digital telephony network. The echo canceller at the local device can- cels local echoes from the hybrid reflection, the echo canceller at the other end of the call eliminates the echoes you hear, and vice versa. The last part of digital voice technologies is noise generation. As it turns out, silence is not always golden. When speech stops, we generate comfort noise to help assure people that the "circuit" is still live, that the call hasn't dropped. The simple techniques involve playing white or pink noise or replaying the last receiver packet over and over. More sophisticated techniques include options in which the trans- mitter measures the local noise environment. The transmitter then sends a special comfort noise packet as the last packet before silence. The receiver generates noise based on the comfort noise packet. One of the major issues in carrying voice in a packet form, be it IP, Frame Relay, ATM, or MPLS, is delay, which is discussed in the following section. Delay, Jitter Buffers, and Error Concealment Table 9.1 shows some of the myriad sources of delay associated with VoIP. The two columns at the right are based on different codec standards. As you can see, the device sample capture introduces a bit of delay, followed by encoding delay, pack- etizing and framing delay, queuing delay, access uplink and downlink as well as transmission over the backbone, and various jitters that might be associated with the transmission, the decoding process, and ultimately, the playout process. The budget that results is fairly high. In G.711 at 64Kbps, the total delay is 94.6 milli- seconds, while with G.729 at 8Kbps, the total delay is 124.1 milliseconds. That's very large, especially when you consider the recommended standards for maxi- mum delay. The ITU-T, for purposes of voice, recommends a maximum delay of 100 milliseconds end to end, with a preference, of course, for much less. Delay is very important in real-time communications. For instance, for video, the ITU-T recommendation is 80 milliseconds maximum en