. The absolute silence of gap may disturb a listener. In this case, often artificial noise is inserted. There are packet loss concealment (PLC) procedures which can camouflage gaps in the output voice signal. The simplest techniques require a little extra processing power, and the most sophisticated techniques can restore speech to a level approx- imating the quality of the original signal. Concealment techniques are most effective for about 40 to 60 msec of missing speech. Gaps longer than 80 msec usually have to be muted. One of the most elementary PLCs simply smoothes the edges of gaps to eliminate audible clicks. A more advanced algorithm replays the previous packet in place of the lost one, but this can cause harmonic artifacts such as tones or beeps. Good concealment methods use variation in the synthesized replacement speech to make the output more like natural speech. There are better PLCs to preserve the spectral characteristics of the talker's voice and to maintain a smooth transition between estimated signal and surrounding original. The most sophisticated PLCs use CELP (codebook-excited linear predictive) or similar technique to determine the content of the missing packet by examining the previous one [11]. Lost packets can be detected by packet sequence numbering. 15.5 MEDIA GATEWAY CONTROLLER AND ITS PROTOCOLS 15.4.5 Echo and Echo Control Echo is commonly removed by the use of echo cancelers and are incorporated on the same DSP chips that perform the voice coding. A good source of information and design of echo cancelers is ITU-T Rec. G.168 [12]. However, most vendors of VoIP equipment have their own proprietary designs. A common design approach is to have the echo canceler store the outgoing speech in a buffer. It then monitors the stored speech after a delay to see whether it contains a component that matches up against the stored speech after a delay. If it does, that component of the incoming speech is canceled out instead of being passed back to the user since it is an echo of what the user ori