Four-wire line

Four-wire lines are used in telecommunications networks to connect the terminal to the switching device . A four-wire cable consists of two pairs of copper wires (two pairs ). To reduce interference, the wires are twisted together , either in pairs or in four .

One pair of wires is responsible for the data transport in both directions. This enables unrestricted full duplex operation, i.e. simultaneous transmission in both directions, via a four-wire line .

Typical application examples for four-wire cables:

• Internal ISDN cabling ( S ₀ bus ) - from the NT to the phone
• ISDN primary rate connection from the exchange to the subscriber
• Computer networks ( e.g. CAT5 cabling )

Analog telephone connections and ISDN basic connections from the subscriber to the exchange are usually designed as two-wire connections, since cables are expensive - a not inconsiderable part of the fixed assets of the subscriber network operator (for example, Deutsche Telekom in Germany ) is buried in the ground - and so with the same Cable can realize twice the number of connections .

However, a two-wire connection does not easily allow simultaneous transmission in both directions. A half-duplex operation - alternately transmitting and receiving - is, for example, telex acceptable devices, but not for phone conversation. To ensure that full duplex operation is still possible using just one pair of wires, special methods have been developed to separate the two channels in the exchange and at the subscriber. The best-known method is the so-called hybrid connection .