ints relating to pair cable are as follows. Far end crosstalk between pairs operating in the same direction Unscreened twisted pair (UTC) as used for telephone cabling, has has to be of the order of 70dB and this is achieved with the use of been evaluated by a number of independent bodies. 10Mbit/s trans- capacitive crosstalk balancing frames during cable installation. mission is technically feasible over distances of up to 100m within the general office environment. Screened twisted pair (STP) can be 15.2.5 Pair cables for digital transmission operated at data rates of tens of Mbit/s over hundreds of metres. The advantage of twisted pair for this application is that it is sufficiently As previously indicated, many 1.5Mbit/s and 2Mbit/s pcm systems low cost, low weight and small size to plan for 'saturation' cabling have been provided over deloaded audio cables with regenerative at the outset and SO avoid expensive upheavals as demand changes. repeaters equipped at the old loading coil sites. For this application the cable loss at 1MHz must not exceed about 30dB. However, the limiting factor on cable fill (the percentage of pairs in a given cable Coaxial pair cables which can be equipped with pcm) is crosstalk within the cable, and within the cable terminations to the repeaters. One solution is to The limitations of balanced pair cables for telecommunication use recode the standard format with a reduced high frequency content at higher frequencies are the crosstalk between pairs, and the lack SO that crosstalk effects are mitigated. This has served to signifi- of predictability in transmission performance. These problems are cantly raise the pcm occupancy on existing cables of unit twin overcome in a coaxial cable. (Schelkunoff). design. Considering the operation of a coaxial cable, at high frequencies For new digital installations, the cable pairs are split into 'go' and skin effect causes the currents to be concentrated on the outer layer 'return' directions by means of an aluminium screen, i.e. a D screen of t