An ATM cell is 5 bytes of header and 48 bytes of data. Look at Figure 5-1 to see the ATM UNI cell format.
The cell format depicted in Figure 5-1 is the User-Network Interface (UNI) cell. The Network-Node Interface (NNI) header is almost identical to this one, except for the GFC field, which has been omitted. Instead, the VPI field occupies the first 12 bits and is thus 4 bits longer, which allows the ATM switches to assign a larger number of virtual path identifiers (VPI).
Table 5-1 shows the name and meaning of each field of the ATM cell header.