Fig. 11.31. ATM header structure.

Various fields of the ATM header are explained below:

Generic Flow Control (GFC). GFC is 4 bit field. It can be used to provide local functions such as identifying multiple stations sharing a single ATM interface. This is currently not used.

Virtual Path Identifier (VPI). It is a 8 bit field used with VCI to identify the next destination of a cell as it passes through a series of ATM switches.

Virtual Channel Identifier (VCI). It is a 16 bit field. With VPI, it is used to identify the next destination of a cell as it passes through a series of ATM switches.

Payload type (PT). It is a 3 bit field. The first bit indicates whether the payload is data or control data. The second bit indicates congestion, and third bit indicates whether the cell is the last cell in the series.

Congestion loss priority (CLP). It is just a one bit field. It indicates whether a cell should be discarded if it encounters extreme congestion. This bit is used for QOS.

Header Error Control (HEC). It is an 8 bit field. This 8 bit CRC detect all single errors and certain multiple bit errors.

The only difference between UNI header and NNI header is that UNI has a GFC field of 4 bit and VPI of 8 bit and NNI has no GFC field but VPI is 12 bit field. The 4 bit GFC field in UNI as used to signal to the user the need for flow control. The NNI uses these bits to expand the VPI field.

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