/mcpSome capabilities that are exchanged and negotiated are page transmission speed, Error Correction Mode (ECM), resolution, page coding, and scan time. Page transmission speed (training) is an important negotiation that determines the speed at which the fax sends its information. Faxes try to train at the highest modulation speed possible based on the parameters exchanged initially. Fax devices retrain to a lower speed if the training at a higher speed fails. Page transmission occurs when the training part of the fax negotiation phase is complete with the use of the previously agreed upon parameters. The page information is coded into scan lines with a standard resolution of 203H x 98V dots per inch. Fax images are typically compressed and encoded with either Modified Huffman (MH) or Modified Read (MR) encoding. MH usually compresses at a 20:1 ratio. MR encoding typically provides a 20 percent compression improvement over MH but is slightly less resilient to error. When page transmission occurs, a bit rate is used that is higher than the initial 300 BPS that is used in the call setup negotiation. The bit rate used for the page transmission is confirmed within training. These are some of the common rates used in fax page transmission: V.27ter – 2400/4800 BPS • V.29 – 7200/9600 BPS • V.17 – 14400 BPS • Note: These V.XX specifications used for page transmission (V.27ter, V.29, V.17) and fax negotiation (V.21) are specifications that define how digital data is to be sent over analog phone lines. Data modems are also able to use these specifications even though most data modems have migrated to much faster speeds. Fax Relay Basics Fax relay is a technique used to overcome the deficiency in high compression voice codecs (G729, g723, and the like) when these codecs try to pass fax traffic. Because a fax call is treated as if it is a regular speech call, the DSP in each gateway is put into voice mode, after which human speech is expected to be received and processed. Within the life of the call, if a fax answer (CED) or call (CNG) tone is heard, the DSP does not interfere with the speech processing. It allows the tone to continue across the VoX call leg.