Transcript Transmission_Media_and_Codes

Transmission Media & Codes
© Prof. Aiman Hanna
Department of Computer Science
Concordia University
Montreal, Canada
C onnect then Communicate!
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There are many factors that controls how devices should be
connected:
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Cost
Capacity of the link / speed
Immunity to noise
Vulnerability to unauthorized listening (security)
Logistics
Mobility
• Once connected communication is guaranteed. Correct?
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Digital vs. Analog Signals
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Different devices may represent or send information differently
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Many devices use digital signals (1 or 0 bit), while others use analog signals
(varying voltage)
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Conversion between digital and analog is possible but complex
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Signals travels from one place to another through a
transmission media
There are 3 types of transmission media:
• Conductive metal
• Transparent glass strand (optical fiber)
• Electromagnetic waves (no physical connection)
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Which media to use depends on many factors
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Two important measures exist: bit rate and
bandwidth
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Bit Rate
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How many bits per second (bps) a link can
transmit
Rates range from 100s bps to billions of bps
(gigabits) and now pushing for trillion bps
(terabit)
Bandwidth
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Period: Time needed for a signal to complete
one cycle
Frequency: Number of cycles per second,
measured in Hertz (Hz)
f = 1 / p
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For example, if p = 0.5 microseconds (μsec)
then f = 2 MHz
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A given medium can accommodate a range of
frequency
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Bandwidth is the difference between the
highest and the lowest possible frequency that
can be transmitted
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For example, telephone signals can handle
frequencies between 300 Hz & 3300Hz; the
bandwidth is hence 3000 Hz
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In other words, very high or very low audible
sounds cannot pass though the telephone
Figure 2.2 - Periodic Signal
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C onductive Metal
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Made of metals such as copper or iron
Examples include twisted wire pair and
coaxial cable
Twisted Pair:
• One of the oldest media; used for the telegraph
• Basically a circuit of power source, switch & sensor
• Closing the switch allows current to run and the sensor
to click
Information on 2-way telegraph can be found at: http://hypertextbook.com/facts/2005/telegraph.shtml
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C onductive Metal
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Twisted Pair
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• Copper is usually used due to its less resistance to
electricity, as well as high resistance to corrosion
• Twisted wires use balanced signals; 180o out of phase
• More twists reduce crosstalk, the electromagnetic
interference between adjacent pairs
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C onductive Metal
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Twisted Pair
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• Although copper has less resistance to electricity, signal will
eventually die (attenuate)
• Repeaters are used to solve this problem
• Two types of twisted pair wires exist: Unshielded & Shielded; they
have different cost & quality
Figure 2.4 – Two Points Connected using a Repeater
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C onductive Metal
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Coaxial Cable:
• Typically transmits information in either baseband or broadband
mode
• With baseband, the cable bandwidth is devoted to a single stream
of data; this is typical in LANs
• With broadband, the BW is divided into ranges; each range carries
separate code information
• Cable TVs use broadband coaxial cable
Figure 2.5 – Coaxial Cable
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C onductive Metal
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Coaxial Cable
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• Two types of coaxial cable exist: ThickNet and
ThinNet
• Coaxial cables accommodate a higher BW and better
error rate than twisted pair, however it is more costly
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O ptical Fiber
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There are several problems with conductive
metals:
• Electrical signals are susceptible to external
interference, such as electric motors, lightening, …
• Weight; cables are heavy and bulky
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Optical fiber is an alternative to conductive
metal
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O ptical Fiber
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Optical fiber uses light, not electricity, to
transmit information
Impervious to electrical noise and capable of
transmitting enormous amount of information
Very thin compared to cables; they can be
bundled together
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O ptical Fiber
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Figure 2.6 – Light Refraction & Reflection
Figure 2.7 – Step-Index Multimode fiber
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O ptical Fiber
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Figure 2.8 – Graded-Index Multimode Fiber
Figure 2.9 – Single-Mode Fiber
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O ptical Fiber
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Optical fiber have many advantages
of over conducting metal:
On the other hand,
• They require that electrical signals
must be converted first to light and
converted back at the other end
• Must be handled with great care; some
of these fibers could be a thin as
human hair
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W ireless Communications
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Physical communication is acceptable in many,
but not all, situations
Wireless communication is an alternative
It involves the sending of electromagnetic
waves
The signal is then received by a receiving
antenna
Broadcast radio & TV transmit signal this way
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W ireless Communications
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Figure 2.10 – Electromagnetic Wave Spectrum
Wavelength = Speed of Light / Frequency
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Low–frequency (high-wavelength) waves traverse very long distances without
much loss
They also require a very long antenna, which even today may represent a health
hazard
There are three important types of wireless communication:
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Microwave
Satellite
Infrared
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W ireless Communications
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Microwave Transmissions
• Typically occur between two ground towers
• Travel in a straight line – does not follow
earth’s curvature
• Cannot travel through solid objects
Figure 2.11 – Parabolic Dish Receiving Signals
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W
ireless Communications
Microwave Transmissions
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• Often through Horn Antenna
• Towers are used as repeaters to solve earth curvature’s
and signal loss problems
Figure 2.13 – Horn Antenna
Figure 2.14 – Microwave Towers as Repeaters
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W ireless Communications
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Satellite Transmissions
• A science fiction in 1945; A common science today!
