SIP stands for Session Initiation Protocol
In both traditional and IP telephony, the network
engineers always have made a clear difference between the 2 different stages of a voice call.
Call Setup Protocol
The first stage of a voice call is ‘call setup’ and it has all the details which are required to get two telephones talking. After the call has been setup, the phone then enters a second stage called ‘data transfer’ by using a completely different family of protocols to really move the voice packets among the phones. In VoIP, SIP is a call setup protocol which operates at the application layer..
Multiple Multimedia Sessions
SIP is an extremely flexible protocol which has a great depth. SIP was designed to act as a general purpose way to set up a real time multimedia sessions among the groups of participants. For instance, other than the simple telephone calls, SIP also can be used to set up a video and audio multicast meetings, or instant messaging conferences.
Another SIP Feature – Text Based Protocol
Another great thing about SIP is that it is a text based protocol which is modeled on the
request/response model used in HTTP.
This makes it a lot easier to debug as the messages are very easy to build (for a developer) and easy to see (for a network manager).
In comparison to the H.323, a SIP is an extremely simple protocol. Nonetheless, it is also has enough powerful features to model the behaviour of a very intricate conventional telephone PBX.
IPv4 and IPv6
A SIP also has the ability to run over IPv4 and IPv6 and it can use TCP or UDP, though the most common implementations use IPv4 and UDP. This reduces overhead, this speeding the performance.
Even though two SIP devices can talk directly to each other, they usually will employ an intermediary system which then acts as the SIP proxy. It is worth noting that SIP proxy just participates in the SIP messages.
Once the call is setup, the phone then sends their voice traffic directly to each other without having to involve the proxy. The SIP proxy can be very helpful in order to offload tasks and to simplify implementation of end station telephones.
For instance, a SIP phone wants to make a call to another phone at the extension of 4094. Even though the phone cannot have a way of translating that 4094 into an IP address or location, it usually just sends its call request to its own SIP proxy.