Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 209
Support for Time-Sensitive Apps

RTP supports sequencing, timestamps, synchronizes different streams (audio or video), and contains information describing RTP’s payload type. Information describing the payload type allows RTP to support multiple compression types such as MPEG and H.261. In fact, an RTP receiver can receive information that is encoded by two different methods and has the ability to produce one single stream from this. This is a process known as mixing. It is explained later.

A digitized audio signal may be produced using an simplex encoding scheme known as pulse code modulation (PCM). Say, for example, that PCM builds 160 byte packets every 20 milliseconds for a sampled voice stream. This information is transmitted through an Internet using IP. The digitizing of the voice signal is very sensitive to time. The reception of the stream of voice must be put back into the original timing in which it was transmitted; otherwise, there will be an uneven flow for voice at the receiver and it will not be received as it was spoken at the originator. IP does not care about timing, sequencing, or delays. It only has to deliver the data. IP will probably deliver these packets at different times and may deliver them in varying order. Therefore, an RTP application must put the packets back in the original order and reapply the timing between the packets. RTP provides information on this but does not accomplish this directly. Timestamps, which mark the relative beginning of the event, are provided with the packet and this provides enough information to the application to rebuild the original audio stream.

Support for Time-Sensitive Apps