SIP: The Ultimate Unified Communications Protocol

Session Initialization Protocol (SIP) has clearly won as the leading IETF (internet engineering task force) standard which everyone is using to build Unified Communication applications. Now that the IETF has clearly defined this protocol and the way it is supposed to function, all software manufacturers using it can ensure that their applications will work with other vendor applications using the same standard.

But how did they get there?

The main reason software manufacturers are supporting SIP is the potential to incorporate future improvements being made by their engineers into the internet standards guided by the IETF. Of course, all vendors who have a chair on the IETF committee must all agree before an idea is accepted and implemented. This check and balance, allows protocol to adapt to future hardware and software improvements that will ultimately change the internet in the future. So this standards-based approach is critical.

Additionally, since SIP uses the internet infrastructure for its operation, it is very easily adaptable for Unified Messaging applications for small, medium, to large company environments but also home users. Some companies have already developed and rolled out their solutions for home owners like Vonage, Skype, as well as all the major cable companies now competing in this fast growing market – all due to the development of the SIP protocol.

Since it is quickly becoming the leading communications protocol used by all UC vendors, it is very important to understand how it works and what support it gives to current and future applications under development. First of all there three types of SIP servers which service SIP phones and they are proxy, registrar, and redirect.

  • Proxy – Acts as an intermediary between a SIP User Agent Client and SIP User Agent Server. In other words it forwards SIP requests which will generally modify it before sending it on.
  • Register – Receives a REGISTER request from SIP Agents which contain IP address and the SIP URI (Uniform Resource Indicator) of the User Agent trying to register.
  • Redirect Server – Receives a request from one user agent trying to communicate with another user agent whom has forwarded their communication requests to another location. Just like when you are setting your cell phone to forward all your calls to your home phone number, your cell phone provider would redirect the call to your home.

The user agent mentioned above is the actual phone device using SIP. A SIP phone device has all of the requirements within it to make and receive a call. To make a call, the phone or user agent will use the client process piece of software code to generate the call request. On the receiving user agent, the server process piece of software accepts the call request. So, UAC (user agent client) makes the call and the UAS (User agent server) receives the request.

If you want to really understand how this protocol functions; you would need to read the IETC working group’s request for comments (RFC) documents #3261 – 3265 (RFC3261 is the main one) that discuss SIP in detail. Now reading RFC’s is not for the novice – they can get pretty technical very quickly – since they are actually written by engineers for engineers. You may want to consider taking a Voice Over IP course where you’ll get detailed information in layman’s terms into the interworking of the protocol and be able to see it in action using leading industry gateways and SIP phone endpoints.

Author: Joe Parlas

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