Specifications

Chapter 9 Implementing Fax over IP on Cisco Voice Gateways
Overview
9-2
Catalyst 4224 Access Gateway Switch Software Configuration Guide
OL-2031-02
Overview
In its original form, fax data is digital. However, to transmit across a traditional PSTN,
it is modulated and converted to analog. Fax over IP reverses this analog
conversion, transmitting digital data over the packet network, and then
reconverting the digital data to analog for the receiving fax machine.
Most Cisco voice gateways currently support two methods to transmit fax traffic
across the IP network:
• Fax Pass-Through—In fax pass-through mode, the gateways do not
distinguish a fax call from a voice call.
• Cisco Fax Relay—In fax relay mode, the gateways terminate the T.30 fax
signaling.
Fax relay mode is the preferred method to transmit fax traffic. However, if a
specific gateway does not support Cisco fax relay, the gateway supports fax
pass-through.
Fax Pass-Through
In fax pass-through mode, the gateways do not distinguish a fax call from a voice
call. The fax communication between the two fax machines is carried in-band
over a “voice” call in its entirety. All Cisco voice gateways support fax
pass-through.
On Cisco voice gateways, you can achieve more reliable fax transmissions using
fax pass-through mode if you disable VAD on the gateway dial peers.
Figure 9-1 illustrates how fax pass-through works. The fax traffic is transparently
carried across the quality of service (QoS)-enabled IP infrastructure, and the data
is not demodulated within the IP network.