VoIP Voice and Fax Signal Processing

15.3 FAX BASIC FUNCTIONALITY AND DETECTING FAX CALL

In this section, a summary on fax functionality mapped to PSTN and VoIP are presented. This section has an extended summary of Chapter 14 for creating continuity to map a PSTN fax call to VoIP. A fax scans a page and generates binary bits of “ones and zeros,” which are modulated using amplitude, phase, and frequency variations depending on the selected modulation. These modulations are within the voice band frequencies of 300 to 3400 Hz. A PSTN network works as a transparent line for the voice band signals between two fax machines. At destination, the fax machine demodulates the analog signal to extract the bit pattern. These bits are finally composed as an image for printing.

A group-3 (G3) fax machine generates a 1100-Hz calling (CNG) tone and a 2100-Hz called terminal identification tone (CED) or answer tone (ANS) at the beginning of the fax call. The CNG name in voice is comfort noise generation of voice processing. In fax, the CNG name is for the calling tone. Some V.34 super group-3 (SG3) fax machines generate a call indication (CI) signal in place of the CNG tone. CI is a V.8 signal used as an alternative to call tone (CT), which is usually generated in the modem call setup phase. On detecting the CNG tone, the receiving SG3 fax terminal responds with an /ANSam tone. Signal /ANSam is a 2100-Hz answering tone with an amplitude-and phase-modulated tone.

The CNG tone is generated from the originating fax machine, and the CED ...