Following the VPI and the VCI is the Payload Type Indicator (PTI). This parameter indicates the type of data found in the payload portion of the cell. Remember that the payload is not always data. There could be signaling information, network management messages, and other forms of data. These are identified by the PTI.

The PTI is followed by the Cell Loss Priority (CLP) parameter. This is used to prioritize cells. In the event of congestion or some other trouble, a node can discard cells that have a CLP value of 1 (considered low priority). If the CLP value is 0, the cell has a high priority and should only be discarded if it cannot be delivered.

The last byte in the header is the Header Error Control (HEC) parameter, which is used for error checking and cell delineation. Only the header is checked for errors. The HEC works like other error checking methods, where an algorithm is run on the header and the value placed in the HEC. ATM is capable of fixing single bit errors but not multiple bit errors.

An ATM node places itself in error correction mode during normal operation. If a single bit error is detected in the header, the data in the header is small enough that the error correctio algorithm can determine which bit is in error and correct it. If a multibit error is detected, the node places itself in error detection mode. Errors are not corrected while in this mode. The node remains in error detection mode as long as cells are received in error. When cells are received without error, the node places itself back into error correction mode.

NEXT PAGE: Routing in ATM - VPI/VCI

Copied with permission,McGraw-Hill Telecommunications from the book Telecommunications Protocols, by Travis Russell, 1997 McGraw-Hill Telecommunications, pages 372-386.