ATM Overview309978-D Rev 01 D-5User-to-Network Interface (UNI)The user-to-network interface (UNI) represents the interface between an end pointof an ATM network and the switch (the user connection).The UNI can also be the interface between an end point of an ATM network and aswitch or between a switch and a router.There are two types of UNI interfaces:• Private UNI -- interface between an end point device and a private networkswitch• Public UNI -- interface between an end point device and the public switchednetworkFor more information about UNI 3.0 and UNI 3.1, refer to ATM User-NetworkInterface (UNI) Specification, Version 3.0 and ATM User-Network InterfaceInterface (UNI) Specification, Version 3.1.ATM Data TransmissionData transmission (also called cell switching) through the ATM network relies onthe establishment of logical connections between ATM entities. ATM is aconnection-oriented service. This means that an ATM entity cannot transmitinformation until it establishes a connection with a receiving entity. Theseconnections consist of virtual channels, virtual paths, and transmission paths(Figure D-2).A virtual channel is a logical connection between two communicating ATMentities. Each virtual channel may carry a different protocol or traffic type. Thevirtual channel transports cells that have a common identifier. The identifier iscalled the virtual identifier (VCI) and is part of the cell header. You can establishpermanent virtual channels or you can set them up as dynamic virtual channels,which allows the network to adjust to the traffic demand.A virtual path is a set of virtual channels between a common source anddestination. The virtual channels in a virtual path logically associate to a commonidentifier. This identifier is called the virtual path identifier (VPI) and is part of thecell header. You can base cells on either the VPI alone, or on a combination of theVPI and the VCI.
