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ISDN Basic Rate Interface (BRI) uses two B channels and a D channel. But only two copper wires. The three channels are defined by the way data is arranged and transported.

A frame is an 'electronic envelope' used to transport data. Frames require a few bits of data to describe their payload and sequencing information so that the enclosed data can be accurately reassembled at its destination. The data inside a frame is called a packet. ISDN data goes from your equipment to the central office and onto the network. If you are connecting to a packet-switched network, the data breaks up into individual frames and traverses the network until it reaches its destination. Data received from the packet-switched network goes to the central office where it is assembled into ISDN frames, and then goes out to the wire to your equipment. You can also make circuit-switched connections, where you dial into a particular network, such as your corporate LAN, and transmit data, which travels in ISDN-formatted frames.

V.120 is a standard for encapsulating asynchronous data communication into ISDN data streams, which are synchronous. Two computers can communicate over an ISDN connection, using their standard, asynchronous-only COM ports and a V.120 adapter, which can be connected externally or internally. In this type of setup, the COM port takes on the role of a network adapter.

ISDN terminal adapters --A device that allows analog voice and data devices to work through an ISDN connection. The terminal adapter can distinguish between ISDN data calls and voice calls from information in the signal (that is, information in the frame). It intercepts the voice calls and redirects them to the appropriate phone port.