With the proliferation of Voice over-IP telephone systems, SIP Trunking and VoIP networks, the issue of Fax transmission has become critical. The truth is that Fax over IP networks doesn’t work very well. Sometimes it seems to work OK, but Fax over IP networks can be very unreliable.
Unforgiven
Voice is a fairly forgiving medium and adapts pretty well to an IP environment. A voice conversation with small amounts of delay, small amounts of jitter or small amounts of packet loss will still be intelligible. But Fax is not at all forgiving. Small amounts of delay, jitter or packet loss can disconnect a Fax session or render the received image useless (e.g., by throwing a random page break in the middle of your document).
Seek Alternate Route
Voice over IP systems and networks are optimized for voice. Fax, if considered at all, is an afterthought. This is true for all equipment vendors and all network providers. So my basic suggestion is to seek an alternative to sending Faxes over an IP network. Use analog lines, transition to an IP Fax service or use scan-and-email. But, if you insist on sending Fax over IP, here are a few suggestions to improve your odds of success.
Codec Choice
The first issue is the bit rate of the codec being used for Fax transmission. Voice over IP systems will often use compression to minimize bandwidth required. These compression codecs will destroy a Fax signal. So, only send Faxes over links that will always use G.711. Or, if your system supports it, turn on G.711 pass-through for Fax calls.
Silence Suppression & Comfort Noise
Fax signals don’t like silence suppression and they don’t like so-called ‘comfort noise’. Most VoIP systems utilize one or both of these strategies. Silence suppression systems can create clipping around the points where they switch on and off. Comfort noise is just that – noise when there should be silence. Fax signals use silence to demarc parts of the transmission. Be sure you can turn both of these options off for your Fax Calls.
QoS for Fax Traffic
Latency (the time it takes a packet to reach its destination) and jitter (the variability in latency) are the enemies of your Fax signal. You must do everything you can to minimize both in order to increase your odds of a successful Fax transmission. Quality of Service (QoS) is the tool to use to prioritize your Fax traffic. But QoS is only effective where you control the network from end-to-end. As I have mentioned many times in this blog, make sure you have selected an appropriate WAN technologyto insure your QoS strategy is effective.
The Standard Stuff
T.38 is the Fax standard intended to address many of these issues. T.38 specifically looks at transmission of Fax signals over an IP network. T.38 incorporates things like Forward Error Correction (referred to as ECM or Error Correction Mode), data redundancy, retransmission and many other techniques to compensate for the vagueness of the IP network connection. There are scenarios where T.38 works very well. When a Fax is sent from one T.38 Fax machine to another, where every node along the intervening IP network understands and correctly implements T.38 and your QoS, results can be excellent. If you are in this situation, congratulations; your Fax-over-IP problems are solved. But for the rest of us who live in the real world, T.38 is often not a complete solution.
Through the Gateway
A more realistic scenario is where a T.30 Fax device sends a Fax (over the PSTN or through a local PBX) to a T.38 Fax gateway. The gateway then converts or encapsulates the T.30 signal into a T.38 data stream. This is then sent either to a T.38 enabled Fax machine or another T.38 Gateway that converts it back to a PSTN (PCM or analog) signal and terminates the Fax on another T.30 device. But T.38 has many weaknesses. There are several flavors of T.38. A TCP version and a UDP version. One that uses standard RTP protocol, one that uses its own UDPTL protocol. And the T.38 spec is quite ambiguous in some areas, leaving it open to the interpretation of each vendor implementing it. Many, many Fax machines are out there that do not speak T.38 at all and gateways are not uniform in their treatment of T.38 traffic. So, if you want reliable, predictable Fax service, I come back to my original recommendation: Seek an alternative to sending Faxes over an IP network.
How are you handling Fax over your IP network? Do you use T.38 compliant Fax machines? How reliable is your solution? Share your thoughts in the comments.
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