• Primarily, it is a microwave transmission in which one of the
towers is a satellite
• In 1957, the Soviet Union launched the Sputnik
Figure 2.15 – Satellite Communications
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W ireless Communications
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Satellite Transmissions
• Now, how can a satellite remain in a fixed position?
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• Kepler’s law is used: P = KD where:
- P is time period needed to rotate around planetary body,
- D is the distance between the satellite and the planet’s center.
The higher D is the longer it takes for a full rotation
- K is a constant
- Now, at what height a satellite will have a 24h period (P)?
- According to Kepler’s Law, the answer is K = 22,300 miles
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above the equator
Geosynchronous Orbit
Three satellites can almost cover the entire earth
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W ireless Communications
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Satellite Transmissions
Figure 2.16 – Satellites in Geosynchronous Orbit
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W ireless Communications
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Satellite Transmissions
Figure 2.17 – Atmospheric Interference as a function of angle of transmission
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W ireless Communications
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Satellite Transmissions
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Satellites must be located apart from each other (with a minimal angel)
Transmission should also be restricted to a specific angel
Figure 2.18 – Satellite receiving more than one signal
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Satellite Transmissions
• Now, there is no interference. Good! But what about
legality?
Figure 2.19 – Satellites receiving one signal
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W ireless Communications
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Satellite Transmissions
• Some applications, such as military surveillance, require
that the satellite does not remain in a fixed position
• Low Earth Orbit
• Low Earth Orbit Satellites can still be used for
communication. How?
Figure 2.20 – Ground station communicating with LEO satellites
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W ireless Communications
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Satellite Transmissions
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Using many LEO that can directly communicate with each other would enable any two
locations in the planet to be connected
This however has not fully been implemented yet
The LLC project by Motorola
Another player is the Teledesic Corporation – 288 satellites (12 groups of 24) – reduced
lately to 30
Figure 2.21 – Two stations communicating via LEO satellites
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W ireless Communications
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Satellite Transmissions
Teledesic Satellite
Constellation of Teledesic Satellites
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W ireless Communications
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Wireless LANs
• Two technologies used by wireless LANs are
Infrared and Radio Waves.
Figure 2.25 – Wireless LAN configuration
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W ireless Communications
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Bluetooth
• Allows devices that are not considered as
typical network devices, such as fridges,
microwaves, coffeemakers, …etc., to
communicate
• This communication can possibly be
between such device and an Internet service
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W ireless Communications
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Free Space Optics
• Wireless optical technology without the use of a fiber,
which is also called as Tera-beam technology
• LifeSpan BioSciences, Merill Lynch, and others used
FSO
• Advantage:
• Disadvantage:
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W ireless Communications
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Does New Technologies obsolete Older ones?
• Twisted Pair, Coaxial Cable, Optical Fiber,
Microwave, Satellite, Infrared, FSO, .. all coexist
• Reasons?
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Codes
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• How is information coded in a format suitable for
transmission?
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Different codes include:
• Morse Code
• ASCII (American Standard Code for Information Interchange),
7-bit code, control characteristics
• EBCDIC (Extended Binary Coded Decimal Interchange Code),
used for IBM mainframes and peripherals, 8-bit code, control
characters
• Baudot code: 5-bit code for each character. It was designed for
the French Telegraph
• BCD (Binary Coded Decimal), which was common in many
early IBM mainframes
• Unicode: New. 16-bit code independent of language or computer
platform. Supports many scripts and mathematical symbols
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Codes
C odes
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Figure 2.26 – Transmitting an ASCII Coded Message
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