Across the Internet even a G.711 codec fax transmission is unpredictable. An excellent discussion of why faxing and modems don’t work well over VoIP can be found here. However, people often get perfectly good results on lightly loaded LANs. It still isn’t perfect, as a burst of data on the LAN can still upset things, but some people get results they can live with.
Fax-over-Internet T.38 pass-thru functionality within ShoreTel has arrived in 11!
In detail:
There are two conceptual methods of carrying virtually real-time fax-machine-to-fax-machine communication across packet networks:
Fax relay, in which the T.30 fax from the PSTN is demodulated at the sending gateway. The demodulated fax content is enveloped into packets, sent over the network, and remodulated into T.30 fax at the receiving end.
Fax pass-through, in which modulated fax information from the PSTN is passed in-band end-to-end over a voice speech path in an IP network. The following two pass-through techniques are possible:
The configured voice codec is used for the fax transmission. This technique works only when the configured codec is G.711 with no voice activity detection (VAD) and no echo cancellation (EC), or when the configured codec is a clear-channel codec or G.726/32. Low bit-rate codecs cannot be used for fax transmissions.
The gateway dynamically changes the codec from the codec configured for voice to G.711 with no VAD and no EC for the duration of the fax session. This method is specifically referred to as codec upspeed or fax pass-through with upspeed.
In addition to the methods for real-time fax transmission, a method called store-and-forward fax breaks the fax process into distinct sending and receiving processes and allows fax messages to be stored between those processes. store-and-forward fax is based on the ITU-T T.37 standard, and it also enables fax transmissions to be received from or delivered to computers rather than fax machines.
T.38 relay:
In fax relay mode, gateways terminate T.30 fax signaling by spoofing a virtual fax machine to the locally attached fax machine. Unlike in-band faxing or fax pass-through, fax relay breaks down the T.30 fax tones into their specific HDLC frames (demodulation), sends the information across the voice network using the fax relay protocol (T.38), and then converts the bits back into tones at the far side (modulation). The fax machines on either end are sending and receiving tones and are not aware that a demodulation/modulation fax relay process is occurring.
Summary:
Fax pass-through is the simplest technique for sending fax over IP networks, but it is not the default, nor is it the most desirable method of supporting fax over IP. T.38 fax relay provides a more reliable and error-free method of sending faxes over an IP network
Fax over IP or SIP trunks! is a post from: VOIP Tech Blog
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