Fax Services over IP Networks
There are three conceptual methods of accustomed fax-machine-to-fax-machine communications
across packet networks:
■ Fax relay: The T.30 fax from the PSTN is demodulated at the sending gateway. The
demodulated fax agreeable is belted into packets, beatific over the network, and
remodulated into T.30 fax at the accepting end.
66 Authorized Self-Study Guide: Cisco Articulation over IP (CVOICE)
Note Cisco IOS supports two types of fax relay: T.38 fax broadcast and Cisco Fax Relay
(which is proprietary).
■ Fax pass-through: Modulated fax advice from the PSTN is anesthetized in-band
end-to-end over a articulation accent aisle in an IP network. There are two pass-through
techniques:
■ The configured articulation codec is acclimated for the fax transmission. This technique
works alone back the configured codec is G.711 with no VAD and no echo
cancellation (EC) or back the configured codec is a clear-channel codec or
G.726/32. Low bit-rate codecs cannot be acclimated for fax transmissions.
■ The aperture dynamically changes the codec from the codec configured for voice
to G.711 with no VAD and no EC for the continuance of the fax session. This
method is accurately referred to as “codec up speed” or “fax pass-through with
up speed.”
■ Store-and-forward fax: Breaks the fax action into audible sending and receiving
processes and allows fax letters to be stored amid those processes. Store-andforward
fax is based on the ITU-T T.37 standard, and it additionally enables fax transmissions
to be accustomed from or delivered to computers rather than fax machines.