Transmission Media Reading Assignment Stallings Chapter 3 Transmission

Transmission Media Reading 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)

• 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.

• 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)

• 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)

– 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

• 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

• 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

– 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

– 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

– 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

• 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