Cisco Fax Relay - Cisco WS C4224V 8FXS - 200Mbps Ethernet Switch Software Configuration Manual

Chapter 9 Implementing Fax over IP on Cisco Voice Gateways

Cisco Fax Relay

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Figure 9-1 Fax Pass-Through
Voice Gateway
Fax
PSTN
Modulated
data
Cisco fax relay does not involve Cisco CallManager; it is a gateway-controlled
fax mode. Most of the fax processing occurs in the digital signal processors
(DSP), requiring only packet switching from the main processor (CPU) and some
limited signaling to switch to fax mode. Therefore, the CPU overhead is very
similar to a normal voice call.
Initially, a voice call is established. Once the V.21 preamble is detected at the
terminating gateway, the originating and the terminating gateway negotiate the
codec type. If the two sides cannot negotiate on a common codec and speed, the
fax fails. If negotiation is successful, fax transmission begins.
Cisco fax relay is supported using MGCP or H.323, depending on the specific
gateway type. The Cisco voice gateways support as many concurrent fax calls as
G.711 voice calls.
As illustrated in Figure
crossing the IP network. The voice gateway at the other end of the IP network
demodulates it for transfer across the PSTN.
Catalyst 4224 Access Gateway Switch Software Configuration Guide
IP
network
Modulated data
Latency, Jitter, buffers,
Packet loss
V
9-2, the voice gateway demodulates the fax data before
Overview
Modulated
data
Fax
V
Voice Gateway
9-3