of an ieee802 3 local area network using 10baset interfaces a hub 16 defining a central location is connected to a typical computer 18 a via a cable that includes two wire pairs 17 a 1 and 17 a 2 each pair is operative to carry data in one direction only one pair say 17 a 1 canning data from the hub 16 to the computer 18 a while the other pair 17 a 2 carries data in the other direction fig3 also shows a telephone set 13 a associated with computer 18 a and preferably near it and a telephone private automatic ranch exchange pabx 11 which is preferably also at the central location the term hub is used herein to represent any digital network concentrating unit and may equally refer to a switching hub a router a server computer or to any digital device having multiple data ports any of these being also referred to herein as a central digital device similarly pabx is used herein to represent any type of central telephone switching unit and will also be referred to as a central telephone device according to the invention a signal transformer is inserted at each end of each wire pair whereby for example transformer 31 a 1 is inserted at the end of wire pair 17 a 1 that is.