Faxing over Broadband: The Problem & The Cure

Broadband is here in a big way, and there is no doubt that the migration from copper to broadband is huge for phone companies/carriers alike.

With this migration to broadband companies are finding that the voice portion for the most part is fine, until they try to send or receive a fax over that Broadband channel. Perhaps that first fax session or two will be perfectly fine. But sooner or later you will start to notice that faxing just doesn’t seem to work as well as you remember it working on the good old copper phone line.

So what’s going on?

The problem rests is one of two matters or both. First is lossy” audio compression (codec). The second issue is “jitter”.

Why is lossy compression and jitter a problem for faxing? Understanding the mechanics so to speak of is the first step.

A fax is typically two fax machines communicating passing image data from the sender to the recipient.

This type of data being digital must be modulated to communicate the data over audio lines “channels” such as a phone line. The key is that the audio signal is converted into an audio wave and then decoded at the receiving end.

This is the beeping and screeching you hear when a fax tone is audible.

So one machine is basically for lack of better words talking and the other is listening.

Now that we have the basics, what is lossy compression and jitter and how does it affect broadband fax machine calls?

When the data is sent it must be compressed. Compression is when a string of data is taken and then converted into a smaller string. On the other end this compressed data must be decompressed so that the information can be retrieved.

The varied compressing of audio data is called “codecs”. Some codecs are called “lossless” which means that there is no loss in detail of the data when undergoing the compression and decompression exchange.

Others are called “lossy” which means that there is some loss of detail information when undergoing the compression and decompression stages; basically detail is sacrificed in order to achieve a tighter compression.

So it is obvious then that faxing cannot happen reliably without issues on lossy codecs because it will consistently remove data.  So you have to go with a lossless channel which does use more bandwidth than a lossy one.

The most common means of transport used in the industry is called T.38, the issue is that T.38 was devised for transmission/termination over a closed controlled lossless environment and when it is used over an environment like the open internet, WiFi, or satellite they lossy nature of this medium makes it a very unreliable means of fax transport. But for years this was the only alternative.

One of the results of using T.38 on broadband channels is an event known as “jitter” resulting in:

Message signal interruption.

Loss of Synchronization

Broken signal data

During a broadband call the audio data is streaming between the machines. This data is called “packets”; each packet has the data sent over the internet indicates who sent it and where it should go. While being sent over the internet the data must take what is called “hops” through each station that can potentially send the packet through a different route to reach the final destination.

It is not uncommon that a station is unable to handle a packet at any given time. Resulting in delay of the packet routing. In addition depending on what type of packet the sender used, a station can even ignore and not relay the packet at all.

Broadband like a large percentage of “streaming” communication, using the type of packet that the internet stations have the discretion to ignore. This is done to keep the audio from pausing at various points and to keep the audio as real­-time.

This typically works out perfectly fine for voice audio because the human ear does not pick up on the missing audio. And some Broadband equipment, noticing the missing audio, will even synthesize some audio to fill in the gap (this is called a “jitter buffer”). However, both missing and synthesized audio constitute a corruption of the audio data from when it left
the sender, and there is no immediate way for the receiver to recover the missing audio data. This is why the jitter you see represented shows up as gaps of silence.

An intelligent jitter buffer like T.38 uses will help to fill ­in the gaps enough to prevent premature detection of a signal­ end, however, there still will be missing audio. This missing audio is still makes it difficult for fax to work over Broadband. The other issue is the increased bandwidth that has to be used to maintain this buffer.

Which means  your provider up sell you to use more bandwidth in the business world this translates into more overhead and less cash at the end of the day in your pocket.

Another issue is that fax machines use an error correction method (ECM) to request, retransmit, and recover any image data that did not make it through. But because of the inherent nature of ECM and the potential to disrupt T.38 most T.38 installs recommend that this valuable feature be turned off on the machine.
HTTP(S) is the cure?

HTTP(S) uses a request-response paradigm that works well with the distributed nature of the Internet, as a request or response might pass through many intermediate routers and proxy servers. Also a request includes not only the requested content but also relevant status information about the request. This self-contained design allows intermediary servers to perform value-added functions such as load balancing, caching, encryption, and compression.

This means that HTTPS ensures secure and reliable last mile connectivity for fax machines and servers when faxes are being transferred over the Internet.

HTTP(S) uses chunked transfer encoding which allows a server (our in our case a specific Fax ATA to our cloud) to maintain an HTTP persistent connection for dynamically generated content. Chunked encoding has the benefit that it is not necessary to generate the full content before writing the header, as it allows streaming of content as chunks and explicitly signaling the end of the content, making the connection available for the next HTTP request/response. This means that packet loss does not affect HTTPS transfer.

As such HTTPS suffers from none of the issues t.38 has with the open Internet. Latency, packet loss, jitter and congestion simply do not interfere with HTTPS packets as they do with T.38.


by Randy Simmons, FaxSIPit VP of Sales