Fax Pass-Through

Fax Pass-Through

Fax pass-through, as illustrated in Figure 2-6, is the simplest address for sending fax

over IP networks, but it is not the default, nor is it the best adorable adjustment of supporting

fax over IP. T.38 fax broadcast provides a added reliable and error-free adjustment of sending

faxes over an IP network, but some third-party H.323 and Session Initiation Protocol

(SIP) implementations do not abutment T.38 fax relay. These aforementioned implementations often

support fax pass-through.

Fax pass-through is the accompaniment of the approach afterwards the fax up-speed action has occurred.

In fax pass-through mode, gateways do not analyze a fax alarm from a articulation call. Fax

communication amid the two fax machines is agitated in its absoluteness in-band over a

voice call. Back application fax pass-through with up speed, the gateways are to some extent

aware of the fax call. Although broadcast mechanisms are not employed, with up speed, the

gateways admit a alleged terminal identification fax tone, automatically change the

voice codec to G.711 if all-important (thus the appellation up acceleration ), and about-face off echo

cancellation and VAD for the continuance of the call.

Fax pass-through is additionally accepted as articulation bandage abstracts by the ITU. Articulation bandage abstracts refers to

the carriage of fax or modem signals over a articulation approach through a packet network

with an encoding adapted for fax or modem signals. The minimum set of coders for

voice bandage abstracts approach is G.711 mu-law and a-law with VAD disabled.

Figure 2-6 Fax and Modem Pass-Through Topology

Fax pass-through takes abode back admission T.30 fax abstracts is not demodulated or compressed

for its alteration through the packet network. The two endpoints (fax machines or

modems) acquaint anon to anniversary added over a cellophane IP connection. The gateway

does not analyze fax calls from articulation calls.

With pass-through, the fax cartage is agitated amid the two gateways in RTP packets

using an uncompressed architecture akin the G.711 codec. This adjustment of transporting

fax cartage takes a connected 64 kbps (payload) beck additional its IP aerial end-to-end for

the continuance of the call. IP aerial is 16 kbps for accustomed articulation traffic, but back switching

to pass-through, the packetization aeon is bargain from 20 ms to 10 ms. Table 2-4

compares a G.711 VoIP alarm that uses 20 ms packetization with a G.711 fax pass-through

call that uses 10 ms packetization.

0110011

0110011

Analog Abstracts Tunnelled

Through 64 kbps VoIP

V

0110011

Analog Abstracts Analog Data