[VL2006] - 72: SIP's Expanding Functionality

Tue Jan 27 15:52:50 EST 2009

VL on VoIP

*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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