/mcpdial-peer is configured to use the default G.729 voice codec that compresses voice to 8 kbps, the fax rate voice setting would not allow fax calls to exceed this codec bandwidth. The fax would be limited to a bandwidth of 7200 BPS, even if it tried to initially negotiate at a higher bandwidth of 14400 BPS or 9600 BPS. A common complaint is that faxes that had completed within a certain time when connected via the PSTN now take twice as long. If a low bandwidth codec such as g729 has been configured with the default fax rate voice setting, this behavior is expected. With the fax rate command, it is possible to configure fax transmissions to use a bandwidth greater than the codec compression. The command fax rate 14400 allows fax calls to negotiate to a maximum of 14400 BPS regardless of the voice codec configured. This configuration resolves the problem of longer completion times. The main purpose served by the fax rate command within VoX networks is to provide deterministic bandwidth usage per call. The fax rate voice setting is the default because it ensures that both voice and fax calls use the same amount of bandwidth within the VoX network. This consideration is understood when the fax rate is changed to something greater than that of the codec bandwidth. In addition, some fax machines can operate more stably at a rate different from the default. In this case, the fax rate command can be used to test operation at different speeds. Note from the router output that fax relay can also be disabled if you issue the fax rate command. A valid troubleshooting technique is to disable fax relay and configure high bandwidth codecs such as G711. This technique is discussed in the "Troubleshooting" section under 6. Disable Fax Relay and Change Codec for Passthrough. fax-relay ECM disable Command The fax-relay ECM disable command is available for Cisco proprietary fax relay only and is issued to disable Error Correction Mode (ECM) negotiation between a pair of fax machines. ECM ensures that the faxed pages are transmitted error free and is a feature that is usually found on higher end models. Unfortunately, ECM has a low tolerance (approximately two percent) for jitter and packet loss, but when this negotiated feature is enabled, it can result in a higher fax failure rate in lossy VoX networks. Incomplete output on the termination fax is a symptom of failures due to packet loss. If both fax machines agree within the fax negotiation phase, ECM is enabled, but within fax relay the routers demodulate the fax tones into their true HDLC frame format. As a result, the routers are able to intercept and overwrite the field in the frame that indicates ECM status. If a fax machine transmits that it is capable of ECM, the router can change this parameter so that the other fax machine believes that ECM is not supported. Both fax machines are then forced to disable ECM, which means the fax data must be transmitted with standard T.4 data. Fax reliability is increased greatly with ECM disabled, even with much higher packet loss (about 10 percent)