Transcript Transmission Media Reading Assignment : Stallings Chapter 1

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