ayout deadline) arrives. Al- ally contain only a limited number of frames in order to though this introduces additional delay for packets ar- reduce the packetization delay. riving early, this mechanism ensures continuous media The network delay comprises the propagation delay playback. The buffering delay is the time a packet is held and the queuing delay across all links in the transmission in the buffer before it is played out. path. The propagation delay, which is a constant for a fixed Note that a trade-off exists between the average path, depends on the packet size and the speed of links, as buffering delay and the number of packets that have to be well as the length of the links. The queuing delay occurs dropped because they arrive too late (late loss). Sched- when a packet is queued behind some other packets uling a later deadline increases the possibility of playing waiting to be transmitted over the same link. The queuing out more packets and results in a lower loss rate, but at the Sender Playout Receiver buffer Encoder Packetization Network Depacketization and decoder dpack Figure 3.6-9 Total end-to-end delay in a typical real-time communication system. Real-time communication over networks CHAPTER 3.6 waveform repetition simply repeats the information Table 3.6-3 Coding delays for sample speech coders. contained in the packets prior to the lost one [20, 21]. A Speech Encoded Frame Look-ahead Decoding more advanced loss-concealment technique using time- coder bit rate size (ms) delay (ms) delay (ms) scale modification is described in [22] and [23] and can (kbit/s) be used in conjunction with adaptive playout scheduling in a low-latency scenario [17]. Waveform repetition G.711 0.125 typically does not introduce any algorithm delay as G.729A timescale modification typically does. However, it does not provide as good a sound quality [24]. G.723.1 5.3/6.4 18.75 For video communication, postprocessing is typically applied at the client side for error concealment and loss cost of a higher buffering delay. V