d-to-end delay. And when we get into some of the interactive applications such as teleimmersion or videogames, the tolerance for delay is only 30 to 50 milliseconds maximum end to end. There is a lot to consider in adequately addressing the QoS for voice from the standpoint of delay, both end to end and jitter. Chapter 9 IP Services Table 9.1 Standards for IP Voice Delay G.711/64Kbps G.729/8Kbps Source of Delay Budget (in ms) Budget (in ms) Device sample capture Encode delay (algorithmic delay + processing delay) Packetization/framing Move to output queue/queuing delay Access (up) link transmission Backbone network transmission Access (down) link transmission Input queue to application Jitter buffer Decode processing delay Device playout delay Total 124.1 As mentioned earlier in this chapter, speech is inherently isochronous. It occurs in real-time, requires a fixed amount of bandwidth, and is not very tolerant of delays or losses. Transmitters emit packets at fixed intervals, but packet net- works have queues. As a result, the receivers see interarrival jitter. What do we do about this? The solution is jitter buffers. There are two varieties: fixed jitter buffers and adaptive jitter buffers. Fixed jitter buffers work as follows: A playout point is set to some fixed number of packets in the future, and the hope is that no packets arrive later than the playout point. Late arrival equals lost packets, SO there is a del- icate tradeoff between delay-induced impairment and loss-induced impairment. Fixed jitter buffers are useful when delay doesn't matter, as with Internet radio, or with nonstop sources, such as fax. With adaptive jitter buffers, the ideal is to con- verge to minimal delay with zero loss. You need breathing room to move the play- out point to address talk spurt boundaries or silence. Adaptive jitter buffers can do fancy audio processing, such as speeding up or slowing down the rate. They rely on algorithms that adapt the playout point close to measured jitter, and if loss occurs, they back off. Anothe