SIP: a skill for the future

Linking the Internet and conventional telephony, SIP will be the key to 3G services, writes Nick Langley.

Linking the Internet and conventional telephony, SIP will be the key to 3G services, writes Nick Langley.

What is SIP?
Session Initiation Protocol (SIP) is used for setting up communications sessions - such as conferencing, telephony, whiteboarding and instant messaging - on the Internet. It bridges the gap between the Internet and conventional telephony.

Commercial SIP-based products and services already include IP (Internet Protocol) phones, PC clients, SIP servers and IP telephony gateways. It has huge potential for use in third-generation (3G) wireless networks, in mobile applications, and in providing the essential infrastructure for Internet telephony, including quality of service and security. Multimedia applications for SIP include spontaneous video calling and picture caller ID. SIP is gradually replacing an earlier protocol, H.323.

Where did it originate?
SIP was developed by the Internet Engineering Task Force (IETF), which also created the Web protocol HTTP.

What's it for?
At a recent comms show in the US, SIP was demonstrated at work in softswitches and softswitch-compliant media servers, "presence" products (which detect whether people you know are online at the same time as you) for mobile and fixed networks, IP gateways, desktop phones and application servers.

What makes it special?
SIP is a standard protocol which addresses the needs of IP telephony from a wholly Internet-based perspective, rather than evolving from the hierarchical and deterministic world of legacy telephony systems. It has the potential to break up the traditional private branch exchange-style phone system into a number of commoditised, easily obtainable components that can be mixed and matched.

This means customers will no longer be locked into the very expensive value chains of conventional telephony. This could be further bad news for already beleaguered telecoms equipment suppliers - although some of the leading suppliers are in the vanguard of SIP development.

How difficult is it?
SIP itself is an extremely simply protocol, but its applications can be complex. See, for example, the IETF paper An Application Server Component Architecture for SIP at

Where is it used?
Initially SIP is being used to provide IP telephony services which parallel those available in conventional telephony. Network providers Level 3 and WorldCom both offer SIP-based voice over IP services. But more revolutionary SIP infrastructures and applications are being developed.

Not to be confused with
The Society of Internet Professionals; the Stop Internet Plagiarism campaign.

What does it run on?
The leading SIP suppliers include Convedia, Hotsip, Lucent, Mitel, Nortel, Pingtel and Radvision. BT is using it for instant messaging. Others that have signed up include Nokia and Deutsche Telecom.

AOL Time Warner has adopted SIP for the Instant Messaging and Presence Leverage (Simple) messaging protocol.

Perhaps the most significant boost for SIP comes from Microsoft, which has included a SIP stack in Windows XP and plans to make SIP a main plank of its .net technology.

What's coming up?
SIP will be embedded in 3G phone handsets and personal digital assistants.

In time, a SIP-based infrastructure may be used for both wireless and wireline phones on public networks.

SIP training is available from conventional and Internet telephony equipment suppliers and independent training organisations such as Learning Tree (

The jobs market
"With this sort of momentum behind it, demand for SIP knowledge will increase as the telecommunications industry emerges from recession, and related skills will inevitably be in short supply," says E-Skills UK. Few jobs request SIP as yet, and with the continued layoffs and recruitment freezes in the comms sector, investing in this skill may require faith.

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