A window into the technology and business of fax...

Fax, VoIP, and the Perils of Poor Audio Quality


So once you have well-functioning hardware such as the IQ Express and well-functioning software such as Mainpine's IQFSP with Windows Fax Service or HylaFAX+, what is left that could possibly go wrong for reliable fax operations?  Just as important to fax reliablity as the hardware and software is the line audio quality delivering the fax audio to the modem.

The Mainpine support department sees this story repeatedly: a customer felt like they were paying too much for their PSTN phone service, this customer was wooed by promises and advertising of low-cost service by some VoIP service provider, a few faxes worked fine after switching, but ultimately things are not going very well.  This story can be replayed with various deviations, for example, maybe the change in telephone service occurred unwittingly when a business relocation occurred.  Maybe the change in telephone service occurred when a new office phone system was installed.  Maybe faxing worked well for months or even years and only now started having problems.  Whatever the case, Voice-Over-IP telephone service (VoIP) does not work very well for fax, and here's why...

VoIP service utilizes a customer's internet connection.  So instead of using the Public Switched Telephone Network (PSTN) as with traditional telephony the audio for a VoIP phone call travels over the IP network known as the internet.  The PSTN and internet work in considerably different fashions.

Imagine a bunch of interwoven water pipes that connect a set of homes to a common water system.  Imagine in one scenario where various valves and knobs on the pipes could be turned such that a water connection could be made through the water system between two different homes.  In that scenario imagine that those valves and knobs block-out all other water paths such that there is only one path for the water to follow, and that all water put in at one end passes through the water system and will invariably come out at the other end in the same order as the water went in.  This is the PSTN.  When a call is made on the PSTN there is a dedicated channel between the two ends which cannot be interrupted by other traffic elsewhere on the PSTN.

Imagine that same set of homes and that same common water system with interwoven water pipes, but this time imagine that in the water are labels on each molecule with its intended destination, that down each pipe goes molecules headed to various different destinations, and instead of valves and knobs at the intersections there are little robot devices called "routers" whose job it is to sort through all of those molecules and direct them down a suitable pipe towards their intended destination.  Due to the difficult task the routers have, they are endowed with some relevant liberties.  For example, if the first pipe-of-choice is too full for the water the router can send it down another pipe to a different router that will ultimately send it on to the destination in a different way.  And if some router is just too busy to handle all of the water traffic at the moment it can either attempt to return the water back to its source or simply spill the excess out of the system onto the ground.  This is internet.  When a call is made on the internet (VoIP) the call passes through numerous network routers which have to manage a lot of traffic from all sorts of sources and may have to make decisions on how to route the traffic, how much of the traffic to route, or whether to route it at all.  The receiver will get the various components of the audio stream and will have to reassemble them as best it can in a timely manner before delivering it to the modem.  Sometimes the audio stream will be missing little bits of the audio.

The little bits of missing or mis-ordered audio are commonly called "jitter".  If there is an extremely large amount of jitter then it may sound like the person on the other end is trying to talk with a mouthful of water, but often the jitter is undetectable by the human ear and brain.  However, fax machines and modems as well as data modems cannot manage well with jitter.  The reason that fax and data cannot tolerate disruptions in the audio stream such as jitter is because every single packet of audio data (a water molecule in the narrative) contains necessary information which is not replicated in the adjacent packets.  In voice communications a single spoken word is composed of sounds each of which may last for hundreds of audio packets.  So if a few go missing it's not a big problem since our ears and brain can "smooth-over" the missing bits.  However, this is often not the case for data communications.  If packets of audio go missing then that translates into bits of data that are missing.

Depending on when the audio corruption occurs in the fax protocol the reaction and response may be different.  However, the most-common reaction is that the missing bits of audio are interpreted as silence.  Since fax communications are half-duplex (only one end speaks at a time) any beginning of silence is interpreted as a signal to the other end.  If this accidental signal occurs at an extremely unfortunate time, then it may trigger an ultimate end to the fax session altogether.  If the fax endpoints are not using error correction mode (ECM) then it may be even more likely that the fax will fail to complete.  And worse yet is if one or both of the endpoints are not programmed with robust recovery procedures, then failure to complete that fax is almost certain even with only a minor audio disruption.

Casual users of fax will easily observe the effect of line audio quality problems as black lines, streaks, truncated pages, or missing portions of the fax image on the page that commonly occur when ECM is not used on noisy connections.

And VoIP jitter is not the only source for audio problems for fax.  Other problem sources can potentially be...

  • Call waiting service on the fax line.  (The call waiting "beeps" will disrupt audio.)
  • DSL service on the fax line.  (Even with a DSL filter the net effect of the DSL signal being added-to and subtracted-from the line results in audio quality degradation.)
  • PBX or other office phone system problems.  (Not all office phone systems are designed and tested to support fax.)
  • Electrical interference on the fax line.  (Electrical currents can be induced by some kinds of electrical equipment such as fluorescent light ballasts, power adapters, and generators.)
  • Poor wiring.  (Phone line wiring that is not properly connected can go undetected by human ears.)
  • Other problems at the telephone company.  (There is a lot that goes on there and a lot can go wrong.)
  • Maybe your telephone company or long-distance service provider uses VoIP even if you don't.  (They want to save on their costs, too.)

So when troubleshooting fax communications problems make sure that you have all three:  good hardware, good software, and a good fax line.  Once those three components come together reliable fax operations will follow.

To learn more about problems with VoIP jitter and fax have a look here and here.