One address to rule them all

By Tim Dean

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The precursor to the modern day business card could be considered to be the trade cards used in England in the 17th century. They served not only as useful reminders of a business' services but often included a map to help you find their office or store. London lacked a formalised street numbering system up until the late 1880s, so a few landmarks would surely have helped in remembering the location of a particular shopfront in the sprawling alleys of London.

The business cards we use today have evolved a great deal compared to those original trade cards, although they still serve a similar purpose - to assist our memory. It was hard enough to remember a shop's address in London, but it's arguably even harder to remember all the contact details with which we're bombarded today.

Look at a modern day business card and you'll likely find at least six or seven discrete addresses or contact numbers. Most business cards will list a street address, phone number, fax number, mobile number, email address and web address. That's a lot to keep in mind.

The good news is that things are about to get a lot simpler, and business cards a lot less cluttered. And it's all because of SIP (Session Initiation Protocol).

More convergence
SIP is that final piece of the puzzle that will enable all our communications devices, such as our home and mobile phones, email, instant messaging and even future video phones, to operate as if they were all running over the one seamless network.

We've already covered a great deal of convergence technology on these pages, and boiled it all down to that humble wonder, IP (Internet Protocol). However, IP is only one piece of the puzzle. IP forms the backbone of convergence but it needs other protocols running over the top to actually enable applications to run. In other words, IP just lays down the road but you need other protocols to define the lanes across which bundles of data can steer.

This is where SIP steps in. SIP is a glue that sticks a whole range of other protocols and technologies together and helps them work seamlessly.

With SIP in full flight you can immediately see if the person you want to call is online or whether they're away. And if you do call, it will automatically be routed through to the most appropriate device, whether that's the phone on their desk, their mobile or through to voicemail if they're on another call. And best of all, you only have to remember one 'contact detail': their SIP address.

Slim and simple
As the name suggests, SIP is about initiating sessions. One of the most misunderstood facts about SIP is that is actually doesn't carry data, such as VoIP packets, itself. SIP is not a wrapper that goes around data, all it does is establish a connection between two end points and then lets another protocol, such as RTP (Real-time Transport Protocol) for VoIP, take over. In fact, the SIP data might even take an entirely different route across a network or the Internet than the actual data it's assisting.

SIP was also designed with our good old PSTN (Public Switched Telephone Network) in mind. When you make a phone call with your landline there are a few things you expect to hear. The first is a dial tone. Then when you dial a number, it will ring on the other end, and you'll hear that ring fed back to your handset. And if the person you're calling is already on the phone, you get a busy signal.

SIP aims to give us all these intuitive functions on all of our communications devices, but there's a big difference in the way it achieves these things. The PSTN network, using the current SS7 (Signalling System #7) system, has all the intelligence in the network itself. The end points, our telephones, are fairly dumb. This requires a significant amount of complexity and overhead - and cost - in the network.

Instead of taking this SS7-like approach, SIP aims to be as simple as possible and places all the intelligence in the end points. Given the computing power found in even entry-level mobile phones these days, this is not such a problem.

There is some intelligence in the network, although this is primarily focussed around tracking down users. Because a SIP address could be attached to multiple devices, and these could all be in different locations, the network needs to have a way of finding the correct end point.

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