The End System Identifier(ESI) is the ATM terminal address. It is a six- byte or 48-bitdata field within the ATM addresses, which together with the one-byte selector( SEL) forms the user part of the address.
With its 48 bits, the ESI data field corresponds to the MAC address in local networks and is used to distinguish between different nodes. The End System Identifier (ESI) is also used in the address according to IEEE 802.
The ESI signal is important for detecting the network constellation. Namely, as soon as a station is connected to the ATM network, it sends its device-specific ESI signal to the ATM switch, which then responds with the network part. The terminal adds the network part to its address and sends the complete ATM address to the ATM switch.