SBC as P-CSCF co-located with ATCF and E-CSCF

The ATCF is generally expected to be co-located with an existing functional element in the IMS architecture, such as the P-CSCF, IBCF or MSC server. The most likely option for ATCF co-location is the P-CSCF, since it is already in the path of all UE signalling with IMS. The IBCF is only in the path for a session when the UE is roaming, and the MSC server is only involved in a session during and after SRVCC. Thus the IBCF and MSC server are not co-located with the ATCF. The E-CSCF may reside within the P-CSCF to support IMS Emergency procedures.

ATCF and E-CSCF integrated with P-CSCF :  In this case IMS-AGW would act as ATGW (and also since the SBC5xx0 acts as integrated P-CSCF and IMS-AGW, Ribbon ATCF/ATGW solution would also be integrated in same box i.e ATGW interface is NOT exposed externally).

E-CSCF is responsible for routing emergency call requests to the nearest PSAP based on the callers location information, as well as other information, such as the type of emergency service being requested.  E-CSCF receives from the P-CSCF request for emergency session establishment, and, if required, contacts the LRF to validate the location information given by the UE and routes the request to the appropriate destination.

Figure 1: SBC as P-CSCF along with co-located ATCF and E-CSCF

At a high level, the functions performed by the ATCF are:

• Allocate a STN-SR.
• Include itself for the SIP sessions.
• Instruct the ATGW to anchor the media path for originating and terminating sessions.
• Keep track of sessions (either in pre-alerting state, alerting state, active or held) to be able to perform Access Transfer of the selected session.
• Perform the Access Transfer and update the ATGW with the new media path for the (CS) access leg, without requiring updating the remote leg.
• After Access Transfer, update the SCC AS that Access Transfer has taken place to ensure that T-ADS have the information on the currently used access.
• Handle failure cases during the Access Transfer.

At a high level, the functions performed by the E-CSCF are:

• Receive an emergency session establishment request from a P CSCF or an S-CSCF.
• If location information is not included in the emergency request or additional location information is required, the E CSCF may request the LRF to retrieve location information.
• If required, the E-CSCF requests the LRF to validate the location information if included by the UE.
• Determines or queries the LRF for the proper routing information/PSAP destination.
• Route emergency session establishment requests to an appropriate destination including anonymous session establishment requests.
• Subject to local regulation, the E-CSCF may send the contents of the P-asserted ID or UE identification to the LRF.
• Based on operator policy, the E-CSCF may route the emergency IMS call to ECS for further call process.
• For supporting SRVCC, the E-CSCF forwards the session establishment request to the EATF in the serving IMS network for anchoring.

For configuration details, see Configuring SBC as P-CSCF Co-located with ATCF and E-CSCF