The Session Initiation Protocol (SIP) is an application-layer
control protocol that can establish, modify, and terminate multimedia
sessions or calls. Like MGCP, SIP is text-based. SIP
came out of the Internet Engineering Task Force (IETF) in
1998 as an RFC. It has rapidly gained widespread support,
including Microsoft’s announcement that SIP will be supported
in the next generation Windows XP product.
SIP uses a “request-response” model like that used in
Hypertext Transfer Protocol (HTTP). There is one major difference
between MGCP and SIP—a call agent is not necessary
to mediate between clients. An SIP interface is shown in
Figure 2-19.
The usefulness of SIP for multimedia is almost limitless.
Sessions can be unicast or multicast and include multimedia conferences, distance learning, VoIP, or similar applications.
Some examples of multicast protocols include email, news
groups, Web pages, and the like. SIP also supports the ISDN
and Intelligent Network telephony subscriber services for personal
mobility, which is important for the Wireless Internet.
SIP is reliable, scalable, and can be used with other protocols.
Development is fast because it is very similar to HTTP,
thus making the addition of feature-rich applications very quick
to implement. Initially H.323 and MGCP may be the protocols
of choice for tomorrows’ media gateways and soft switches but
SIP and Megaco will be strong contenders as they mature.
No matter which protocol or protocols become the favorite,
soft switches will be flexible enough to adapt. This adaptability
makes network service providers very happy. Unlike old legacy
switches, this new breed of switches will be quick to accept
added features or changed services without waiting months for
a manufacturer to modify the switch design. 104