Crosstalk

Written on 6:46 AM by ooe

Crosstalk causes by far the largest contribution to capacity limiting noise for DSL systems, so it is worth examining it in a little more detail. There are two very different types of crosstalk in multi-pair access network cables.

NEXT is interference that appears on an adjacent pair located at the same end of the cable as the source of the interference. Its level is substantially independent of the length of the cable.

FEXT on the other hand is interference that appears on another pair at the opposite or far end of the cable to the source of the interference. Its level is attenuated at least as much as the signal itself if both have travelled the same distance.

NEXT affects any systems that transmit in both directions at once (e.g. echo-cancelling systems), and where it occurs it invariably dominates over FEXT.

NEXT can in principle be eliminated by not transmitting in both directions in the same band at the same time, separating the two directions of transmission either into non-overlapping intervals in time or into non-overlapping frequency bands. This converts duplex transmission into independent simplex transmissions, avoiding NEXT at the cost of a reduced bandwidth in each direction. At high enough frequencies the advantage of transmitting against FEXT noise rather than NEXT noise becomes so great that it can outweigh the disadvantage of reduced bandwidth.