he characteris- tics of voice traffic. First of all, voice traffic is an isochronous traffic flow, meaning it involves real-time communications that are delay sensitive as well as loss sensi- tive. Voice has a low bandwidth requirement, but as a tradeoff, it requires very high Digital Voice Technologies A number of digital voice technologies are enabling IP voice as we know it today. The primary component is the codec (coder-decoder), which is used to digitize the voice stream. Two broad categories of codecs are used: Waveform coders-Waveform coders directly encode speech in an efficient way by exploiting temporal and/or spectral characteristics. There are sev- eral categories of waveform coders. The first is time domain, or predictive, coders, and these include the well-known standards G.711, known as Pulse Code Modulation (PCM; u-Law in the North American and Japanese stan- dards, using 56Kbps, and A-Law for those following the ITU standards, at 64Kbps), and G.726, known as Adaptive Differential PCM (ADPCM), which encodes voice at a reduced rate of 32Kbps, 24Kbps, or 16Kbps. Two other types of waveform coders are G.722 subband coders and transform coders, which are used to convert between different digitizing techniques. Source coders, or vocoders-Vocoders estimate and efficiently encode a parametric representation of speech. The standards under this category include G.728, which is also known as Low-Delay Code-Excited Linear Prediction (Low-Delay CELP) and supports digital voice at 16Kbps; G.729, known as Conjugate Structure Algebraic CELP, which reduces voice down to 8Kbps; and G.723.1, known as Multipulse Maximum Likelihood Quanti- zation, which carries voice at 6.4Kbps. Another element of digital voice technologies is echo cancellers, which are used to eliminate echoes. The voice echo path is like an electrical circuit: If a break or a cancellation is made anywhere in the circuit, an echo canceller can eliminate the echo. The easiest place to make the break is with a canceller looking into the local analog/