Information Technology Reference
In-Depth Information
hungry, and are sensitive to delay, jitter, and packet loss (packet redundancy can be used to mitigate this). Therefore, fax passthrough
generally is not considered to be the most desirable method of transporting fax over an IP network, but it can be useful if a voice/fax
gateway does not support T.38.
Store-and-Forward Fax
Store-and-forward fax over IP networks operates like e-mail. A local fax machine sends a fax to a server, which sends it on to
intermediate servers, and it is finally sent to the intended remote fax. Store-and-forward also allows delivery of faxes to PCs and other
devices besides fax machines.
T.37 is an ITU standard for store-and-forward fax transmissions. When using this method of fax transmission, a voice/fax gateway
receives a fax transmission from a connected Group 3 fax machine and converts it into a TIFF attachment for an e-mail message (this
gateway is referred to as an on-ramp gateway). The e-mail message is then sent over the network, and the remote voice/fax gateway
converts the e-mail message TIFF attachment back into a fax message and sends it to a connected fax machine. (This gateway is
referred to as an off-ramp gateway.)
Modem Transmission over an IP Network
One method of transporting modem signals over an IP network is modem relay. In this case, a voice gateway demodulates signals sent
by a connected modem and sends them over an IP network using the Simple Packet Relay Transport (SPRT) protocol. The remote
voice gateway receives the SPRT protocol packets, reconstructs the modem signals, and sends them to a connected modem.
Modem passthrough is an alternative method of transporting modem signals over an IP network, and it is similar to fax passthrough.
In this case, a voice gateway receives modem tones from a connected modem, switches to a high-bandwidth codec, such as G.711
with no VAD, and sends modem signals across the IP network to a remote voice gateway. The remote voice gateway reconstructs the
modem signal and sends it to a connected modem. Like fax passthrough, modem passthrough is sensitive to delay, jitter, and packet
loss in the IP network (although packet redundancy can be used to address packet loss).
