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How Fax Works In A VoIP Environment — And Why It Can Fail To Work…

There are two primary difficulties in transmitting fax over an IP network or VoIP service.

The first problem:  Fax cannot be compressed so it must be digitized for transmission over IP as a full rate 64Kbps data stream.  That may not matter for occasional use, but it adds up in a high volume faxing environment, especially at peak hours when everyone else is trying to send their documents or make calls at the same time. Most VoIP calls are compressed to 32kbps or less, so fax consumes at least twice the bandwidth of a compressed VoIP call.  In addition, there is the IP packet overhead, which increases the required bandwidth to around 88kbps, or at least 175% more bandwidth than a VoIP call.

Second, Fax has little tolerance for packet delay and packet loss. One of the virtues of IP is that large data files can be compressed and “packetized,” broken down into smaller discrete packets of information and then transmitted over the Internet. This technique attaches a ‘header’ containing destination and source IP addresses to each individual packet (like the “to” and “from” address on an postal envelope) — and including information about the packet’s place in the larger sequence of the data (“I’m the fourth piece in a seven-piece series that makes up part of this fax”), and where it’s going (“Here’s the IP address of the computer where I’m supposed to be delivered”).

What this means is that IP allows the network to find the fastest, clearest route for each individual packet to reach its destination. This sometimes means that pieces of the transmission — such as an email message — arrive out of sequence. The process still works reliably, though, because the header information helps the system put the document back together almost immediately upon arrival at the recipient’s end. If some packets get lost along the way, they can be retransmitted until the full message is assembled. 

This works well for documents and email, where a few seconds of delay is not noticed, but not so well for ‘real-time’ communications, and for Fax the delay could be deadly.

You’ve no doubt been on a phone call where someone cuts out momentarily and you miss a part of a word or two. That’s a packet(s) not reaching the other end of its VoIP journey to your phone or arriving too late, in which case the packet will be discarded. In those cases, all you have to do is ask the other person to repeat their last sentence. And believe it or not, in other cases a word or two is dropped and your brain is able to interpolate the missing information without your even realizing it consciously. This is why voice managed to make the transition to IP despite the imperfections.

A fax, by contrast, cannot be compressed and cannot tolerate even a tiny percentage of packet loss — even a 1% packet loss, and more than a couple seconds of delay, can cause the connection to time-out and the fax to fail. It also cannot tolerate a break in the packet sequence which could result in more delay. The recipient’s fax machine might very well read any of these issues as a problem with the inbound fax, and kill the entire transmission.

The second problem:  Fax transmissions have low tolerance for interoperability issues. The hundreds of millions of active fax machines in the world use several different fax protocols — T.30, T.38 and G.711 being the primary ones, and speeds like V.14 or V.34, while VoIP typically uses G.729 to compress calls and save bandwidth.

When a fax is sent over an analog network like the phone system, the two fax machines communicate with each other and agree on the type and speed of transmission. But when the fax is being transmitted over VoIP, any gaps in the tones create the same problems for the recipient’s fax machine.

If a fax travels over a VoIP network from a machine using one protocol and arrives at a machine that expects another, this can cause gaps in the fax’s analog tones as the system tries to work out the protocol issues. The fax machines misinterpret the gaps and lose synchronization with each other.

For example, when the VoIP network is set to use G.729 compression, it has to switch to G.711 for uncompressed transmission when a fax is sent.  The brief breaks on the fax tones that occur as the VoIP system tries to negotiate between the two protocols can cause the fax to fail.  And the longer the fax, the less likely it is to make it through.

T.38 may save the day, someday. The newer T.38 protocol was intended to transmit faxes directly over IP (FoIP), so the fax doesn’t need to be converted to an audio stream first. In theory, two T.38 capable fax machines should be able to communicate over VoIP.

But T.38 must be on both ends of a network to work, and many service providers never implemented the protocol. If the fax has to traverse networks that do not support T.38, it will need to be transcoded, which can add latency, increase cost, and may cause the call to disconnect. In addition, the spec has been implemented in various ways by manufacturers, so that one machine’s T.38 may be incompatible with another vendor’s equipment. The result is a failure to communicate.