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