Information Technology Reference
In-Depth Information
Real-Time Fax
Real-time fax over IP networks operates in a manner similar to regular fax transmissions: The fax machines involved in the
transmission synch up, and then the fax data is sent between them over the intervening IP network. If the remote fax machine to
which a transmission will be sent is busy, the local fax machine will receive a busy signal and can retry later.
There are two methods of transporting fax in real time across a network: fax-relay and fax passthrough.
When using fax-relay, the T.30 fax signal from a connected fax machine is demodulated by the sending voice/fax gateway
(originating gateway [OGW]) and sent over the IP network to a remote voice/fax gateway (terminating gateway [TGW]). The remote
voice/fax gateway then reconstructs the T.30 fax signal and sends it to a connected fax machine.
There are two fax-relay mechanisms: Cisco fax-relay and T.38 fax-relay.
Cisco fax-relay is an older method of transporting fax transmissions between fax machines connected to Cisco voice/fax gateways.
When using this method, a voice/fax gateway terminates T.30 fax tones from a local fax machine, and then sends the fax data across
an IP network by breaking the tones into HDLC frames, and then transmitting them using RTP. The remote voice/fax gateway then
reverses the process, re-creates the T.30 fax tones, and sends them to a connected fax machine.
T.38 is an ITU standard that provides for real-time (fax-relay) transport of Group 3 fax over an IP network. T.30 fax signals received
from a connected fax machine are demodulated at a local voice/fax gateway and encapsulated into IP packets for transport over a
network to a remote voice/fax gateway. At the remote voice/fax gateway, the T.30 fax signal is then reconstructed and played out to
the connected fax machine.
T.38 includes a mechanism by which a voice/fax gateway can inform a remote voice/fax gateway of its desire to change the media
type from voice to data. T.38 also provides for the transport of fax over an IP network using either TCP or UDP, though the use of
UDP is more prevalent.
When using fax passthrough, modulated fax data is sent in-band across the IP network by a voice/fax gateway using a voice codec
(such as G.711 with no VAD or echo cancellation).
Also, when using fax passthrough, T.30 fax calls are not distinguished from regular voice calls; they are simply sent in-band over
an IP network. In this case, a voice/fax gateway detects fax tones and switches to a high-bandwidth codec such as G.711 with no
VAD (which prevents distortion of the fax signal). For this reason, fax messages sent using fax passthrough are relatively bandwidth
