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.)