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The E-CSCF and EATF may reside within the P-CSCF to support Emergency and related SRVCC procedures. Sonus 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.

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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 Sonus 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.

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

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