The User Network Interface (UNI) is the customer interface. It occurs in all network concepts and forms the interface between the provider 's devices and the user's terminals. Depending on the network concept, the UNI interface supports different services and protocols. User interfaces exist in access, city and wide area networks, in optical networks, Carrier Ethernet, Next Generation Networks (NGN), ISDN or ATM.
In Carrier Ethernet, the User Network Interface (UNI) forms the customer or provider side interface between the Metro Ethernet network and the subscriber device, the Customer Equipment (CE). In this case, the UNI interface uses the standardized Ethernet data rates of Ethernet, Fast Ethernet, Gigabit Ethernet, and 10 Gigabit Ethernet.signaling specifications adopted by the ATM Forum. They compare to the Q. 2931 signaling protocol for ITUB-ISDN signaling. compatibility with Q.2931. This version is not compatible with version 3.0, but it forms the basic specification for all future versions, which will in any case be downward compatible with V3.1. The specification for UNI signalling with version 4.0 has also been adopted.