Hi Neil,

The max transfer rate for a fax call is usually not dependent on the DSP hardware but instead the transfer rate is more dependent on the transport method selected by the user and programmed into the DSP.

The DSPs used in Cisco voice products are able to handle the maximum G3 fax speed of 14.4 kbps and SG3 (V.34) speed of 33.6 kbps. To obtain the maximum G3 fax speed you can choose from the various fax relay and passthrough options available. However, be aware that the "fax rate" command can limit the maximum fax speed depending on the configured voice codec. To ensure that the maximum G3 fax speed is achieved for both Cisco fax relay and T.38 fax relay make sure that the "fax rate" is set to 14400.

To achieve the maximum SG3 speeds over IP, then the only option is NSE-based passthrough. However, you should be aware that support for SG3 over T.38 fax relay should be released sometime next year.

Regards,

David