In a VoIP G.711 call, several modules such as echo canceller, VAD/CNG, PLC, and adaptive jitter buffer (AJB) will be used for voice enhancements. For a successful fax and modem pass-through call, these modules operations have to be properly controlled. The guidelines for a successful pass-through call are given below [URL (Cisco-fax4)].
Codec Switching. A compression codec has to be switched or renegotiated for the G.711 codec with either PCMU or PCMA. In some countries, default preferences will be selected based on the local country PSTN standard. In Europe, PCMA is preferred, and in North America, PCMU is preferred.
Direct Current (DC) Blocking High Pass Filters. In some voice processing implementations or as part of codec processing, DC blocking high-pass filters are used. In pass-through mode, these filters have to be bypassed to minimize distortions. In some designs, these filters are used after characterizing for fax and modem pass-through modes of operation.
Packetization. For the available bandwidth, selected packetization has to be as small as possible. In pass-through mode, 10 ms packetization will be the preferred option, and more than 20 ms should not be considered. Higher packet intervals like 30 and 40 ms will create a long silence in the case of packet drop. Modem and fax will create a disconnect on such long interruptions or will fall back to slower speeds of 2400- or 4800-bps support.
Echo Cancellation. An ...