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CHAPTER
6-1
Cisco ATA 186 and Cisco ATA 188 Analog Telephone Adaptor Administrator’s Guide for MGCP (version 3.0)
OL-4803-01
6
Configuring and Debugging Fax Services
The Cisco ATA provides two modes of fax services that are capable of internetworking with Cisco IOS
gateways over IP networks. These modes are called fax pass-through mode and fax mode.
With fax pass-through mode, the Cisco ATA encodes fax traffic within the G.711 voice codec and passes
it through the Voice Over IP (VoIP) network as though the fax were a voice call. This mode uses the
Cisco proprietary fax upspeed method.
With fax mode, the Cisco ATA presents itself as a device capable of using only G.711 codecs; therefore,
no codec renegotiation or switchover is required. This places minimum functionality and configuration
requirements on remote gateways. Fax mode is recommended for environments in which G.711 fax
upspeed is not available for the supporting Cisco gateways.
This section contains the following topics:
• Using Fax Pass-through Mode, page 6-1
• Using FAX Mode, page 6-6
• Debugging the Cisco ATA 186/188 Fax Services, page 6-7
Note The term Cisco ATA is used throughout this manual to refer to both the Cisco ATA 186 and the
Cisco ATA 188, unless differences between the Cisco ATA 186 and Cisco ATA 188 are explicitly
stated.
Using Fax Pass-through Mode
Fax pass-through mode allows for maximum codec flexibility because users may set up a voice call
using any voice codec, then renegotiate to a G.711 codec for the fax session. To use fax pass-through
mode, first configure the Cisco ATA and supporting Cisco gateways to support the Cisco-proprietary
G.711fax upspeed method. Then, disable fax relay on the far-end gateway—either for the entire gateway
or for the dial peer engaged in the fax call with the Cisco ATA.
The fax upspeed method allows you to use low bit-rate codecs such as G.723 and G.729 for voice calls,
and G.711 codecs for fax calls. With a fax call, the Cisco ATA detects a 2100-Hz CED tone or V.21
preamble flag, then informs the remote gateway of its intent to switchover to G.711 via a peer-to-peer
message. This type of message, carried as a Named Signaling Event (NSE) within the RTP stream, is
used for all fax event signaling. The Cisco ATA can initiate and respond to NSEs and can function as
either an originating or terminating gateway.