SIP and SIP-I: Session Initiation Protocol
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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:
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