SIP server (9) ACK Jsmith (10) RTP Audio/Video data user agent user agent Figure 3.6-8 A call setup process using SIP. delay component has to be minimized and trade-offs have delay is a random variable depending on the packet size, to be considered in optimizing the overall system design. traffic load and characteristics of the route, and the Encoding delay is introduced during the data com- scheduling scheme. Advanced resource allocation and pression process. For speech coders, encoding delay scheduling schemes such as Resource Reservation Pro- usually includes the frame size and look-ahead delay. tocol and Differentiated Services enable prioritization of Look-ahead delay is the time spent in processing part of audio and video packets and can efficiently reduce the next frame SO that a correlation between successive queuing delay for these real-time data streams. frames can be exploited. Typically, more advanced codecs Varying queuing delay, typically caused by congestions achieve higher compression efficiency at the cost of of links in the route and related to the queuing mecha- higher encoding delays. Decoding delay is introduced nisms, introduces delay jitter, which is usually unknown during the data decompression process. Table 3.6-3 lists and random. Due to delay jitter, IP packets are sent pe- the coding delays for some common speech coders. riodically but are received in irregular patterns. For this Packetization delay is the time spent in collecting reason, a playout buffer, also referred to as a dejitter sufficient data frames to form the payload of an IP buffer, is employed at the receiver to absorb the delay packet. Since the packet headers have a fixed size, jitter before media are output. When using a playout a larger payload size reduces the header overhead and buffer, packets are not played out immediately after improves the transmission efficiency. However, due to being received but are held in a buffer until their the stringent latency requirement, the payload can usu- scheduled playout time (pl