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Troubleshooting Fax Relay
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Use debug commands on the gateways to determine what is happening.
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Describing the details of debug outputs is beyond the scope of this document, but described below
are the basic transactions that are commonly seen when debugging T.38 fax relay. For more in-depth
information on T.30 messaging or for information on messages that are not listed below, see the T.30
specification.
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Trace information may look different in different versions of Cisco BTS 10200 Softswitch software, but
you should be able to understand whether the fax codec was loaded correctly and which fax protocol was
used by the output following the "FAX" keyword.
If the fax codecs were not loaded correctly, there is probably a software problem. Make sure that the
latest release is used and, if there is still a problem, open a case with the Cisco TAC.
Cisco BTS 10200 Softswitch T.38 Fax Relay Support
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Try different hardware—
If you have another voice gateway with FXS ports available to you, try to connect it directly
with the Ethernet crossover cable to each of the voice gateways and send a fax using the fax
machine connected to the FXS port. This procedure will help determine if there are problems in
the packet network such as queuing, fragmentation, or prioritization.
The fax transmission starts with the exchange of DIS/DCS (CSI,DIS / TSI,DCS) messages:
DIS is the initial message stating the capabilities of the answering end. The accompanying
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CSI frame has the phone number.
DCS defines the transmit parameters and starts an image transfer sequence. The
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accompanying TSI frame has the phone number.
The fax machines go into training mode and can make several attempts to agree on a
transmission speed. For instance, the fax machines may first train to a speed of 9600 bps, fail,
and then train to a speed of 7200 bps.
Successful training is followed by a CFR message.
Transmission starts following the CFR message.
If there is a high number of errors, a good fax analyzer will detect them.
If the terminating fax machine considers the error rate too high, it will terminate the connection.
MCF is the normal response to an end-of-image message sequence by the receiving end. It
indicates that the image was received with less than five percent of lines in error. It is normally
followed by the DCN (disconnect) message.
If there is no MCF message, the transmission was not completed successfully. It may suggest a
high error rate caused by one of the following:
Digital line errors (clocking, cabling)
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VoIP packet loss (queueing, prioritization, fragmentation, compression)
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Hardware fault
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Cisco IOS/DSPW incompatibility (rarely occurs) ¨
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If the DIS or DCS messages are retransmitted several times, it may be that they are not passed
across the VoX connection correctly or only passed in one direction—a software or
configuration problem.
If training repeats multiple times, each time at a lower speed and then the transmission fails, it
may be that the fax codec was not loaded and the voice gateways handle the fax transmission
as a normal voice conversation, again a configuration or software issue.
Cisco BTS 10200 Softswitch T.38 Fax Relay