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Chapter 7
Configuring and Debugging Fax Services
Debugging FAX Pass-through Mode
When the Cisco ATA is configured to use fax pass-through mode, the fax call session can be established
with an arbitrary voice codec. Once the voice call has been established, fax machines can signal their
presence by means of a CED tone or V.21 preamble flag, after which the gateways send NSE packets to
initiate switchover.
For fax pass-through mode, check the Cisco ATA debug log to verify that it is acting as an originating
Note
gateway as well as a terminating gateway.
Terminating-Gateway Example
When the Cisco ATA is used as a terminating gateway for a fax session, make sure the following
conditions are true:
•
•
An example debug log for a terminating gateway scenario is show below:
[0]Tx MPT PT=100 NSE pkt c0000000
[0]codec: 4 => 0
[0]Rx MPT PT=100 NSE pkt c0000000
The NSE response to the CED tone event is not mandatory; some gateways may not send back an NSE
Note
response.
Originating-Gateway Example
When the Cisco ATA is used as an originating gateway for a fax session, make sure that the following
conditions are true:
•
•
•
An example debug log for an originating gateway scenario is shown below:
[0]Rx MPT PT=100 NSE pkt c0000000
[0]Tx MPT PT=100 NSE pkt c0000000
[0]codec: 4 => 0
[0]Rx MPT PT=100 NSE pkt c0000000
[0]Rx MPT PT=100 NSE pkt c0000000
If your gateway is using a legacy IOS software image, it may not send NSE packets but instead may rely
Note
on a straightforward codec switchover mechanism. In this case, a codec switchover event occurs rather
than an NSE packet transaction.
Possible Reasons for Failure
If your Cisco ATA does not receive CED-tone-event NSE packets and codec switchover does not occur,
the failure may be due to the following reasons:
•
OL-4804-01
The Cisco ATA transmits CED-tone-event NSE packets.
The encoder switchover to G.711 occurs during the NSE-packet transaction.
The Cisco ATA receives and responds to CED-tone-event NSE packets.
The NSE payload type is the same for the received and transmitted NSE packets.
The encoder switchover to G.711 occurs during NSE-packet transaction.
The terminating gateway is not configured with fax/modem pass-through.
Cisco ATA 186 and Cisco ATA 188 Analog Telephone Adaptor Administrator's Guide for H.323 (version 3.0)
Debugging the Cisco ATA 186/188 Fax Services
7-11
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