VoIP Voice and Fax Signal Processing

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FAX OPERATION ON PSTN, MODULATIONS, AND FAX MESSAGES

Fax is an abbreviation for facsimile. To access the public switches telephone network (PSTN), an analog two-wire TIP-RING interface is used for voice and fax communication. The PSTN may use analog, or a combination of analog and digital operations, for end-to-end transmission of voice and fax signals. The fax data that are in bits and the digital numbers that form inside the fax machine are modulated and created as digital samples. These samples are converted to an analog voice band of a 300- to 3400-Hz signal before delivering on the TIP-RING interface. TIP-RING interfaces are connected to the PSTN lines. High-level representation of a PSTN-based fax call is represented in Fig. 14.1. In PSTN calls, fax and voice share the same telephone lines. At the digital hierarchy of PSTN, fax or voice samples are sent as G.711 compression bit streams and take 64 kilobits per second (kbps) per call. The PSTN central office (CO) or digital loop carrier (DLC) converts an analog signal to a digital bit stream.

Fax data are divided into four groups depending on the image formatting and handling method used inside the fax machine. Each group of fax machines will take different time durations to send the same A4 size fax page or document.

Group-1 (G1) became a fax recommendation in 1968 for analog facsimile devices to communicate over analog telephone lines. Group-1 is based on the ITU-T-T.2 recommendation. It takes six minutes to transmit ...