a/b Interface

Despite having only two wires, the a/b interface allows full-duplex transmission. A hybrid circuit is used to split the two directions of transmission. The frequency range of the a/b interface available for calls ranges from 300 Hertz to 3,400 Hertz. The twin wire supplies the analogue device with the direct current electricity required for the main basic functions. A call alternating current is used to indicate a call on the device. Dual-tone multi-frequency signaling (DTMF for short) is typically used for dialing and to provide the switchboard or the telephone system with the number being called. So-called pulse dialing, still supported by most phone systems and analogue networks, stems from the days of dial plates. The a/b interface will additionally transmit the telephone number of the party calling. It can be displayed on the phone of the party receiving the call. This process is called CLIP (Calling Line Identification Presentation) and uses dialing pauses to display the number.

In Voice over IP (VoIP) telephony, voice information is transmitted over an IP network in form of digital data. However, the a/b interface is also still common in IP telephony. Many VoIP systems also feature a/b ports for connecting analogue phones, therefore allowing these to still be used on a digital network. Several new features of VoIP telephony, however, cannot be used with analogue devices. To take full advantage of all the VoIP features, using IP-compatible devices is recommended.