Transcript Chapter 4 - William Stallings, Data and Computer

1
Transmission Media
Data and Computer Communications
by William Stallings
Eighth Edition
Networks and
Communication
Department
Click to4edit Master subtitle style
Chapter
Overview
transmission medium: is the physical path between transmitter and
receiver.
transmission media can be classified as :
1- guided media- wire :
electromagnetic waves are guided along a solid medium, such as copper
twisted pair, copper coaxial cable, and optical fiber
2- unguided media – wireless:
wireless transmission occurs through the atmosphere, outer space, or water.
Overview
characteristics and quality determined by medium and signal
 in unguided media - bandwidth produced by the antenna is more
important
 in guided media - medium is more important
key concerns are data rate and distance
Design Factors
1.Bandwidth
higher bandwidth gives higher data rate
2.transmission impairments
eg. attenuation, limit the distance.
For guided media, twisted pair generally suffers more impairment than
coaxial cable, which in turn suffers more than optical fiber.
Design Factors
3.Interference
Interference from competing signals in overlapping frequency bands
can distort or wipe out a signal. Interference is of particular concern for
unguided media.
For guided media, interference can be caused by emanations from
nearby cables. For example, twisted pairs .
4.number of receivers in guided media
-in guided media
-each attachment introduces some attenuation and distortion on the line,
limiting distance and/or data rate .
-more receivers (multi-point) introduces more attenuation, limiting
distance and data rate.
Electromagnetic Spectrum
Transmission Characteristics of Guided Media
Frequency
Range
Typical
Attenuation
Typical
Delay
Repeater
Spacing
Twisted pair
(with loading)
0 to 3.5 kHz
0.2 dB/km @
1 kHz
50 µs/km
2 km
Twisted pairs
(multi-pair
cables)
Coaxial cable
0 to 1 MHz
0.7 dB/km @
1 kHz
5 µs/km
2 km
0 to 500 MHz
7 dB/km @ 10
MHz
4 µs/km
1 to 9 km
Optical fiber
186 to 370
THz
0.2 to 0.5
dB/km
5 µs/km
40 km
Twisted Pair
physical description:
•A twisted pair consists of two insulated copper wires arranged
in a regular spiral pattern.
•The twisting tends to decrease the crosstalk interference
between adjacent pairs in a cable.
•Twisted pair is much less expensive than the other
•easier to work with.
Twisted Pair
Application:
•Most common medium
• Telephone network
• Between house and local exchange
-Within buildings .
• For local area networks (LAN)
Twisted Pair
Transmission Characteristics:
analog
 needs amplifiers every 5km to 6km
•digital
 can use either analog or digital signals
 needs a repeater every 2-3km
•limited distance
•limited bandwidth (1MHz)
•limited data rate (10 to 100MHz)
•susceptible to interference and noise
•
Unshielded vs Shielded TP
Twisted pair comes in two varieties:
1.
unshielded Twisted Pair (UTP)
 ordinary telephone wire
 cheapest
 easiest to install
 suffers from external Electromagnetic interference (EMI)
2.
shielded Twisted Pair (STP)
 metal braid or sheathing that reduces interference
 more expensive
 harder to handle (thick, heavy)
UTP Categories
Cat 3 (up to 16 MHz)
— Twist length of 7.5 cm to 10 cm
Cat 5 (up to 100MHz)
— Twist length 0.6 cm to 0.85 cm
— Tighter twist -> more expensive, But better performance
Cat 5E (Enhanced), Cat 6 & Cat 7
C ate gory 3
Class C
C ate gory 5
Class D
C ate gory 5E
C ate gory 6
Class E
C ate gory 7
Class F
Ban dwi dth
16 MHz
100 MHz
100 MHz
200 MHz
600 MHz
C able Ty pe
UTP
UTP/FT P
UTP/FT P
UTP/FT P
SSTP
Li n k C ost
(C at 5 =1)
0.7
1
1.2
1.5
2.2
Coaxial Cable
physical description:
•like twisted pair, consists of two conductors.
•It consists of a hollow outer cylindrical conductor that surrounds a
single inner wire conductor
•Coaxial cable can be used over longer distances.
Coaxial Cable
Application:
• Television distribution
• Long distance telephone transmission
• Short distance computer systems links
• Local area networks
Coaxial Cable
Transmission Characteristics:
Coaxial cable is much less susceptible to interference and crosstalk
than twisted pair
•
Coaxial cable is used to transmit both analog and digital signals.
