FAX
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What is the problem?
Fax is a reality in today's office. If is very convenient to print out a page, sign it and send it to a business partner. Customers expect that with a modern telecom equipment this works the same way.
When fax was invented, things such as packet loss were not a problem. The connection between the two fax machines were connected through a switched network. Fax was designed to deal with bad connection and clicks, but it was not designed to deal with a loss of a complete piece of the transmission. However, this is exactly what happens when a packet gets lost.
What is the solution?
The solution has the name "T.38". This is a technical standard that describes how the two endpoints should report packet losses and resend the missing packets.
T.38 must be supported by both endpoints. The endpoints are usually FXS gateways which are connected to fax devices. But it could also be a fax device that talks SIP natively. Althought these devices are quite rare today, they might become more and more popular as the PBX are talking SIP.
How does it work?
In the beginning of a call, it is not clear if the call will be a fax transmission or a regular telephone call. Therefore, a fax transmission starts in the normal voice mode. During this phase, the sending fax station sends those strange fax recognition tones. As soon as the receiver detects that the sender wants to send a fax, it tries to re-negotiate the used codec to T.38. With this change comes a change of the used ports, as the T.38 actually does not even use RTP.
The PBX does actually not interfere with T.38. It sits just in the middle and makes sure that the two fax endpoints are able to renegotiate the codec and the ports.
Which devices can I use?
There are a couple of devices available that support FAX. Check our if your vendors explicitly supports T.38. Just the "supports FAX" is not enough as some vendors try to solve the T.38 is a too simple way - by just using their own proprietary implementation which does not operate with anything else.
Can I use the PBX to detect FAX?
At the beginning of the FAX transmission, the FAX sender uses a special tone (CNG, 1100 Hz) to indicate that this is a FAX. The PBX is able to detect this tone and route the call accordingly. Because that tone is not a DTMF tone, the PBX uses the name "F" to identify the tone.
That means, if you use the pattern "F" in the auto attendant and program you FAX as a direct destination, the PBX will route FAX messages coming in on the central line to the extension that receives the FAX (usually a T.38-enabled ATA).
