SIP was designed to solve only a few problems and to work with a broad spectrum of existing and future IP telephony protocols. To this end SIP provides four basic functions. SIP allows for the establishment of user location (i.e. translating from a user’s name to their current network address). SIP provides for feature negotiation so that all of the participants in a session can agree on the features to be supported among them. SIP is a mechanism for call management – for example adding, dropping, or transferring participants. And finally SIP allows for changing features of a session while it is in progress. All of the other key functions are done with other protocols.
Yes this does indeed mean that SIP is not a session description protocol, and that SIP does not do conference control. SIP is not a resource reservation protocol and it has nothing to do with quality of service (QoS). SIP can work in a framework with other protocols to make sure these roles are played out – but SIP does not do them. SIP can function with SOAP, WSDL, HTTP, XML, VXML , SDP, UDDI and an few others. Everyone on of these is as important as the next.
There is no question that SIP was designed to be a modular component of a larger IP telephony solution and thus functions well with a large number of these IP related protocols. But SIP is even friendlier as it “plays nicely” with protocols that are often viewed as overlapping in function. For the near term we can expect that SIP will have to coexist with overlapping protocols such as H.323 and MGCP.
H.323 networks are already deployed in many parts of the world. Network operators are interested in growing network capability with coexisting SIP networks. SIP to H.323 translation products are already available. MGCP and MEGACO can also benefit from SIP as by themselves they aren’t enough to build a complete IP telephony system. These protocols sit architecturally below SIP and can benefit in functionality by in effect being controlled through SIP.
Clearly, SIP is an important protocol that is becoming widely deployed. SIP is a catalytic protocol that delivers key signalling elements, which can turn a voice over IP network into a true IP communications network – a network capable of delivering the next generation of converged services. SIP’s strength comes from doing what it does best, and that is integrating well with the rest of the other protocols in the converged protocol arena.
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