pagation in a copper cable is approximately 200,000 km/sec. The three main types of copper cables are shown in Figure 4.27. 4.7.1.1 Twisted Pair twisted pair consists of two insulated copper wires that are typically 0.4 to 0.6 mm thick or about 1 mm thick if insulation is included. These two wires are twisted together to reduce external electrical interference and interference from one pair to another in the same cable. The twisted pair is symmetrical and the difference in voltage (or to be more accurate, electromagnetic wave) Transmission Examples of usage: Electrical signal Twisted pair: Subscriber loops; telephone, ISDN; and ADSL and twisted pair LANs Open-wire lines: Telephone subscriber loops Electrical in rural areas; signal low capacity telephone FDM carrier systems A pole Coaxial cable: Analog and digital high Electrical capacity systems in the signal telecommunications networks; Inner broadcast radio and TV conductor antenna systems; coaxial LANs Outer conductor and sheath Figure 4.27 Copper cable as a transmission medium. between these two wires contains the transmitted signal. Twisted pair is easy to install, requires little space, and does not cost a lot. Twisted pairs are used in the telecommunications networks in subscriber lines, in 2-Mbps digital transmissions with distances up to 2 km between repeaters, in DSLs up to several megabits per second, and in short-haul data transmissions up to 100 Mbps in LANs. Unshielded twisted pair (UTP) cables used in LANs are categorized as UTP Cat 3, 4, and 5. Cat 3 is a voice-grade cable designed for voice fre- quency applications, such as local loops. The characteristics of Cat 5 cable are specified up to a 100-MHz frequency and they are suitable for high-speed LANs operating at 100 Mbps or 1 Gbps (see Chapter 6). 4.7.1.2 Open-Wire Lines The oldest and simplest form of a two-wire line uses bare conductors sus- pended at pole tops. The wires must not touch each other, otherwise short circuit occurs in the line and communication will be interrupted. New open-wire