SIP stands for Session Initiation Protocol, and is one of many ways to do Voice over the Internet Protocol, but its applications reach far beyond mere Voice. SIP is to real-time communications what e-mail is to message delivery, and is slated to power the next major revolution in ways we humans communicate … in real-time. on the Internet. For free. Or Cheap. Want to start playing? Start here.
Skype, while very powerful, remains a closed ecosystem that relies on everybody using the same software which complies to a closed protocol, within a system owned and operated by one entity who calls all the shots.
SIP is an open protocol, and Voice over IP through SIP relies on a set of other open protocols and standards, all of which work hand in hand to make real-time communications work. Anybody today can become an e-mail provider, write their own e-mail server software, or e-mail user program. The same holds true for real-time communications through SIP.
What are real-time communications? Voice. Text. Video Conferencing. iChat uses SIP for Voice and Video. While text still goes through AOL’s closed protocol.
It’s been around for a good 10 years, but hasn’t had a chance to make it onto the mainstream until a few technical challenges could be overcome, some of the more prominent ones being Network Address Translation and higher bandwidth. The STUN protocol which came out in 2003, provides developers with a standard way to circumvent Network Address Translation issues.
Our ducks are now in a row. We’re ready. How does it work?
Your “SIP program” registers its online presence with a “SIP Proxy”: “Hey, i’m on my home network right now, and I can be reached at this IP address”. When you arrive at work, your SIP program will now say “Yoohoo, i’ve moved, i’m now here!”.
If someone wants to call you, they’ll type your SIP address in their SIP program. The SIP provider will help this person’s SIP software get in touch with your computer‘s SIP software, partly thanks to some STUN magic thrown in the middle. A SIP address looks exactly like an e-mail address, and, with some providers such as EarthLink, can very-well be one and the same. In my case, you can send me an e-mail at firstname.lastname@example.org or plug email@example.com ( or sip:firstname.lastname@example.org ) in your SIP program to call me up. If i’m not online or available, you’ll hear my voicemail, which will then be delivered as a .wav attachment to my e-mail address … which Mail.app plays inline just fine!
You don’t even need a “SIP Provider” to do SIP. If you know your party’s IP address or host name, if their SIP software is properly configured, you can plug their IP address into your SIP program to give them a ring.
Having a SIP address just gives you a more universal way for people to get in touch with you.