ATM cell header
The Flow Control field was reserved for compatibility with IEEE 802.6 DQDB MAN and is always set to zero in the User-Network Interface (UNI)
In the Network-to-Network Interface (NNI) this field is used to carry 4 more bits of VPI the (the most significant 4 bits)
Consequently, VPI has 8 bits in UNI and 12 bits in NNI
The VCI field always has 16 bits
Payload type has three bits indicating the following:
- user-generated cell
- forward congestion notification
- type indicator for higher layers (last cell in AAL 5)
Cell Loss Priority may be set by the ATM network to mark the packet ”discard-eligible” (DE)
The 8-bit CRC only covers the cell header