ATM packet is also known as CELL. It has a fixed size of 53 bytes, which consist 48 bytes of payload and 5 bytes of header. CELL header is further divided into types of format:-
1. UNI Cell Format
2. NNI Cell Format
UNI Cell Format aka User Network Interface, which is directly connected to any ATM switch and responsible to send and receive atm cells.

NNI Cell Format aka Network to Network Interface; the interface between any two ATM switches.