Bits
Byte in header
ATM cell header structure
8
7
6
5
4
3
2
1
GFC* / VPI
VPI
VPI
VCI
VCI
VCI
PTI
HEC
GFC  Generic Flow Control    HEC Header Error Control
VPI   Virtual Path Identifier    PTI Payload Type Indicator
VCI   Virtual Channel Identifier    CLP Cell Loss Priority
* GFC at User to Network Interface (UNI)
CLP