Architecture of an SIP VoIP Call
Connecting a VoIP Call
If you're a business thinking of making the shift to an Internet-based communication system, you might be wondering as to how this is achieved. It may seem incredibly complex especially since it will also be integrating with the traditional PSTN phone system with each VoIP device having its own telephone number etc. In reality, the broad structure of voice over IP is very similar to how e-mail works. At least, that is the procedure for SIP to SIP communications. When the person at the other end is connected to a plain old telephone system, an extra wrinkle is added to the process.
The first step of course is to have an account with an SIP provider. There are several ways to do this. We have free SIP services as well as paid ones who provide the features that the business will be looking for including Direct Inward Dialing and others. Your SIP provider will give you an "SIP URI" that will be your address on the Internet for all VoIP communications. Much like an e-mail address. In fact, business users often have the same e-mail and SIP ID. This reduces confusion and streamlines communication.
The benefits of having an SIP URI is that it allows any other SIP client in the world to communicate with you for free over the Internet, no matter where they are. Just like e-mail does. It means that everyone in your company regardless of their geographical location has instant access to everyone else at no cost. This is made possible because your SIP provider knows which telephone numbers correspond to which SIP addresses. So when you dial-in a telephone number of someone in your company, the provider looks up their address in an internal database and makes a call over the Internet without resorting to the PSTN phone system at any step.
Interconnecting with Older Networks
So far, we know how VoIP works when both individuals are making use of an SIP client. But most of the time in the real world the other person who is either making the call or to you to whom you wish to speak will be utilizing the traditional PSTN phone system. How does VoIP work its magic in this case? Well, we have something known as a VoIP Gateway. The SIP service will carry your traffic as far as possible over the Internet and only drop down to the regular phone network when it has to. The benefit of this is that it avoids a large number of fees especially when the call crosses national borders. This is why VoIP calls and especially international ones are so inexpensive.
One thing to keep in mind is that one SIP provider does not know the telephone numbers handed out by another. This means that went wto SIP clients belonging to different services try and talk, the connection will go over the PSTN network and not the Internet. There are attempts being made to create a public database mapping all known VoIP telephone numbers with their SIP URI but a few large corporations are not playing ball as they benefit from their customers being locked in.