There are two kinds of interfaces in ATM. An interface that connects two or more networks, called Network to Network Interface ( NNI ) and an interface to connect the user to the network, called User to Network Interface ( UNI ) . It is envisioned that the ATM network service providers may offer several types of interfaces to their networks. One interface that is likely to be popular with companies that build routers and bridges for local area networks is a Frame based interface. One or more of the IEEE 802.X or FDDI frames may be supported at the UNI, with frame to ATM cell conversion and reassembly being done inside the UNI at the source and destination end points respectively. Thus a gateway host on a local area network might directly connect its ethernet, token ring, fddi, or other LAN/MAN interface to the UNI, and thus bridge two widely separated LANs with an ATM backbone network. This will preserve the existing investment in these standards and equipments, and enable a gradual transition of the ATM networks into the market place.
Typically, an ATM network will require a network management agent or proxy to be running at every UNI which can communicate and exchange administrative messages with the user attachments at the UNI for connection setup, tear down, and flow control of the payload using some standard signalling protocol. A direct user attachment at the UNI is likely to cost more and be more complex, than a user attachment to something which in turns interfaces to the UNI.