VoIP Voice and Fax Signal Processing

16.4 OVERVIEW ON REDUNDANT AND DUPLICATE FAX PACKETS

Duplicate packets are the same packets sent multiple times with the same sequence number. Redundancy packets use an increasing sequence number and secondary payloads. Some implementations provide indicator packets as single events. Similar issues for single packets are presented in Chapter 4 while dealing with isolated voice activity device (VAD) packet transport on the network. In this situation, duplicate packets are used to emulate the benefits of redundancy.

In a redundancy-based scheme, packets are sent with present and previous payloads. The redundancy technique is given in RFC2198. Redundancy techniques are popularly used in fax pass-through and T.38-based fax data transmission. Fax signals are composed of phase and frequency modulations. In fax transmission, packet loss concealment (PLC) techniques are not useful because of the limitations in PLC implementations to recover phase and frequency modulations.

Table 16.5(a) shows the basic principle of redundancy. In this example, each frame is considered for a duration of 10 ms. In row-1 of the table, frames marked as 10, 11, 12, and 13 are transmitted for every 10 ms from the transmitter side to the IP network. In row-2, packets with redundancy = one that groups a current 10-ms frame and previous 10-ms frame as one payload. In row-3, the previous two frames and the current frame is transmitted as one payload for redundancy = two. Redundancy = one takes care of one packet ...