-analog signals
•
amplifiers every few km
•
closer if higher frequency
•
up to 500MHz
-digital signals
•
repeater every 1km
•
closer for higher data rates
•
Optical Fiber
physical description:
An optical fiber is a thin (2 to 125 µm), flexible medium capable
of guiding an optical ray. Various glasses and plastics can be
used to make optical fibers
Optical Fiber
Application:
• Long-haul trunks
• Rural exchange trunks
• LANs
Optical Fiber
Benefits:
•greater capacity
 data rates of hundreds of Gbps
•smaller size & weight
•lower attenuation than for coaxial cable or twisted pair
•electromagnetic isolation
•greater repeater spacing
Optical Fiber
Transmission Characteristics:
uses total internal reflection to transmit light
can use several different light sources:
 Light Emitting Diode (LED)
 cheaper, lasts longer
 Injection Laser Diode (ILD)
 more efficient, has greater data rate
Optical Fiber Transmission Modes
There are three fundamental types of fiber optic cable in use:
1. step-index multimode
Light from a source enters the cylindrical glass or plastic core. Rays at
shallow angles are reflected and propagated along the fiber. variety of angles
will reflect. With multimode transmission, multiple propagation paths exist,
each with a different path length
2. graded-index multimode
light in the core curves helically because of the graded index, reducing its
travel distance. Graded-index fibers are often used in local area networks.
3. single-mode
there is a single transmission path with single-mode transmission. Singlemode is typically used for long-distance applications, including telephone
and cable television
Optical Fiber Transmission Modes
Difference between Guided Media
Wireless Transmission techniques
Microwave
 Frequency :2GHz to 40GHz
 Used for point to point communication
 Used for satellite communication
Radio wave
 omnidirectional
 30MHz to 1GHz
Infrared wave
 Used for local point-to-point and multipoint applications
 3x10^11Hz to 2x10^14 Hz
Antennas
Electrical conductor used to radiate or collect electromagnetic
energy
-transmission antenna
 radio frequency energy from transmitter
 converted to electromagnetic energy by antenna
 radiated into surrounding environment
-reception antenna
 electromagnetic energy impinging on antenna
 converted to radio frequency electrical energy
 fed to receiver
-Same antenna is often used for both purposes
Types of Antenna
1- Horn antenna
2-Dish antenna
Wireless propagation methods
1-Ground propagation
• Follows contour of earth
• This effect found in freq below 2MHz
• Example :AM radio
Wireless propagation methods
2-Sky propagation
• Signal reflected from ionosphere layer of upper atmosphere
• Frequency from 2 to 30 MHz
• Useful for long distance communication
• Example : BBC world service, Voice of America
Wireless propagation methods
3-Line of sight propagation
• Point to point
• Frequency above 30Mhz communication
• Example : satellite ,TV
Terrestrial Microwave
Physical description:
•The most common type of microwave .
•used for:
1- long haul telecommunications
2- short point-to-point links
•line of sight transmission
•use a parabolic dish to focus a narrow beam onto a receiver antenna
•an alternative to coaxial cable or optical fiber
•requires far fewer amplifiers or repeaters than coaxial cable over the
same distance
Terrestrial Microwave
Transmission characteristic:
At higher frequency more attenuation
•Attenuation increase with rainfall
•
Satellite Microwave
Satellite is relay station
•Used to link two or more earth stations
•
Satellite communication configuration:
1-Point to Point Link
2-Broadcast Link
•
Satellite Point to Point Link
Satellite Broadcast Link
Broadcast Radio
use broadcast radio, 30MHz - 1GHz, for:
 FM radio
•is omnidirectional
•Communication is line of sight
•Can penetrate walls
•Less sensitive to rainfall
•suffers from multipath interference
•
Infrared
infrared light
•within line of sight (or reflection)
•are blocked by walls , Thus the security and interference
problems encountered in microwave systems are not present.
•typical uses
 TV remote control
•
Refraction
velocity of electromagnetic wave is a function of density of
material
C= 3 x 108 m/s in vacuum, less in anything else
• When an electromagnetic wave moves from a medium of one
density to a medium of another density,
• its speed changes.
•
The effect is to cause a bending of the direction of the wave.
• Wavelength get smaller
•
Refraction
Index of refraction (refractive index) is:
 n=sin(incidence)/sin(refraction)
 also n= C/V
Summary
looked at data transmission issues
frequency, spectrum & bandwidth
analog vs. digital signals
transmission impairments