VoIP Voice and Fax Signal Processing

7.7 DTMF REJECTION

During a voice call, DTMF tones are sent to the destination to allow various interactive operations. In PSTN calls, DTMF is not distorted as compared with VoIP. In VoIP, an end-to-end call may use a compression codec such as G729AB, which results in distortion of the DTMF signal. In packet delay or lost situations, DTMF transmission is disturbed and may not necessarily be detected as a valid digit at the destination. Packet loss creates discontinuity in DTMF transmission even with G.711-based VoIP calls.

The preferred approach in VoIP is to send DTMF as out-of-band packets with redundancy. The digit value, and its power, duration, digit starting, and digit ending information are sent as message packets to the destination. RFC2833 [Schulzrinne and Petrack (2000)] mainly provides guidelines for sending digits as packets. Digits sent this way are called out-of-band packets or out-of-band digits. At the destination, these packets are used to regenerate DTMF digits with right parameters. Before this method can be used, initial negotiation is needed to establish that both VoIP end terminals support out-of-band digit transport through RFC2833.

Additional difficulties with out-of-band support exist. At the source, DTMF detection takes several tens of milliseconds, and during this period, a significant part of the DTMF tones have already transmitted in RTP packets. These in-band tones (inside the IP voice packets) can reach the destination with possible distortion depending ...