Acronym

Name

Width

Description

VPI

Virtual Path Identifier

8 bits in UNI 12 bits in NNI

This field identifies a group of connections which share a common path and are used to provide the same quality of service.

VCI

Virtual Channel Identifier

16 bits

This field identifies a specific connection existing within a virtual path.

GFC

Generic Flow Control

4 bits

This field passes rate control information over a User Network Interface (UNI) from the network towards the end-system.

HEC

Header Error Control

8 bits

This field stores a checksum that is calculated over the entire header. This field supports the correction of single-bit errors and the detection of multiple bit errors in the cell header. This value is recalculated every time a field value changes in the cell header.

PT

Payload Type

3 bits

This field identifies special user data cells and also enables control cells.

CLP

Cell Loss Priority

1 bit

This single bit field enables low priority cells to be distinguished from high priority cells within the same connection. This bit is used to explicitly indicate cell loss priority and may be set by the user or by the network service provider to indicate lower priority cells. Cells with CLP set are subject to discard when congestion is experienced.