VoIP Voice and Fax Signal Processing

14.5 FAX AND MODEM TONES BASICS

Group-3 fax machines work up to 14400bps. A fax call starts with a 1100-Hz calling (CNG) tone from the originator and a 2100-Hz ANS acknowledgment tone from the responder. These tones are used to detect the fax call in VoIP. The V.21 preamble is generated immediately after the ANS tone from the responding fax machine. The V.21 preamble indication is also used in addition to the CNG/ANS tones for detecting the fax call. In Fig. 14.4, the tones are indicated in the beginning of the call. SG3 fax machines use a V.34 modem for high speed that generates an answering tone consisting of amplitude and 180° phase reversal modulation (/ANSam). In this case, the fax detector has to support the detection of an /ANSam detection indication to switch to a fax call. Some fax machines send modem tones resulting from various options and violations in implementations. To cater to these impairments, it is necessary to analyze the combinations of these tones, timings, and V.21 preamble detection to arrive at a final decision on fax and modem. Many deployments do not use a modem in VoIP. It is suggested to favor the decision to fax when any fuzzy conditions occurs between fax and modem detections.

14.5.1 CNG Tone

CNG is the calling tone generated as a first indication of a fax call from the originating fax machine. The CNG tone is a pure tone with a frequency of 1100 ± 38 Hz (tone frequency tolerance is ± 38 Hz) with 0.5 seconds ON and 3 seconds OFF. The timing tolerance ...