10.1 Overview
29.1623GPPInterworking between the IM CN subsystem and IP networksRelease 17TS
10.1.1 General
The present specification describes Ix signalling procedures and their interaction with SIP signalling in the control plane, and with user plane procedures. Each scenario or "use case" is described in a separate subclause within subclause 10.2. 3GPP TS 29.238 [25] maps these signalling procedures to H.248 messages and defines the required H.248 profile (which provides details of used packages and parameters).
10.1.2 Network model
Figure 10.1.2.1 shows the network model. The broken line represents the call control signalling. The dotted line represents the user plane. The IBCF uses one context with two terminations in the TrGW.
Figure 10.1.2.1: Network model
10.1.3 Example Call Flow
10.1.3.1 Basic Procedures
10.1.3.1.1 Call Establishment
Figure 10.1.3.1.1.1 depicts the signalling flow for a call setup either from or toward an external network.
1. The IBCF receives an SDP offer in SIP signalling.
2. The IBCF detects that one of the CS-TrGW functions is required, e.g. NAPT/NAT.
3. The IBCF sends a H.248 ADD command to create the outgoing termination and to request resources to execute TrGW function.
4. The TrGW creates the outgoing termination.
5. The TrGW replies to IBCF with a H.248 Add reply command and provides the local address and port of the outgoing termination.
6. The IBCF replaces the IP address inside the SDP using the information coming from TrGW.
7. SDP offer is sent to the network at the outgoing side.
8. SDP answer is received by IBCF.
9. The IBCF sends a H.248 MOD command to configure the outgoing termination with address and port information received in the SDP answer.
10. The TrGW configures the outgoing termination.
11. The TrGW replies to IBCF with a H.248 MOD reply command.
12. The IBCF sends a H.248 ADD command to create the incoming termination and to request resources to execute TrGW function.
13. The TrGW creates the incoming termination.
14. The TrGW replies to the IBCF with a H.248 Add reply command and provides the local address and port of the incoming termination.
NOTE 1: Steps 12 to 14 may also be executed after step 2.
15. The IBCF replaces the IP address inside the SDP using the information coming from TrGW.
16. SDP answer is sent to the network at the incoming side.
Figure 10.1.3.1.1.1: IBCF and TrGW interaction at Call establishment.
When creating the termination towards the IMS network or towards external networks, the IBCF may also indicate that the IP Interface Type is "MboIP".
NOTE : Other values may be indicated by a CS-IBCF, as detailed in 3GPP TS 29.235 [29].
The IP Interface Type allows the TrGW to collect statistics per interface type associated with the RTP bearer termination. The provision of these statistics is outside of the scope of this specification.
10.1.3.1.2 Call Release
Figure 10.1.3.1.2.1 depicts the signalling flow for a call release.
1. The IBCF identifies that the call is to be release. Typically this will be by the receipt of a SIP BYE request.
2. The IBCF sends a H.248 SUB command to release the outgoing termination.
3. The TrGW destroys the outgoing termination.
4. The TrGW replies to IBCF with a H.248 SUB reply command.
5. The IBCF sends a H.248 SUB command to release the incoming termination.
6. The TrGW destroys the incoming termination.
7. The TrGW replies to IBCF with a H.248 SUB reply command.
NOTE 1: Steps 5 to 7 may also be executed before steps 2 to 4 or in parallel with steps 2 to 4.
NOTE 2: Rather than releasing the two terminations separately, the IBCF may request the TrGW to release both terminations in a single request.
Figure 10.1.3.1.2.1: IBCF and TrGW interaction at Call release