated call clearing or in clause 7.3.3, 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.

The Paging is defined in the clause 6.1.1.4 of TS 23.205.

The MSC Server shall send a SETUP message towards the UE. According to TS 23.108, the MSC server can either apply the early access bearer assignment procedure, i.e. the MSC does not include the signal IE in the SETUP to request that the user is alerted only after the setup of the access bearer as specified inTS 24.008), or the MSC server can apply the late access bearer assignment procedure, i.e. the MSC includes the signal IE in the SETUP to request that the user is immediately alerted. For basic call setup the MSC server should apply the early access bearer assignment procedure to ensure that the terminating UE is not alerted until the end to end bearer is established.

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.

The MSC server requests the MGW to prepare for the network side bearer establishment using the Reserve and Configure RTP Connection Point procedure (bullet 4 in Figure 6.2.1.3.14.2-1). The MSC server shall select the codec for the network side bearer connection before it performs the Reserve and Configure RTP Connection Point procedure. Within this procedure, the MSC server shall request the MGW to provide a local user plane IP address and UDP port and may indicate that the IP interface type is for Nb over IP with SIP-I based Nc. The MSC server shall also provide the MGW with the selected codec and with the remote user plane IP address and port information that was received from the preceding node in the SDP offer. The MSC server shall include in the SDP answer (e.g. 183 SESSION PROGRESS) to the preceding node: the user plane IP address and UDP port received from the MGW, the selected codec and any additional accepted payload types.

If the early access bearer assignment procedure is used (see TS 23.108), the access bearer assignment shall be started only when the core network bearer is established, as described by the following conditions.

1. Either:
   1. The incoming INVITE(IAM) indicated that remote preconditions had not been met, and new SDP has been received indicating that preconditions have been met, or
   2. The incoming INVITE(IAM) did not include any preconditions information or indicated that remote preconditions had been met;
   and
2. The incoming side RTP connection point has been successfully reserved and configured in the MGW.

The access bearer assignment is defined in the clause 6.2.1.2.5 of TS 23.205.

There is no specific framing protocol initialisation requirement in SIP-I based CS network. The MGW terminates the Iu Framing Protocol towards the Iu interface and will receive an IuUP Initialisation from an Iu-CS interface towards the radio interface and shall process it according to the procedures in TS 25.415 and TS 29.232. No information from the framing protocol initialisation needs to be interworked towards the Nb interface of a SIP-I based CS core network. Information within subsequent Iu UP payload PDUs and RTP PDUs shall be interworked according to the procedures in TS 29.414.

For a speech call, when the MSC server receives an Alerting message, it shall request the MGW to provide a ringing tone to the calling party using the Send Tone procedure (bullet 11 in Figure 6.2.1.3.14.2-1).

For a speech call, when the MSC server receives a Connect message, it shall request the MGW to stop providing the ringing tone to the calling party using the Stop Tone procedure (bullet 13 in Figure 6.2.1.3.14.2-2).

In combination with the Reserve and Configure RTP Connection Point procedure and with the Prepare Bearer procedure or Reserve Circuit procedure, the MSC server should 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 4, and bullet 9 or 10 in Figure 6.2.1.3.14.2-1). When the MSC server receives the Connect message, it shall request the MGW to both-way through-connect the bearer using the Change Through-Connection procedure (bullet 13 in Figure 6.2.1.3.14.2-2).

The MGW may use an interworking function that is based on the PLMN Bearer Capability, TS 24.008, 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 13 in Figure 6.2.1.3.14.2-2).

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

A voice processing function (i.e. echo cancellation) 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 13 in Figure 6.2.1.3.14.2-2).

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 7.2.4, visited MSC server initiated call clearing or in clause 7.2.5, 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.

Figure 6.2.1.3.14.1 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 6.2.1.3.14.2-1 shows the message sequence example for the basic mobile terminating call. In the example the GMSC server requests from MGWa the seizure of the outgoing side bearer termination before forwarding the INVITE to the MSC server. After the outgoing side bearer termination is seized the GMSC server sends the INVITE with supported SDP to the MSC server. The MSC server requests from MGWb the seizure of the network side bearer termination when Call Confirmed message is received from the UE. The MSC server sends the SDP answer (183 SESSION PROGRESS) to the GMSC. On receipt of the SDP answer, the GMSC requests from MGWa the through connection of the outgoing side bearer termination and the seizure of the incoming side bearer termination. The GMSC server sends the SDP answer (183 SESSION PROGRESS) to the external network. On receipt of the second SDP offer (UPDATE) from the external network, the GMSC server forwards this to the MSC server. The MSC server requests seizure of the access side bearer termination. For a speech call the MSC server requests MGWb to provide a ringing tone to the calling party at alerting. At answer the MSC server requests MGWb to both-way through-connect the bearer. For a speech call the MSC server requests MGWb to stop the ringing tone to the calling party at answer. When the MSC server receives an answer indication, for circuit switched data call it may request the activation of the interworking function in both bearer terminations. The (G)MSC server may request the activation of the voice processing functions for the bearer terminations.