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CH. 4 Transmission Media 4.1. Guided Transmission Media • Twisted Pair (Table 4.1, Figure 4.2) – Two insulated copper wires arranged in a regular spiral pattern. – Many twisted pair can be bundled together in a protective sheath. – The most common medium for analog and digital data. – Used for telephone wiring. – Tends to be limited in distance, bandwidth, and data rate. 4.1 Twisted Pair (cont.) • Categories of Twisted Pair for Data Transmisson • ANSI Standards • Table 4.2—Twisted Pair Categories and Classes • Figure 4.5 Category 6A Channel Requirements 4.1 Transmission Media (cont.) • Coaxial Cable – Made of an outer cylindrical conductor and a single inner wire conductor, separated by an insulator. – The outer conductor is covered by a jacket or shield. – Used for long-distance communication lines, cable TV, and LANs. – Tends to have a higher bandwidth than twisted pair. 4.1 Transmission Media (cont.) • Optical Fiber – Conducts an optical ray (ie. light). – Can be made from glass and plastic. – The core is surrounded by glass or plastic cladding and a protective jacket. – Has high bandwidth, low attenuation, small size, is light weight, and is not susceptible to electromagnetic interference. – Used for trunks, local loops, and LANs. 4.2 Wireless Transmission Media • Antennas – An antenna is an electrical conductor or system of conductors used for radiating electromagnetic energy or for collecting electromagnetic energy. – An isotropic antenna radiates power in all directions. – Parabolic reflective antenna are used to focus received signals and to transmit a parallel beam without dispersion (Fig. 4.5). – Then gain equation is given by equation 4.1. 4.2 Wireless Transmission Media(cont.) • Terrestrial Microwave – Parabolic dishes (3 meters in diameter) usually located at substantial heights. – Line-of-sight transmission. – Loss = 10 log10(4pd/l)² dB. – Rain attenuates the signal. – Table 4.6 shows typical microwave performance. 4.2 Wireless Transmission Media (cont.) • Satellite Microwave (Fig. 4.8, 4.9) – – – – Used to link two or more ground stations. Uplinks and downlinks use different frequencies. A single satellite might have several transponders. Used for television, long-distance telephone transmission, and private networks. – Very small aperture terminals (VSATs) have reduced the cost of satellite communications – Optimal transmission frequencies are 1-10 G Hz but saturation is causing higher frequencies to be used. 4.2 Wireless Transmission Media (cont.) • Broadcast Radio – Omnidirectional. – Typical frequencies range from 30 MHz to 1GHz. – Less sensitive to rain than microwave radio. – Multipath interference is a prime source of impairment. 4.2 Wireless Transmission Media(cont.) • Infrared – Transceivers modulate noncoherent infrared light. – Transceivers must be within line-of-sight or each other or must be able to receive reflections of the signal. – Infrared does not penetrate walls, and hence has a security advantage over radio waves. 4.3 Wireless Propagation • Ground Wave Propagation – Radio waves follow the contour of the earth, and can travel beyond the horizon. – Frequencies up to 2 M Hz. – AM radio is an example. 4.3 Wireless Propagation (cont.) • Sky Wave Propagation – A signal from an earth based antenna is reflected from the ionized layer of the upper atmosphere – Used by amateur radio, CB radio, and international broadcast radio. 4.3 Wireless Propagation (cont.) • Line-of-Sight Propagation – For signals above 30 MHz. – Optical line of sight: d = 3.57 x Sqrt(h) where d is distance and h is height. – Radio line of sight: d = 3.57 x Sqrt(K h), where K is an adjustment factor (of about 4/3). 4.4 Line-of-Sight Transmission • LOS Impairments – Free Space Loss--the signal disperses with distance. – Atmospheric Absorption – Multipath Reflection (Fig. 4.13) – Refraction (bending of waves through the atmosphere.)