VoIP Voice and Fax Signal Processing

5.3 TRANSMITTER- AND RECEIVER-BASED TECHNIQUES

In the transmitter-based scheme, the transmitter assumes that a certain packet drop could occur in the end-to-end operation. The transmitter sends additional payloads or modifications that help the receiver (decoder/destination) to recover the lost packets. The popular options for these payloads are redundancy and FEC. Retransmission with the TCP-based method is possible for listening and for the broadcast mode of voice communication, but it is not used in interactive voice conversations.

5.3.1 Retransmission or the TCP-Based Method

Retransmission is a TCP-based method. In principle, retransmission-based methods work for any end-to-end media, signaling, or data packets. A lost packet is identified at the destination, and a request is initiated to the transmitter for retransmission of the lost packet. The longest delays of 300 to 400 ms will happen with inter-regional voice calls. As per calculations given in RFC0793, RFC2988 [Postel (1981), Paxson and Allman (2000)], inter-regional calls create a retransmission time-out of 1.0 to 1.6 seconds. Interactive audio applications have tight latency (delay) bounds. End-to-end delays need to be as low as possible and preferably in the upper limit range of 150 to 250 ms with the exception of intermediate satellite links and hard-to-reach areas. For this reason, interactive voice applications typically do not employ retransmission-based recovery for lost packets. Fax tolerates more end-to-end ...