ure 3-16). In the case of encoded audio, timestamps inform the receiver when to play the payload of the RTP packet through the speaker. A field called payload type describes which kind of data is transported in the RTP packet (such as audio encoded using a PCM codec). Besides bearing information about payload, RTP headers also contain identifiers for the source originating the payload. Let's revisit the example of voice conversation and unpack all the operations involved. The sender produces RTP packets with encoded audio as the payload. The receiver implements a buffer to store incoming RTP packets. They are ordered according to sequence number. An RTP packet is removed from the buffer when its timestamp indicates that it is time to play back its payload. If the timestamp of an incoming RTP packet indicates that its payload should have already been already played, the packet is discarded. There- fore, buffers should be long enough SO that packets have time to arrive Chapter 3 Figure 3-15 Effect of jitter in Hello voice transmissions. The Internet llo ... he before they have to be played. Additionally, buffers should be short enough so that the inevitable delay doesn't make normal conversation impossible. What if the packet has not arrived by the time it should be played? Some other sound is played instead. Interpolation techniques are available to create a smooth sound transition that is able to hide these gaps from the recipient. The Internet Multimedia Conferencing Architecture Figure 3-16 RTP couteracts the Hello effects of jitter and the arrival of out-of- sequence packets. Packet no: 1 Packet no: 2 Timestamp: 0 Timestamp: 1 The Internet Hello Packet no: 2 Packet no: 1 Timestamp: 1 Timestamp: 0 Real-Time Transport Control Protocol The previous example had just one media stream: an audio stream. Time- stamps can also be used to recover the original timing relationship of the data in multiple media streams; they perform this task for each media stream individually. For instance, in a videoconference, the times