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.
ATM is based on pure circuit and packet switching technique. ATM connection is having VPI and VCI concept. VPI stands for Virtual Path Identifier and VCI stands for Virtual Connection Identifier. The VPI/VCI field is 24 bits long in UNI 28 bits in NNI. VPI field is 8 bits long in UNI and 12 bits long in NNI. The VCI field is 16 bits long in UNI as well as in NNI. It means we can have only 2^8=256 VPI connections in UNI and 2^12=4096 connections in NNI. Each UNI and NNI VPI is having 65536 VCI connections.
VPI connections can be assigned from any number but some limitation is there for assigning VCI connections. VCI 0 – 15 are reserved by ITU-T, 16 – 31 are reserved by ATM forum and 32 to 65535 are free and could be used by users.
Virtual connection is the combination of both VPI and VCI. VPI and VCI values are always local to the ATM devices and have nothing to do with the remote. Intermediate ATM switches translate the incoming and outgoing VPI and VCI values along with their ingress and egress ports. This operation is referred as label swapping.