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

Line Audio Quality Testing


Last year we discussed "VoIP and the Perils of Poor Audio Quality".  In that article we discussed various kind of factors that can influence audio quality.

When Mainpine support is called upon to troubleshoot and diagnose problems with failing faxes, partially-received faxes, handshaking issues, or transmission time issues Mainpine technicians perform "line audio quality testing" and in such cases the result is almost perfectly consistent: poor line audio quality.  We will then advise the customer to bring the matter up with their phone service provider, long-distance service provider, and their local telephone company to test the line and make sure that the factors discussed in last year's article are not involved.

Unfortunately, it's not terribly uncommon for a customer to go through that process with the telephone service providers only to be told that there is no problem with the line.  Then the customer comes back to Mainpine support - now slightly aggravated with frustration - looking for us to resolve the problem, perhaps concluding on their own that the modem hardware must be faulty because the telephone service provider insisted the line was not at fault.  Furthermore, the customer maybe connected an analogue phone to the line and made some test calls that sounded just fine because no noise was heard on the call.  Mainpine's diagnosis is challenged, and the questions become, "Why do you believe that there is a line audio quality issue?  How do you come to that conclusion?"

I'll answer these questions, but beware... this may get a bit technical.  You've been warned.

Fax is a communication of data.  Traditional fax involves the communication of that data on phone lines, but in order to do so that data must be modulated into audio by the transmitter and then demodulated back into data by the receiver.  In order for this to happen adequately the medium on which the audio is carried must be "lossless", that is, the medium must not disturb the audio in such a way that the demodulated data differs from the data before modulation.

Have I put you to sleep yet?

So the basic method of determining adequate line audio quality is to compare the original data with the data that was received at the other end.  Fax protocol affords us a very good tool for this purpose in ECM (error correction mode).

In ECM the fax image data is broken-up into 64-kilobyte sections known as "blocks" and then further from there into 256-byte chunks called "frames".  Each chunk of frame data is followed by a 16-bit (two byte) checksum value.  A checksum is a number that can be computed algorithmically (mathematically) using the source data to verify the integrity of the data.  So, the sender transmits a block of frames along with their checksums to the receiver, and then signals to the receiver how many frames were sent in total.  The receiver then is expected to use the checksums to verify the integrity of the frames received and then signal back to the sender which frames were not received correctly.  The sender, then, will retransmit the frames that were received incorrectly, and the receiver will again check their integrity and signal back to the sender accordingly.  This process goes on until all of the data is received correctly or until one of the fax protocol drivers (the fax software or the modem firmware) is convinced that there is no way to communicate the data correctly and aborts the fax session.

Mainpine support technicians use this fax protocol interaction to determine line audio quality.

In a perfectly lossless scenario the data received would always match the data sent, and there would never be any need for retransmissions.  This kind of reliability is not unheard-of; it usually involves scenarios where the customer is connected directly to the PSTN (public switched telephone network) through a service provider that uses only PSTN circuits (such as AT&T or Verizon) instead of VoIP circuits (such as Comcast and Time Warner).

However, not all scenarios are perfect.  Local phone systems (PBXes), minor sources of electrical interference, imperfect telephone switches, and other factors do have an influence on the audio quality.  So Mainpine considers anything under a 2% frame retransmission rate as representing "good" audio quality; anything under a 4% frame retransmission rate represents "fair" audio quality; and anything above 4% frame retransmission rate is considered "poor".  Now, a 95% success-rate may seem very good, I mean, in school that's an "A" grade after-all, right?  But it's not that way for fax frame transmission rates.  It is Mainpine's experience that customers with greater than 4% frame retransmission rates report consistently high failure rates for their faxes, partially-received faxes, and inability to send faxes with many pages.  For reference, a common frame transmission failure rate due to VoIP service is around 10%.  So 90% success is not good when it comes to fax.

If you think about it in a bigger terminology it may make more sense... Who would want to routinely have to retransmit one out of every twenty faxes sent?  Would that level of performance be considered acceptable?

Indeed, fax customers demand a greater reliability than this.  Such reliability is certainly attainable.  Mainpine technicians are trained to assist their customers to reach that kind of dependable fax performance.  Line audio quality testing is part of that service.