Faxing over IP networks does not work reliably. There are many technical reasons why; I won’t go into them here. This page provides a pretty detailed explanation about why trying to transmit analog modem signals over an IP network will not work — variable jitter, insufficient bandwidth, silence suppression and many other factors in VoIP call handling will work together to destroy your faxes. There are two main solutions in the FoIP (Fax over IP) space:
- T.37 (store-and-forward): Use e-mail as the IP transport medium. T.37 defines a protocol by which faxes are converted to an e-mail message and then delivered to a T.37 endpoint – whether that is someone’s email box, or a device capable of translating the attachment into a fax image and then sending it to the target fax machine using the PSTN.
- T.38 (real-time fax): Use either “Internet-capable” (T.38) fax machines, analog telephone adapters, or a combination of T.38 aware/compatible devices to transmit faxes using special UDP packets.
Neither of these mechanisms is particularly elegant. In fact, the adoption rate of T.38 is quite low among ATA makers, and many implementations are buggy. Also, the fact that T.38 must be implemented on both ends of a call is another nail in the coffin.
Let’s step back a moment here and reconsider what we want to do. Suppose I am a business owner considering (or having switched) to a VoIP network, but I still have my old (non-T.38 capable) fax machine. I want to send faxes to any other fax device in the world, and I don’t care whether the receiver’s equipment is T.38 capable or not. I am willing to invest in a T.38 ATA, and assume that I can do so without too much cost or effort, and that it will work reasonably well. What do I do?
At the moment, not much — it’s probably faster and more reliable to set up an analog phone line just for sending and receiving faxes. But I have a business idea, as follows: set up a company whose job it is to simply receive faxes from any and all subscribing parties. Every subscriber would use the same inbound fax number; e.g. 1-888-INET-FAX. Subscribers would also use a special fax cover sheet (prepended to the body of the fax) to distinguish their faxes from other subscribers; on the special cover sheet, the ultimate target fax number would be filled out. The cover sheet could be a preprinted form like a Scantron optical answer sheet; the intermediary system would read the target fax number from the cover sheet, discard the sheet from the fax, and then retransmit the rest of the fax to the target using the PSTN over a bank of regular analog phone lines.
If you think this is a good idea, feel free to steal it and start your own company. I won’t ask for royalties! I just want faxing to work over IP networks!