Buffer synchronization and startup can be handled by means of time stamps plus an a priori constraint on the maximum buffer size, rather than explicit use of the VBV values.

In the system timing model diagram, the encoder buffer and decoder buffer represent the only delay elements of the timing model.  Since the encoder is instantaneous, and inserts access units at a constant rate, while the decoder similarly extracts access units at a constant rate,  the sum of encoder buffer delay and decoder buffer delay is a constant. Note that the total delay is constant even if the encoder buffer output does not have a constant bit rate, because the transmission medium is modeled as an instantaneous transfer from the encoder buffer to the decoder buffer. The constant total delay and instantaneous transfer across the transmission medium also means that the decoder buffer fullness is the mirror image of the encoder buffer fullness.

In general, the encoder buffer output is not at a constant bit rate, and if a  constant rate is needed by a transmission medium, it is achieved in the transport layer by insertion of null packets or opportunistic data.

First we will study the case of constant bit rate transmission, and then comment on the variable bit rate case.

Subtopics are:

• Constant Bit Rate Operation

• Variable Bit Rate Operation

• Startup

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