The currently defined structure of an ATM cell at the user-network interface is indicated in the diagram below:
BIT
8 7 6 5 4 3 2 1
|-------------------------------------------|
| GFC | VPI | 1
|-------------------------------------------|
| VPI | VCI | 2
|-------------------------------------------|
| VCI | 3
|-------------------------------------------| OCTET
| VCI | PT | CLP | 4
|-------------------------------------------|
| HEC | 5
|-------------------------------------------|
| Cell Payoad (48 Octets) | 6-53
|-------------------------------------------|