/mcpMake sure these commands are entered on both gateways. These commands disable fax relay, disable echo cancellation, and force the call to use a high bandwidth codec without VAD. The router then samples the tones like a normal voice call, and, with the high bandwidth codec (G.711), the most precise sample possible is captured. The tone to be replayed on the other side is as accurate as possible. The caveat to this step is that, because G.711 is a 64 kbps bandwidth codec, each call consumes up to 80 kbps (for VoIP) when 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 fax passthrough workaround for the fax relay problem. Secondly, and more significantly, if bandwidth consumption is an issue, the problem has been isolated to the fax relay software, and a TAC case is to be opened. If this test fails, it is likely that whatever causes the fax calls to fail with fax relay also causes the failures with this test. What comes to mind first is that the network can have a large amount of jitter or packet loss. 7. Check for Packet Loss on the VoX Network The easiest and most accurate way to determine if there is a packet loss is to do this: Disable VAD on the VoX dial-peers. 1. Make a voice call between the same ports where the fax machines are connected. (Fax machines can serve as ordinary phones, or you can connect the handsets to the same ports where the fax machines are connected). 2. When the call is connected, do this: 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, which means that no packets are lost. If the counters are not equal, packets possibly become lost. Type the show voice dsp command several times at 30-second intervals to determine if the difference increases and packets are lost. a. Issue the show voice call summary command to see which port (and time slot if applicable) is allocated to the voice call. Type terminal monitor and then issue the show voice call command with the voice port (and time slot if applicable) 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 less than 20. If the counter(s) is higher than 20, investigate the packet loss. b. 3. If the network appears to be lossy, it is not reasonable to expect fax relay to work reliably. It is possible to disable ECM, but further investigation is probably needed to ensure that QoS is provisioned end-to-end so that the Voice and Fax relay traffic has priority and is never lost within the congestion. The Related Information section contains more information about how to troubleshoot voice quality problems. 8. Disable Fax Relay ECM (Cisco Proprietary VoIP Only) For networks with packet loss and lots of jitter, disable ECM to improve fax relay calls. Issue the command fax-relay ECM disable (discussed in more detail in the Configuration section of this document) to turn off ECM so that a larger amount of jitter and packet loss can be tolerated. Issue the fax-relay ECM disable command to improve fax relay performance in lossy networks, but this command is also recommended for basic troubleshooting. Even if there is not a noticeable jitter problem in the network, this command can sometimes help determine fax relay problems. This command is available under VoFR and VoATM dial-peers but currently works only for VoIP.