SBC can be deployed on the access edge as well as on the peering edge in an IMS/VoLTE network. On the access edge, the functional roles include: P-CSCF (IMS-AGW), E-CSCF, ATCF (ATGW) and EATF. On the peering edge the functioal roles include: IBCF (TrGW).
The Proxy Call Session Control Function (P-CSCF) is the first point of contact in an IMS network. It is responsible for security associations towards the UE. At the time of UE registration, it inserts itself into the path so that it is in the signaling path for all subsequent messages. P-CSCF determines whether the UE is roaming or not. For a non-roaming UE i.e. a UE in the home network, it forwards the registration messages towards the I-CSCF and directly contacts the S-CSCF for all subsequent messages. For a roaming UE, it selects a peering SBC (i.e. IBCF) and forwards the SIP messages (including registration, call-related and non-call-related) towards the same.
The following are the deployment scenarios where SBC acts as Access SBC for ATCF provisioning and EATF provisioning respectively: