CAN YOU FAX THAT TO ME NOW?
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,
Ensure adequate bandwidth on all link segments that will carry voice or fax.
Prioritize fax and voice with class of service configurations in LAN switches.
Obtain priority service classes from WAN carriers with Service Level Agreements.
Select queuing algorithms that always put voice and fax first.
Set a largish and fixed-size jitter buffer at the receive end to absorb differences in clock frequencies between send and receive gateways.
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 popular pulse code modulation (PCM, G.711) was part of the legacy network in which fax originated. PCM should work at standard speed (pages per minute) if the packet loss is below 1%. Bandwidth needed will be the 64 kbit/s of PCM plus the several headers to encapsulate image data into packets. With IPv6 headers a PCM connection will require over 100 kbit/s.
Voice compression with another codec that requires only 32, 16, or 8 kbit/s will sound OK for speech but will not reproduce the phase shifts in the tones of faster modems. The fax transmission will slow to a simpler modem format (taking longer per page). The greatest level of compression seldom allows a fax connection to set up at all.
Fax relay (T.38) decodes the modem sounds from the sending fax machine to the binary image data (and signaling information) which are encapsulated and sent as IP. At the receiving end a modem in the fax gateway regenerates audio tones, from the data in the packet payloads, which are played back to the fax machine.
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.