Inexpensive long-distance telephone rates abound. Historically, before VoIP services became widely used by average long-distance carriers, and when the PSTN dominated telecommunications, most US consumers expected about 10 cents per-minute long-distance toll charges. However, over the last ten years consumers – both residential and business – have largely migrated to much less-expensive long-distance carrier services that can run 5 cents per-minute or less. The cost difference between the 10 and 5 cent per-minute services is primarily fueled by telecommunications competition leveraging VoIP services.
While this shift towards less-expensive VoIP services may arguably be great for voice communications it is a difficult situation for fax users. The perils experienced with VoIP telephone services when used for fax communications were discussed over two years ago in “Fax, VoIP, and the Perils of Poor Audio Quality”. Consequently, fax users who are conscientious about their operations must be very particular to use telephone services which deliberately support and are tested for fax use.
Since introducing the Mainpine On-line Fax Service, we often get asked by interested potential customers why the outbound pricing is 10 cents per-minute when they can readily find 5 cent per-minute long-distance for their own business use. The answer is that ultimately the 10 cent rate is actually less-expensive than the 5 cent rate – when used for fax.
How can this be?
Recognize that fax is primarily image data and secondarily handshaking data. Imperfections in the fax call audio quality translate directly into imperfections in the data received. Imperfections in the image data will result either in fax image distortion or, more likely, in increased transmission times as fax error correction mode is employed to retransmit portions of the image that did not make it through correctly. Imperfections in the handshaking data are much more perilous. Incorrect handshaking signals are met with great inconsistency by various fax devices; generally there is only some limited amount of tolerance built-into their operations, but this tolerance often expires very quickly before the fax fails with a protocol error.
In scenarios of perfect call audio quality there is never any data loss, never any data retransmissions, and never any protocol errors. As we cannot always control the remote endpoints, and as we cannot always control the carriers used in-between, the perfect scenario is not 100% obtainable for most cases. In our “Line Audio Quality Testing” Mainpine draws this maximum retransmission rate for a “good” line at 2% with no protocol failures. In contrast, a typical fax retransmission rate for a 5 cent per-minute VoIP long-distance service is usually around 10%, and a similar rate of protocol errors.
Now we need to do some math, but before we do there is something significant to consider: not all fax sending software treat protocol failures the same when they later reattempt the fax. Some software, like HylaFAX, will start re-sending on the first unsent page of the fax. Other software, such as Microsoft Fax (a.k.a. Windows Fax Service or Windows Fax and Scan), will resend the entire fax beginning at the first page. So these are the two primary scenarios to evaluate, and for brevity we'll just call them “HylaFAX” and “MS Fax”.
For our calculations we'll say that the average fax is two pages long and that each page would normally take 40 seconds to transmit in ideal conditions where no retransmissions or errors occur. (These assumptions can vary considerably, of course, depending on the particular use.)
Due to the handshaking protocol that surrounds image data retransmission, the retransmission of 10% of the data does not increase the total transmission time by only 10%. Instead, the additional protocol communication time will often match or exceed the time to retransmit the data. So we'll say a 20% increase in transmission time is necessary on the 5 cent per-minute service.
Then, if 10% of the fax attempts fail due to protocol failure there will necessarily be time spent in retransmitting the pages of the fax image data (whether it be the page or the entire fax) which were initially sent but which did not make it through entirely before the protocol failure. For the HylaFAX case this will result in 20 seconds (one-half page, an average between nothing and a full-page) for every ten faxes. Since the average fax is two pages long and each page averages 40 seconds to transmit, that's 20 seconds additional time spent per 800 seconds or 3%. For the MS Fax case this will result in 40 seconds (an average one-half of the 2-page fax) to retransmit the partially-transmitted fax for every 800 seconds... or 5%.
So the sum of the 20% increase with the 3% or 5% increase will be a 23% or 25% increase, respectively. For brevity we'll just go with 25% since it's easier to use.
This 25% transmission time increase also represents a decrease in throughput capacity. A decrease in capacity means that additional lines will be needed to service the same traffic. If a business fax line is available 8 hours per-day for 5 days per-week and costs $50 per-month for merely the service subscription then that is 3 cents per-minute. With a 25% reduced capacity that becomes nearly 5 cents per-minute. So the difference caused by the poor call audio quality is 2 cents per-minute.
It's also fair to conclude that this capacity degradation applies to both sides of the call, as the receiver also suffers a capacity degradation due to the sender's use of phone services that provide poor call audio quality. So the entire opportunity cost is 2 cents per-minute paid for by the sender and 2 cents per-minute paid for by the receiver, or 4 cents per-minute total.
So now we can bring it together. 5 cents per-minute increased by 25% is over 6 cents per-minute. Add-in the four cents per-minute of opportunity cost. Not surprisingly, we arrive at more than 10 cents per-minute – more than the the good-quality service – and with more difficulty. Feel free to place a monetary value on that added difficulty to conclude that there is no savings to be found in low-cost long-distance services with poor fax audio quality.