Specifications

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Chapter 9 Implementing Fax over IP on Cisco Voice Gateways
Overview
Figure 9-1 Fax Pass-Through
Cisco Fax Relay
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 9-2, the voice gateway demodulates the fax data before
crossing the IP network. The voice gateway at the other end of the IP network
demodulates it for transfer across the PSTN.
V
V
Modulated data
Modulated
data
Modulated
data
IP
network
Latency, Jitter, buffers,
Packet loss
Fax
Fax
Voice Gateway
Voice Gateway
PSTN
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