Cisco Fax Relay

Cisco Fax Relay

Figure 2-43 depicts a VoIP network set up for fax relay. Initially, fax calls are digitized representations of the contents on paper. The digitized bit stream is then converted to analog for transmission over voice circuits. If Cisco equipment treated fax calls like voice calls, the analog waveform would then be converted to G.711 PCM at 64 kbps and subsequently compressed before transmission across the VoIP network. Treating fax calls like voice calls is impractical because there are too many conversions and because the coding and compression schemes are designed to convey human speech, not fax modem tones.

Figure 2-43. Fax Relay

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Using Cisco fax relay, as shown in Figure 2-44, the DSP (digital signal processor) chip first sets up the call as an end-to-end voice call. The DSP then recognizes the tones as those coming from a fax machine. The local DSP assumes the role of a fax modem, converting the analog data back to the original digitized bit stream. Acting as the fax modem, the DSP is downshifted in speed to 9.6 kbps to save bandwidth over the IP path. The bit stream is then packaged in VoIP packets and identified as a fax. The remote DSP assumes a similar role and converts the bit stream to analog for reception by the remote fax machine. Cisco fax relay is a proprietary protocol supported only on Cisco voice equipment. Cisco Systems pioneered this protocol in the early 1990s before standards were developed and ratified.

Figure 2-44. Cisco Fax Relay

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