Mobile communication systems typically offer a constantbitrate radio link for real-time media applications as long as the
radio resources of the cell do not have to be reallocated. Consequently, the bitrate variation of video results into varying
delay, which can be smoothed out by buffering in the receiver.The Hypothetical Reference Decoder (HRD) models, specified in the video coding standards, constrain the variation of the video bitrate among other things. Video bitstreams are associated with an initial buffering delay, specified relative to the operation of the HRD. The initial buffering delay is
sufficient for smoothing out the bitrate variation when transmission over a constant-bitrate channel is assumed. In some applications, such as video telephony, it may be more acceptable to allow some amount of jitter in output timing rather than a constant initial delay, to minimize the end-to-end delay. The amount of tolerable latency depends on the application and user expectations. For example, in broadcast applications, startup latency should enable subjectively reasonable channel switch time.