There are many questions that come our way from customers who aren't quite sure how Voice over IP works and why they are experiencing the issues they are. Its actually not that complex, but at the same time, fault finding isn't a walk in the park. In this article we will attempt to explain SIP in a way that anyone can understand, and secondly we'll give some pointers for common issues.
SIP (Session Initiation Protocol)
You often hear people talking about SIP but very few of them actually know what it is, and how it works. SIP is simply a signalling protocol that is involved in setting up and tearing down voice and video calls. SIP came about as a standard to allow different systems to interconnect at a time when proprietary protocols were the norm. Let me take you through a basic call setup.
SIP INVITE - We're asking the system we want to communicate with IF there is a given number on that server and if it will accept a call from us and we tell them what we can talk. We get back a few different answers from, NO, BUSY, OK but you'll need to authenticate and YES.
SIP AUTH - This is where we provide a password hash to authenticate. If you have a softphone for example and you are trying to make an outgoing call, the server will undoubtably ask you to autheticate before placing the call.
SIP INVITE RESPONSE - If we've got the go ahead to connect, we now need to know what protocol we are going to use for the actual voice or video. We've already sent what we can talk and here the called system tells us what it can talk and gives us its choice. We don't absolutely have to select this choice but we can't just decide to use something else without first proposing that, let's assume we agree.
Now at this point, from the two INVITE messages we now know what protocol we're using for voice and optionally video, and we know the IP Address of the remote server for voice, and they know ours so all that's left is to setup the actual call and the role of SIP is done.