7.3.3 Recovering from Packet Loss

Recovering from Packet Loss

Schemes that attempt to preserve acceptable audio quality in the presence of packet loss. Such schemes are called loss recovery schemes. Retransmitting lost packets is generally not appropriate in an interactive real-time application. Retransmitting a packet that has misssed its playout deadline serves absolutely no purpose. And retransmitting a packet that overflowed a router quee cannot normally be accomplished quickly enough.

Two types of loss anticipation schemes are forwards error correction (FEC) and interleaving.

Forward Error Correction (FEC)

The basic idea of FEC is to add redundant information to the original packet stream. For the cost of marginally increasing the transmission rate of the audio of the stream, the redundant information can be used to reconstruct approximations or exact versions of some of the lost packets.

Interleaving

Interleaving can significantly improve the perceived quality of an audio stream. It also has low overhead. The obvious disadvantage of interleaving is that it increase latency. This linits its use for interactive applications althoughit can perform well stored adio. A major advantage of interleaving is that it does not increase the bandwidth requirements of a stream.

Receiver-Based Repair of Damaged Audio Streams

Receiver-based recovery schemes attempt ot produce a replacement for a lost packet that is similar to the original. This is possible since audio signals, and in particular speech, exhibit large amounts of short-term self-similarity. These techniques work for relatively small loss rates length of a phoneme these techniques break down, since whole phonemes may be missed by the listener.

Perhaps the simplest form of receiver-based recovery is packet repetition. Packet repetition replaces lost packets with copies of the packets that arrived immediately before the loss. It has low computational complexity and performs reasonably well. Another form of recerver-based recovery is interpolation, which uses audio before and after the loss to interpolate a suitable packet to cover the loss. Interpolation performs somewhat better than packet repetition but is significantly more computationally intensive.