The present document contains a detailed description of the SCC AS restoration procedure to be used in the eSRVCC network specially. The purpose of the SCC AS restoration procedure, as described in the present document, is to ensure that the originating or terminating session is continued or resumed as soon as possible when some the SCC AS in the SCC AS pool is out of service. The document defines the SCC AS failure scenarios, possible solutions and solution analysis. For the related procedure and functionality of S-CSCF, SCC AS, ATCF and HSS in the IMS and eSRVCC environment, it is based on the requirement in 3GPP related specifications As the introduction of eSRVCC, the SCC AS shall exchange eSRVCC related parameters with ATCF and HSS during initial registration and re-registration procedures as described in TS 24.237, which is different from all other ASs.

1. ATCF reports its ATCF-Management-URI and ATCF-Path-URI towards the SCC AS, and the SCC AS stores it for later routing of MESSAGE.
2. SCC AS shall send MESSAGE to ATCF with its own ATU-STI based on ATCF-Management-URI received, in order to ensure the routing of eSRVCC procedure triggered by ATCF later.
3. SCC AS shall also update the received ATCF-Path-URI of ATCF towards HSS, HSS forwards it to MME, and stores it for subscriber downloading to SCC AS later.

The present document analysis SCC AS failure implications in eSRVCC network, it studies possible solutions and determines whether any should be standardize.