After a few years as the subject of intense debate, research and development work in both the Internet and telecommunications communities, Session Initiation Protocol (SIP) is now starting to emerge into both public and enterprise networks. But what exactly is it, what applications will it support and, perhaps more importantly in today’s context, what will it mean for the future revenues of the communications industry?  

For a start, its rare for a communication protocol to receive the kind of attention that SIP is generating. For most of its history, telecommunications has fortunately kept its underlying complexity safely hidden from its users in direct contrast to IT. The signalling systems that deliver advanced voice services such as conferencing and ring back, or that support more advanced enterprise applications such as call centres, have remained a specialist domain. 

To exploit the possibilities of next generation, all packet fixed and wireless communications networks and tie these directly in with the applications that are now appearing on our desktops and mobile devices, such as web, multimedia, chat, presence, location and messaging services – it became clear to the experts in the late 1990s that a new approach was needed. The result – already dubbed by some as potentially capable of doing for next generation networks what HTML did for the internet – is SIP. Like many aspects of telecommunications, perceptions of SIP’s future role largely depend on where you’re standing in the industry. For the Voice over IP sector, it represents an opportunity to add value to the basic low cost connectivity benefits of the technology, growing functions, features and entirely new and easier ways of generating interactions with other services.

For Communications Service Providers it offers a chance to bridge the circuit and packet infrastructure divide, opening up the field of telecommunications service creation to a new community of developers who won’t need traditional telecoms expertise. As a result, it could generate new revenue streams and protect revenues in an increasingly competitive marketplace. 

For the end user – in both business and private life SIP holds the promise of bringing together at a high level the varied communications services and tools that we currently use, delivering appropriate services to the appropriate device and seamlessly integrating different media into an easily manageable, consistent and coherent whole.

So how does SIP work. One of the most important points about SIP is that it is ASCII, not binary-based, and is similar to HTTP or SMTP. While delivering some functions that are similar to H323, it carries far less message overhead and is an end-to-end, edge protocol – not a network-centric approach. One formal description positions SIP as ‘a control protocol for creating, modifying and terminating sessions with one or more participants, with sessions including multimedia conferences, Internet or other IP network telephone calls and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or via a combination of these. SIP supports session descriptions that allow participants to agree on a set of compatible media types and also supports user mobility by proxying and redirecting requests to the user’s current location. 

In practice, this translates into a potentially fascinating set of applications. Imagine publicly accessible ‘buddy lists’ that could help you book appointments with the doctor or plumber. How about being able to preview a film via your 3G handset and then order it to be sent to your home digital TV over broadband? How about setting up ad hoc voice and video conferences, irrespective of whether the other participants are on the PSTN, using VoIP-enabled PCs, WLAN linked PDAs or mobile phones? 

In an increasingly joined-up world, we need technological tools to do the joining up. SIP, so far, looks like being the right protocol, in the right place and at the right time for the next stage of industry growth.

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