a / b interface

This article or paragraph presents the situation in Germany . Help us to describe the situation in other countries.

In telecommunications, the a / b interface is the interface for connecting analogue terminals , for example telephones , fax machines or modems , to a local exchange of the telephone network or to a telephone system .

The interface connects a terminal device to the switching facility via a two-wire line ( twin wire ). The two wires are referred to as "a-wire" and "b-wire"; hence the name of the interface.

Originally, the a / b interface was provided as a general network termination in the old analog landline network, while in today's networks it is provided via converters in the digital subscriber exchange or via end devices installed on the subscriber side (telephone systems, terminal adapters , analog telephone adapters , integrated access devices , mobile radio -Gateways etc.) is implemented.

Services and parameters of the a / b interface

• Full duplex - voice channel with a frequency band of 300  Hz . To 3400 Hz The direction of separation in the terminal device is a hybrid circuit .
• Remote supply of the basic functions of the terminal with direct voltage :
  • Usually 60 V (50 V to 72 V) in the German public telephone network 
    • Tolerance range of the open circuit voltage : without additional supply 20 V - 75 V, with additional supply up to 105 V.
    • Polarity : a-wire negative compared to the reference potential (the b-wire at ground potential ). The polarity is irrelevant for modern analog devices. However, if the polarity is incorrect, the alarm clock may sound briefly when you pick up older telephones (e.g. type 611). For testing purposes, the subscriber circuit in the exchange can change the polarity of the connection to be measured. The subscriber line can thus be measured using the passive test termination .
  • With telephone systems usually 48 V, 32 V or 24 V.
• Signaling
  • Call signaling with alternating ring voltage :
    • In the German public network approx. 50 V / 25 Hz, tolerance range 32 V to 75 V.
    • In telephone systems also 50 Hz
  • Transfer of the caller's number ( CLIP ) between the first and second call
  • Occupancy detection (picking up the telephone receiver )
  • Electoral procedure:
    • Multi-frequency dialing (DTMF, tone dialing): The dialing method used today
    • Pulse dialing (IWV, pulse dialing): Originally used by number switches (dials). Today it is out of date on the public network and still supported by most modern telephone systems
    • Diode-earth method (DEV): Obsolete key dialing method in telephone systems. Requires an earth connection via an additional wire. Is no longer used.
  • Transmission of control signals from the participant, e.g. B. by means of the inquiry button
  • Transmission of the audible tones on the voice channel
  • Transmission of the charge impulse to the subscriber. With internal a / b interfaces of telephone systems, not available in all telephone systems

Connection technology

Telephone sockets are used to connect the end devices to the a / b interface , in structured cabling also RJ11 sockets , more rarely also RJ45 sockets . Formerly known as were in West Germany for this connector doses (VDo) or outlets (ADo) used; These socket types have not been used since 1989, but can still be found in old installations.

Individual evidence

1. ↑ 1 TR 110-1 - Technical description of the analog dial-up connections (1 MB). Deutsche Telekom AG, Darmstadt