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A real-time fax process consists of five phases:
1. Fax call setup phase. This phase is similar to the process of a telephone call setup. The
difference is that the fax tones identifying the sending/receiving terminals are included.
2. Prior-messaging phase. During this phase, fax faculty negotiation and training are performed.
3. Messaging phase. During this phase, fax packets are transmitted in accordance with the T.4
procedure, and packet transmission is controlled (including packets synchronization, error
detection and correction, and line monitoring).
4. Post-messaging phase. During this phase, control operations such as packet authentication,
messaging completion, and multi-page continuous transmission are performed.
5. Fax call release phase. During this phase, the fax call is released.
Introduction to fax methods
T.38 fax
The device supports two fax protocols: T.38 protocol and standard T.38 protocol. The standard T.38
protocol should be selected for interworking with leading fax terminals in the industry. Because most
leading fax terminals in the industry do not support the local training mode, the end-to-end training
mode must be selected for interworking with them.
Pass-through fax
The fax pass-through technology was developed primarily for the purpose of compressing and
transmitting T.30 fax packets that cannot be demodulated through packet switched networks. With
this technology, the devices on two sides can directly communicate over a transparent IP link, and
the voice gateways do not distinguish fax calls from voice calls. After detecting a fax tone in an
established VoIP call, the voice gateway checks whether the voice codec protocol is G.711. If not,
the voice gateway switches the codec to G.711. Then fax data is transmitted as voice data in the
pass-through mode.
In the pass-through mode, fax information is in the format of uncompressed G.711 codes and is
encapsulated in RTP packets between gateways, and a fixed bandwidth of 64 Kbps is occupied.
Although the packet redundancy mechanism can reduce the packet loss ratio, the pass-through
mode is subject to factors such as packet loss ratio, jitter, and delay. Therefore, it is necessary to
ensure synchronization of the clocks on both sides. Fax pass-through is called voice band data (VBD)
by ITU-T. That is, fax or modem signals are transmitted over a voice channel using a proper coding
method. So far, the codecs supported are only G.711 A-law and G.711 µ-law. In addition, when the
fax pass-through function is enabled, the voice activity detection (VAD) function must be disabled to
avoid fax failures.
You can implement the fax pass-through function on the voice gateway in two ways:
• Configure the fax to operate in pass-through mode on both sides.
• Negotiate the codec as G.711 and disable fax forwarding. Then, disable the VAD function to
avoid fax failures. This method is used for the voice gateway to interwork with other devices in
the pass-through mode.
SIP modem pass-through function
The SIP modem pass-through function is mainly used for remote device management. Because the
VoIP network has replaced part of the traditional PSTN, VoIP devices are required to support the
modem pass-through function, which can help remote PSTN users to log in to internal network
devices through dialup.
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