Packet loss concealment (PLC) is a technique to mask the effects of packet loss in VoIP communications. When the voice signal is sent as VoIP packets on a TCP/IP network, the packets may (and likely will) travel different routes. A packet therefore might arrive very late, might be corrupted, or simply might not arrive at all. One example case of the last situation could is when a packet is rejected by a server which has a full buffer and cannot accept any more data. Other cases include network congestion resulting in significant delay. In a VoIP connection, error-control techniques such as ARQ are not feasible and the receiver should be able to cope with packet loss. Packet loss concealment is the inclusion in a design of methodologies for accounting for and compensating for the loss of voice packets.
Zero insertion: the lost speech frames are replaced with zero
Waveform substitution: the missing gap is reconstructed by repeating a portion of already received speech. The simplest form of this would be to repeat the last received frame. Other techniques account for fundamental frequency, gap duration etc. Waveform substitution methods are popular because of their simplicity to understand and implement. An example of such an algorithm is proposed in ITU recommendation G.711 Appendix I.
Model-based methods: an increasing number of algorithms that take advantage of speech models of interpolating and extrapolating speech gaps are being introduced and developed.