SIP Standards Work Progressing;

What to Look For in Phone Features

By William Flanagan

Remember the PBX features war? Shortly after switch technology converted from analog to digital, PBX vendors sought to one-up competitors by adding special features that made their products unique.  My personal favorite is the "Executive Barge-In Lockout Override."

We're now engaged in a digital technology shift, from Time Division Multiplexing (TDM) to packet voice (specifically, VoIP). Having the basics in place and standardized, vendors once again are going for unique product positioning with additional features, often tied to proprietary protocols (typically variants on standards) and hardware IP phones. Can you spell déjà vu all over again?

The first effort could probably be called a success.  Analog PBXs worked with most any phone that could produce dialing (pulse or tone)--a phone didn't need even that to receive calls, only a ringer.  [Aside: a "candlestick" phone from the 1920's will still work on an anlog line.]  Digital PBXs required a specific telephone to support the hundreds of feature they offered.  Some vendors went so far as to require different phones for different switches within their own product lines.

Now comes before you SIP (Session Initiation Protocol, but you can think of it also as the Set of Internet Protocols that includes RTP, RTCP, UDP, SDP, STUN, TURN, ICE, etc. as well as SIP itself), pleading to be the ultimate solution.  It's not quite there--yet. SIP is a relatively young technology (compared to the telephone, or even to the digital PBX)--it is still under development, adding new features all the time.  So far, however, the feature count isn't anywhere near the 400-500 on a mature PBX.

So what's a vendor to do?  Here's where we experience déjà vu.  We're in another features competition.

A very deliberate aspect of SIP is that it extends easily.  The message formats resemble HTTP, which many people know.  There is a simple mechanism for creating new messages and a requirement that devices simply ignore messages they don't understand.  This environment allows a vendor to add a feature to it's call control server and telephone set very quickly, perhaps in response to a single customer requirement.  Once again telephony vendors are adding custom features to be competitive.

It's not all the same this time.  Since digital PBXs appeared, standards have become much more important to customers.  So these new features are being standardized.  The process to create a standard is slower than creating the feature, but it is happening. Last year there were over 50 projects to add SIP extensions specific to media authorization, privacy, presence, and so on.

Eventually, most of these extensions should become standards, for example an RFC published by the IETF.  Some will remain proprietary, either because only one vendor wants them or a vendor limits distribution via patent protection or trade secret treatment.

For buyers of IP telephony systems, the situation requires a choice between:

--the smaller set of standardized features supported by all vendors, which allows a certain amount of mixing telephone hardware and call-control software; and

--an extended SIP set of a particular vendor that includes a feature the customer really wants, and which also may lock that customer into that vendor's product.

Customers of digital PBXs accepted the lock-in to a particular digital phone--didn't have much choice.  Today there seems to be at least some resistance to a similar lock-in for IP phones because new ideas and products that embody them are coming from all sides, not just the main vendors.  Cutting yourself off from even part of the stream of innovation doesn't have a strong appeal in a rapidly developing market.

There's tons of details on the Web. A simple search for SIP extensions found over 4 million links. If you'd like some help to sort through it, call us.

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