VITE SIP INVITE SIP INVITE SIP INVITE SIP 200 OK SIP 200 OK SIP 200 OK SIP 200 OK SIP 200 OK SIP ACK SIP ACK SIP ACK SIP ACK SIP ACK Figure 9.17. Session establishment for service continuity emergency call 272 VoLTE and ViLTE 9.2.2.6.2. Access transfer for an emergency call The procedure of access transfer for an emergency call is described in Figure 9.18. P-CSCF I-CSCF E-CSCF Server PS-CS inter-system handover SIP INVITE (URI E-STN-SR, SDP MSC GW) SIP reINVITE (URI PSAP, SDP MSC GW) SIP 200 OK SIP 200 OK SIP 200 OK SIP 200 OK SIP ACK SIP ACK SIP ACK SIP ACK SIP BYE SIP BYE SIP BYE SIP 200 OK SIP 200 OK SIP 200 OK Figure 9.18. Access transfer for an emergency call 1) The MSC server entity sends the SIP INVITE request to the EATF entity. The URI identity of the query contains the E-STN-SR number of the EATF entity, configured at the MSC server entity. The body of the SDP message contains the characteristics of the RTP stream provided by the MSC GW entity. 2) The EATF entity transfers the SDP message received to the entity in charge of the interconnection with the emergency center, in a SIP reINVITE message. 3) The EATF entity issues the SIP BYE request to complete the initial dialogue in the P-CSCF entity, and possibly in Alice's UA entity if it has carried out for the SIP stream the PS-PS inter-system handover simultaneously with the PS-CS inter-system handover. Short Message Service 10.1. Message structure A short message service (SMS) over the SGsAP protocol allows for a mobile connected to the 4G network to send and receive an SMS in CS (Circuit-Switched) mode. A short message service over the session initiation protocol (SIP) is a supplementary telephone service offered by the IMS. The SMS message is generated and interpreted by the short message application layer (SM-AL) between the mobile and the SMS service center (SMS-SC) (Figures 10.1 and 10.2). The short message transport layer (SM-TL) ensures reliable transmission of SMS messages between the mobile and the SMS-SC entity (Figures 10.1 and 10.2). The sho