SBC as Standalone EATF

The E-CSCF and EATF may reside within the P-CSCF to support Emergency and related SRVCC procedures. Ribbon provides the P-CSCF, E-CSCF and EATF functions on the SBC and partner with IMS core and LRF or ECS vendors for complete solution.

Emergency Session Transfer Number for SRVCC (E-STN-SR): A number used in the session transfer procedure for emergency calls, pointing toward the serving (visited if roaming) IMS EATF which handles the IMS emergency session transfer from PS to CS access. The same E-STN-SR is used for all the emergency session access transfers within one PLMN.

As per standards:

Either E-STN-SR is configured locally in MME and transferred to MSC or MME sends an Emergency indication to MSC and lets MSC to utilize its local configured E-STN-SR. This means that in an IMS core network we need to have a single EATF running which is serving that PLMN. So that all the MSC servers in that region are going to point to same EATF for PS to CS handover.

Which means that Ribbon supports:

• SBC as P-CSCF, E-CSCF and EATF combined where EATF interface is exposed to the external E-CSCF. 
  
  OR
  
  
• SBC as P-CSCF and E-CSCF combined talking to an external SBC as EATF.

Figure 1: EATF Deployment

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

• It is a function in the serving (visited if roaming) IMS network.
• Providing the procedures for IMS emergency session anchoring and PS to CS Access Transfer.
• Acts as a routing B2BUA which invokes third party call control (3pcc) to enable Access Transfer.
• Performs the session continuity when the Access Transfer request indicated by the E-STN-SR is received.

For configuration details, see Configuring SBC as Standalone EATF