Data Communications Networking Devices - 4th Ed.pdf

288 ________________________________________________________ LOCAL AREA NETWORKSFigure 3.6Crosstalksince a lower attenuation reading indicates less signal loss and is better than ahigher reading for that parameter.Table 3.6 indicates the EIA/TIA-568 speci®cation limits for categories 3, 4 and5 UTP cable and for IBM cabling system types 1, 2, 6 and 9 cable. Thesecategories represent the primary types of UTP and STP cable used for local areanetwork data transmission.In examining the entries in Table 3.6 note that the attenuation and NEXT of acable must be measured over a range of frequencies. That range is based upon thecable category. For example, since category 3 cable is designed to support signalingrates up to 16 MHz, attenuation and NEXT should be measured up to andincluding the highest signaling rate supported by that type of cable.Coaxial cableCoaxial cable consists of a central conductor copper wire which is then covered byan insulator known as a dielectric. An overlapping woven copper mesh surroundsthe dielectric and the mesh, in turn, is covered by a protective jacket which canconsist of polyethylene or aluminum. Figure 3.7 illustrates the composition of atypical coaxial cable; however, it should be noted that over 100 types of coaxialcable are currently marketed. The key differences between such cables involve thenumber of conductors contained in the cable, the dielectric employed and the typeof protective jacket and material used to provide strength to the cable which allowsit to be pulled through conduits without breaking.Two basic types of coaxial cable are used in local area networks, with the type ofcable based upon the transmission technique employed: baseband or broadbandsignaling. Both cable types are much more expensive than twisted-pair wire;however, the greater frequency bandwidth of coaxial cable permits higher datarates for longer distances than are obtainable over twisted-pair wire.