Copyright Goodheart-Willcox Co., Inc. 58 Networking Fundamentals is marked by the AWG rating 0, and the smallest wire measurement on the gauge is marked by AWG rating 36. Cables are often identifi ed as plenum-rated. Plenum-rated means that the ca- ble has a special type of insulation that will not give off toxic gases should the cable be consumed by fi re. Th e term plenum-rated is derived from the plenum in a build- ing, which is the area above a drop ceiling and under a raised fl oor, as illustrated in Figure 2-12. Cables designed to pass through a building plenum must be plenum- rated. Two types of copper-core cable are used in networking: coaxial and twisted pair. Twisted pair was derived from the telephone industry and was not originally designed for high frequencies. Voice signals are relatively low frequency when com- pared with other forms of electrical signals, such as radio, and did not require ca- ble designed for high frequencies. Also, the original networks ran at only 1 Mbps to 4 Mbps. Early versions of telephone-communication cables worked fi ne for these ap- plications. Coaxial cable was designed to carry high-frequency signals and was fi rst used in high-frequency radio communication to conduct a radio signal to and from an antenna. Coaxial cable was also used in early networks. It is used to some degree today because of its shielding eff ect. Th ese two types of cables will be covered in the next section. Coaxial Cable Coaxial cable, or coax, consists of a copper-core conductor surrounded by an in- sulator referred to as a dielectric. Figure 2-13 shows a typical RG-58 coaxial cable assembly. Th e dielectric is covered with two shields: a foil shield and a braided cop- per shield. Th e shields protect the core from electromagnetic interference (EMI) and prevent the cable from transmitting EMI to other cables. Th e entire cable assembly is covered by an insulating outer jacket, which protects the shielding. Coaxial cable is very diffi cult to work with and relatively expensive when compared with some other cable-based media. Tech Tip Do not assume all types of coaxial cable can be used to extend a network. The electrical characteristics of each type of coaxial cable vary greatly, and some coaxial cable types can only be used for specific applications. 2.1, 5.3 NET Figure 2-11 Note the AWG ratings on this wire gauge. The AWG rating of 0 indicates the largest wire size on the gauge whereas, the AWG rating of 36 indicates the smallest wire size on the gauge. Goodheart-Willcox Publisher