VL on VoIP        Issue 86            Oct 2012                           
Return to Archive

Broken POTS:  Lost Faith in High Availability

by William Flanagan, Publisher


by William A. Flanagan, Publisher

Deployments of VoIP and unified communications aren't necessarily entirely unified. Most recent advice has been to keep a POTS line for each facsimile machine because fax over IP isn't reliable. Fax almost always works on the legacy PSTN because the modems in fax machines were optimized by design for an analog channel. If the path includes an IP segment, many things can go wrong. Here's why the odds for success of a fax over IP (FoIP) call are improving.

One appealing feature of facsimile transmission is that it happens in real time. The protocol procedures (defined in ITU T.30) compensate for well known impairments in voice lines: noise, dropouts, fading. Error checking and forward error correction cover short spikes of errors. Retransmission and dropping down to a lower modem speed overcome persistent noise and distortion.

Your next telephone system almost certainly will be VoIP, an environment far different from that of the original fax machines. The most sensitive impairment for IP is packet loss. Depending on the method chosen for a fax call,

        VoIP and Unified Communications              
                          Internet Telephony and the Future Voice Network          
Wiley Interscience published my sixth book early in 2012.  For the list of previous books see the Publications page on our web site.  Together, those five titles have sold over 70,000 copies.  Hope you like this one too.  Order from Wiley.com or your favorite technical bookstore.

a packet loss ratio of more than 1% could cause the call to drop. Fortunately that quality is within reach if the design for an IP network addresses packet loss aggressively. Current best practices include:

How much bandwidth a fax connection requires depends on how it is formatted on the IP network. The simplest technique is to treat fax as voice if the codec will carry modem signals.

The International Interconnection Forum For Services Over IP (i3 FORUM, www.i3forum.org) has researched fax over IP for several years.  The latest report on the topic (Technical Specification for Voice over IPX Service, Release 3.0, May 2012) spells out the bandwidth requirements for various codecs, packet accumulation times, and transmission methods.  It also offers guidelines for maximum latency and other parameters.  It has all the technical detail you could want.

I3Forum identified another cause for the failure of T.38 fax calls in the way fax gateways process certain SIP messages that result when the call originates as voice, which is standard.  Specifically, the IP gateway at the receiving fax machine will send a REINVITE to the sending end when it realizes the call is for fax (on hearing the calling fax's tone).  The REINVITE requests a change from the voice codec (usually PCM) to T.38.  If the sending gateway hears the answer tone from the called fax machine (on the initial PCM voice connection) before the REINVITE arrives, it will drop the PCM connection and attempt to set up a new IP connection for fax.  This move drops the voice call, which looks like a call failure to fax machines, so they hang up.

One of the participants in the I3F testing, Commetrex Corp., has applied for a patent on one solution: have the sending fax gateway refuse the REINVITE if it arrives after the answer tone. The calls stays with the PCM codec. They have put this feature into their software stack for fax.

There may be other solutions that work their way into fax/SIP protocol stacks so in the not distant future we could see much more reliable fax over IP.

Flanagan Consulting Can Help You

     We understand not only the technology of networks, but also
     the surrounding business processes:  procurement, bid
     preparation/analysis, statements of work, financial analysis,
     consensus building around a solution, and more.
     Find out more:  call +1.703.242.8381  or email Bill@Flanagan-Consulting.com

Flanagan Consulting Supports Litigation Professionals
   Several associates are experienced in analysis of patents, trademarks,
   contracts, and other intellectual property related to IT and communications.
   We have assisted attorneys preparing claims, depositions, and testimony.
   Queries to +1.703.242.8381.

Advertise Here...
  ...to reach over two thousand interesting people in Telecom and IT.
  For details, call the Publisher at +1.703.855.0191

Responses to ViewsLetter and Subscriptions
    Mail is welcome when addressed to publisher@viewsletter.com.
Because of the number of 'out of office' autoreplies, replies to
this message are discarded.

Special thanks for supporting ViewsLetter to www.Webtorials.com,
your best source for communications tutorials and white papers.


Flanagan Consulting

Creative Network Solutions
From Desktop to Data Center

3800 Concorde Parkway, Suite 1500, Chantilly, Virginia USA
Ph:  +1.703.242.8381    Fx:  +1.703.242.8391

Flanagan Consulting is a Service Mark of W. A. Flanagan, Inc.

"Beware of false knowledge; it is more dangerous than ignorance."
                                       --George Bernard Shaw