For the past ten-plus years enterprises have been making the switch from TDM to IP telephony. For the most part, this involved replacing older digital telephones with their IP equivalents and converting applications such as voice mail and voice response units to a SIP interface. The benefits of these conversions are significant, and you would be hard pressed to find a new PBX without an IP infrastructure.
However, for most of the early 2000s, enterprise trunks to the public switched telephone network (PSTN) remained unchanged with analog and ISDN/PRI as the norm. Many carriers weren't ready to deliver IP trunks, and those that were didn't offer features that were on par with their TDM sisters and brothers. All of this has changed, though, and enterprises are now rapidly replacing their TDM trunks with SIP trunks.
As it was in the case of IP endpoints and applications, there are many benefits in switching to SIP trunks. Unfortunately, there is one issue that tends to be forgotten: What do you do with all your fax lines? How do you convert them to IP? Is it possible to use SIP trunks for both voice and fax?
Before these questions can be answered, a short primer on fax is necessary.
Fax (short for facsimile) has been around in one form or another since the 1920s, but it wasn't until the mid-1960s that the Xerox Corporation introduced the first commercial fax machine.
Faxes began as analog transmissions (Group 1 and Group 2), but that gave way to digital transmissions (Group 3 and Group 4) in the late 1960s. Fax machines scan documents in the same manner as a frame of analog television, but those scans are then compressed and sent as modem signals across telephone lines.
T.30 is used as the messaging protocol between fax machines. T.30 also supports a handshake mechanism whereby two fax machines exchange information about their capabilities to ensure that the sending fax machine sends data the receiving fax machine understands and can ultimately display.
Faxes are sent and received as modem signals and different ITU standards exist for the different transmission speeds. For instance, V.27 defines faxes sent at 2400 or 4800 bits per second and V.34bis is used for faxes sent at 33600 bits per second. Modem signals and bit speeds are perfectly acceptable when you consider that an analog telephone line is a dedicated, circuit-switched connection with a consistent data flow.
This is not the case with packet-switched IP which may suffer from jitter, delay, lost packets, and packets arriving out-of-order. So, how do you deal with this problem as your enterprise replaces analog and ISDN/PRI trunks with SIP trunks?
Note that G.729 cannot be used to transmit fax. That's because G.729 has been designed to only detect, compress, transmit, and decompress human speech. Now, while I have known some squeaky-voiced people, I have yet to meet anyone that sounds like a fax machine.
Enter T.38 -- a Fax over IP (FoIP) protocol designed to packetize those modem signals in a way that overcomes the "flaws" in an IP network. It is T.38's job to ensure that the fax packets arrive in order, on time, and with as little jitter as possible. In most cases, a fax will begin as T.30 from the sending fax machine, be converted to T.38 by a fax gateway, transmitted across the IP network, and eventually be converted back to T.30 by a gateway (e.g. an Avaya TN2602 board within a G650 gateway) on the receiving end.
Session border controllers (SBC) can play a part with T.38. If you chose to use an SBC for T.38 conversion, you will need to properly engineer it with the right number of hard or soft DSPs. Too few will cause problems with fax transmissions, and too many will be a waste of money.
Newer T.38 fax machines and fax servers exist that eliminate the need to convert to and from T.30, but since many of the older machines are in wide use today, gateways are still necessary.
There is, of course, another alternative and that's to get rid of fax altogether. Frankly, this is my favorite option, but I expect that there are a number of companies that would take issue with that. Fax is still considered a legal document and is used quite extensively throughout the medical and insurance industries. In fact, faxing in insurance claim forms may be the only time I get close to a fax machine these days.
As your enterprise makes its journey from TDM to SIP trunks, it is imperative that you examine your fax usage before committing to a particular SIP architecture. If fax is an important part of your business, you may want to look at T.38 from the standpoint of your communications infrastructure as well as your SIP trunks. While fax-pass-through may work most of the time, its success is based on a near perfect network and that may not be feasible or practical.
Finally, be aware that T.38 might add additional cost to your move to SIP, so don't fail to include that in your decision making process. In the end, though, it is possible to create a system that brings together the power of SIP with older technologies such as fax.
Andrew Prokop writes about all things unified communications on his popular blog, SIP Adventures.
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