S/T and U interfaces are the most popular in integrated services digital network (ISDN) technology. The U interface, called the basic rate interface, connects the ISDN network to the telephone switch at home or office through line termination equipment. The U interface is a single twisted pair of wires that connects to the phone switch.

The S interface connects to a network termination type 2 (NT-2) such as PBX, or it can be connected directly to a TE1 device or terminal adapter. The T interface then connects the NT2 to the network termination type 1. S and T interface are electrically equivalent. The S and T interface are often combined and called an S/T interface. S/T interface uses a four-wire medium. Common devices that connect to the S/T interface are ISDN capable telephones and fax machines, video teleconferencing equipment, bridge or routers, and terminal adapters.

The U interface can support full-duplex data transfer over a single pair of twisted wires. However, only a single device can be connected to it. S/T interface can also provide full-duplex data transmission. The only difference is that it can accommodate up to eight devices. This is because there is a pair of wires dedicated for the transmission and receiving of data.