Transmission Media Reading Assignment : Stallings Chapter 3

1. Transmission MediaReading Assignment : Stallings Chapter 3 • Transmission Media • physical path between transmitter and receiver • electromagnetic wave • Guided transmission • solid medium, copper twisted pair, coaxial cable, optical fiber • data rate mainly determined by medium • Unguided transmission • atmosphere, outer space • wireless transmission • low frequencies : omnidirectional (all directions)

2. high frequencies : possible to focus signal • transmission characteristics determined by bandwidth • Factors determining data rate and distance • bandwidth • greater bandwidth : higher data rate • transmission impairments • attenuation • twisted pair > coaxial cable > optical fiber • interference • by unshielded cables • from competing signals in overlapping frequency bands • number of receivers • Each attachment introduces some attenuation and distortion on the line.

3. Guided transmission media • point-to-point vs. multipoint • Twisted pair • Physical description • least expensive, most widely used • Spiral pattern decreases crosstalk interference. • Neighboring pairs have different twist length (2 - 6 in). • bundled together in hundreds • limitation : data rate and distance • Applications • analog • telephone systems • private branch exchange (PBX) • end office (of phone company) • digital • digital switch or digital PBX (64 kbps) • local area network : 10 Mbps or 100 Mbps (limited length) • long distance (4 Mbps)

4. Transmission characteristics • amplifiers needed • analog signals : 5 - 6 km • digital signals : 2 - 3 km • high attenuation for higher frequency • susceptible to interference and noise • shielding to reduce interference from power lines • twisting to reduce low-frequency interference • different twist lengths • point-to-point analog signals • bandwidth ~ 250 k Hz : a few voice channels • long distance point-to-point digital signals • ~ 4 Mbps • short distance digital signals • ~ 100 Mbps • Unshielded twisted pair (UTP) • Category 3 and 5 UTP • Shielded twisted pair (STP)

5. Coaxial cable • Physical description • Applications • TV distribution (cable TV) • long distance telephone transmission • inferior to optical fibre • short-run computer system links • high speed I/O • local area networks (LAN) • Transmission characteristics • much better than twisted pair in attenuation, thermal noise, and intermodulation noise • analog signals : amplifiers every few km • digital signals : repeaters every km • Optical fiber • Physical description • core • thin strand of fibre of glass or plastic • cladding • glass or plastic • jacket

6. Advantages over twisted pair or coaxial cable • greater capacity • smaller size, lighter weight • lower attenuation • electromagnetic isolation • no radiation of energy, little interference to others, difficult to tap - security • greater repeater spacing • Applications • long-haul trunks • 900 miles, 20 k - 60 k voice channels • replacing coaxial cables in telephone networks • metropolitan trunks • joining telephone exchanges in a metropolitan area • average 7.8 miles, 100 k voice channels • rural-exchange trunks • subscriber loops • from central exchange to a subscriber • still more expensive than twisted pair • local area networks • 100 Mbps, 100 - 1000 stations in a building complex

7. Transmission characteristics • multimode : variety of reflecting angles • single mode : a single angle can pass -- the axial ray • light source • light-emitting diode (LED) : cheaper, longer operational life, greater temperature range • injection laser diode (ILD) : more expensive, higher data rates • wavelength windows • 850, 1300, 1550 nm, all infrared • Wireless transmission • antenna • directional transmission • focused electromagnetic beam, higher frequency • omnidirectional transmission

8. Frequencies • broadcast radio • VHF, part of UHF • microwave • part of UHF, all of SHF • 2 G Hz ~ 40 G Hz • directional, point-to-point • 30 M Hz ~ 1 G Hz • omnidirectional, broadcast • infrared • 3 X 1011 ~ 2 X 1014 Hz • point-to-point, confined areas • Terrestrial microwave • Physical description • parabolic dish, 10 ft in diameter, installed high above ground, rigidly fixed, focused on next receiving antenna / relay tower • e.g., 100 m high => 82 km apart • Applications • long-haul telecommunications service • requiring line-of-sight transmission • voice and TV

9. short point-to-point links between buildings • private short-haul microwave link to bypass local telephone company • also used to propagate cable TV to local installations, and then to subscribers via coaxial cable • Transmission characteristics • Attenuation varies as the square of the distance. • For twisted pair and coaxial cable, loss varies logarithmically with distance. • I.e, repeaters or amplifiers are farther apart for microwave systems. • Attenuation increases with frequency. • Attenuation increases with rainfall. • Assignment of frequency bands is strictly regulated to avoid interference. • Higher frequency => higher potential bandwidth => higher data rate • for short distance • smaller and cheaper antenna

10. Satellite microwave • Physical description • satellite = microwave relay station • earth / ground station • uplink frequency band  downlink frequency band • transponder channels / transponders • point-to-point and broadcast • geostatic satelites : height 35,784 km, period of rotation = earth’s period • 4 angular spacing with respect to earth for 4/6 G Hz band, 3 for 12/14 G Hz band. • Applications • TV distribution • direct broadcast satellite (DBS) : video signal directly to home user • long-distance telephone transmission • point-to-point trunks between exchange offices • private business networks • Divide total capacity into a number of leased channels to individual business users. • Very small aperture terminal (VSAT) : a low cost private network

11. Transmission characteristics • 1 - 10 G Hz • below 1 G Hz : galactic, solar, atmospheric noise • above 10 G Hz : atmospheric attenuation • typical frequency : 4 / 6 G Hz band • uplink : 5.925 - 6.425 G Hz • downlink : 3.7 - 4.2 G Hz • propagation delay : about 0.25 sec from one ground station to another • inherently broadcast nature