Session Initiation Protocol (SIP) is the core networking protocol used within the IP Multimedia Subsystem (IMS) and is one of the more widely known examples of a session control protocol. Session control refers to the process used to create, modify and terminate IP-based communication sessions, and a session can include two-way voice communication, multimedia (text, audio or video) conference collaboration, instant messaging, application sharing and other contemplated but not yet fully specified services. Session control is accomplished through signaling between various network elements and endpoints using a session control protocol.
Although SIP is the most widely known session control protocol, SIP has a major limitation that is of great importance to any GSM-UMTS operator. It does not provide any method of directly inter-working with the Public Switched Telephone Network (PSTN) because it was not created with the intention of it being fully backwards-compatible with legacy PSTN signaling mechanisms.
In addition to SIP, other examples of session control protocols include BICC, SIP-I and SIP-T.
BICC, or Bearer Independent Call Control, is the protocol standardized in the 3GPP Release 4 architecture and deployed in some networks today. BICC, however, is not an optimal choice for ongoing evolution because it has been limited to, and is predicted to remain limited to, operation within a GSM-UMTS context. BICC does not address domains beyond GSM-UMTS such as LTE; as a result, it does not automatically offer the future level of flexibility of continued development and evolution that would accompany the SIP with ISUP encapsulation variants (i.e. either SIP-T, SIP for Telephones or SIP-I, SIP with ISUP encapsulation).
With a technical analysis of capabilities existing within the two SIP technologies with ISUP encapsulation variants, 4G Americas recommends SIP-I as the direction for evolution. There are four areas where SIP-I is better suited for a GSM-UMTS environment than SIP-T: