Cisco BTS 10200 Softswitch Посібник користувача - Сторінка 20

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Troubleshooting Fax Relay

Disable T.38 fax relay and change codec for passthrough.

Step 6

In the previous troubleshooting steps you established that voice calls work, faxes work through PSTN,

and all digital interfaces in the fax relay path are free from errors. This step determines whether faxes

can go through with T.38 fax relay disabled.

Under the dial-peers, enter the following commands on both media gateways :

gateway-name(config)#voice-port 2/0:15

gateway-name(config-voiceport)#no echo-cancel enable

gateway-name(config)#dial-p voice 3

gateway-name(config-dial-peer)#fax rate disable

gateway-name(config-dial-peer)#codec g711ulaw

gateway-name(config-dial-peer)#no vad

These commands disable fax relay, disable echo cancellation, and force the call to use a high bandwidth

codec without voice activity detection (VAD). The gateway samples the fax tones like a normal voice

call. With the high bandwidth codec (G.711), the most precise sample possible is captured and the tone

replayed on the other side is as accurate as possible. Be aware that the G.711 is a 64 kbps bandwidth

codec, so each call consumes up to 80 kbps when the additional transport protocol overhead is added.

If this test is positive, two things have been accomplished:

First, if per call bandwidth consumption is not a major issue for the network, there is now a potential

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fax passthrough workaround for the fax relay problem.

Secondly, and more significantly if bandwidth consumption is an issue, the problem has been

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isolated to the T.38 fax relay software and you should open a case with the Cisco TAC.

If this test fails, it is very likely that whatever is causing the fax calls to fail with T.38 fax relay is also

causing the failures with this test. The network may have a large amount of jitter or packet loss.

Check for packet loss on the network.

Step 7

The easiest and most accurate way to determine if there is packet loss is to do the following:

Disable VAD on the packet network dial-peers.

Make a voice call between the same ports where the fax machines are connected. (Fax machines may

be able to serve as ordinary phones or you might need to connect handsets to the fax ports).

When the call is connected:

If the network appears to be lossy, it is not reasonable to expect T.38 fax relay to work reliably. Further

investigation is probably needed to ensure that QoS is provisioned end-to-end so that the voice and T.38

fax relay traffic has priority and is never lost during congestion.

Cisco BTS 10200 Softswitch T.38 Fax Relay Support

Issue the show voice dsp command. You can see in the output that one of the DSP channels has

the configured codec loaded. Usually the column "TX/RX-PAK CNT" shows that the transmit

and receive packet counters are equal, meaning no packets are being lost.

If the counters are not equal, packets might be getting lost. Type the show voice dsp command

several times at 30-second intervals to determine if the difference increases. If so, then packets

are being lost.

Issue the show voice call summary command to see which port (and timeslot if applicable) is

allocated to the voice call. Type terminal monitor then issue the show voice call command

with the voice port (and timeslot if applicable) specified to get the detailed DSP statistics. In

the "***DSP VOICE VP_ERROR STATISTICS***" section of the output, look for the

counters. They are usually 0 or below 20. If the counter(s) is higher than 20, investigate the

packet loss.

Cisco BTS 10200 Softswitch T.38 Fax Relay