The four-wire line is a connecting line in the telephone connection. It consists of two twisted pairs, one each for the incoming and outgoing signal
. Due to the twisting
inpairs or the twisting of the pairs of cores, the twin cores are less susceptible to interference.
Transmission can take place in full dup lex via the four-wire line, which used to be used in analogue leased lines and private branch exchanges, but is still used today in cabling technology and in the ISDN connection area.