Transcript CSC 335 Data Communications and Networking I

CSC 335
Data Communications
and
Networking
Lecture 2: Transmission
Fundamentals
Dr. Cheer-Sun Yang
Data Communication
Examines how data, in the form of energy,
travel across some medium from a source to
a destination.
A Simplified Communications Model
Data Transmission
• Data
– Entities that convey meaning
• Signals
– Electric or electromagnetic representations of
data
• Transmission
– Communication of data by propagation and
processing of signals
Terminology (1)
• Source – also called sender
• Destination – also called receiver
• Medium
– Guided medium
• e.g. twisted pair, optical fiber
– Unguided medium
• e.g. air, water, vacuum
Terminology (2)
• Direct link
– No intermediate devices
• Point-to-point
– Direct link
– Only 2 devices share link
• Multi-point
– More than two devices share the link
How can data be transmitted?
This question will be our focus in the next
couple of weeks. First, we’ll introduce the
concept of electrical signal. Then, we’ll
focus on the concept of communication
media and how data can be transferred
across such media. Finally, we’ll explain
how transmission forms the basis of data
networking.
Analog and Digital Signals
• Signal – electrical energy measured by the unit of
voltage.
• Digital signals – a sequence of voltage levels.
Graphically, they are represented as a square
wave.
• Analog signals – continuously varying voltage
levels; used in the communication over phone
lines.
• Refer to Fig 2.1 for examples of digital and
analog signals
Continuous & Discrete Signals
Periodic
Signals
Signal Wave
• Peak Amplitude (A)
– maximum strength of signal
– volts
• Frequency (f)
–
–
–
–
Rate of change of signal
Hertz (Hz) or cycles per second or (1/second = Hz)
Period = time for one repetition (T)
f = 1/T ( so T = 1/f )
• Phase ()
– Relative position in time
Some Examples
• If T = 0.5 ms, what is the frequency?
(NOTE: 1/second = Hz)
• If ƒ = 1 MHz, T = ?
Some Units
• Some units:
– Kilo =
– Mega =
– Giga =
10 3
10 6
10 9
Some Other Units
• Millisecond (ms) = 103
• Microsecond (µs) = 106
• Nanosecond (ns) = 109
Varying Sine Waves
Theoretical Basis for Data
Transmission
• Information can be transmitted through a
medium by varying some physical property.
• The physics of the universe (noise,
distortion, attenuation) places some limits
on what can be sent over a channel.
• Purpose of physical layer – to transport a
raw bit stream from one machine to another.
Electromagnetic Spectrum
• Electromagnetic energy – waves created by
moving electrons.
• Electromagnetic wave – a large family of
waves consisting of electric and magnetic
fields that vibrate ate high angles to each
other, both vibrating at the same frequency
• Pertinence to media – asset and hindrance
Electromagnetic Spectrum
• James Maxwell 1865 – predict the existence
of electromagnetic waves
• Heinrich Hertz 1887 – first produced and
observed these waves. That’s why
frequency is measured in Hertz(Hz) – the
number of oscillations per second of an
electromagnetic wave.
How do we transmit these
waves?
• Feed an electrical signal to the antenna of a
transmitter
• The signal makes the atoms of the antenna
vibrate (changing energy levels).
• This change causes the antenna to emit
electromagnetic waves.
Sample Data Representation
• Bits can be sent as a voltage or current through a
wire
• For example:
zero = +1 volts
one = -1 volts
voltage
+1
0
-1
1
0
1
0
0
1
Bandwidth
A given transmission medium can accommodate
signals within a given frequency range. The
bandwidth is equal to the difference between the
highest and the lowest frequencies that may be
transmitted. For example, a telephone signal can
handle frequencies between 300 Hz and 3300 Hz,
giving it a bandwidth of 3000 Hz. This means,
very high- or low-pitched sound cannot pass
through the telephone system. Sometimes,
bandwidth is used to denote the number of bits
that can be transmitted.
Electromagnetic Spectrum
Radio
104
108
Microwave
108
1012
Infrared
1012
1014
Visible Light
1014
Ultra Violet Light (UV)
1015
1015
1016
Electromagnetic Spectrum
Criteria for Media Evaluation
• Bandwidth – difference between highest and
lowest frequencies that may be transmitted.
• Bit rate – expresses the data rate capacity of a
network system.
• Delay – the time period required to send a signal
across a network.
• Cost of medium material.
• Ease of installation and maintenance.
Transmission Media
•
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•
•
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Copper Wires
Glass Fibers
Radio
Satellites
Geosynchronous Satellites
Low Orbit Satellites
Low Orbit Satellite Arrays
Microwaves
Infrared
Laser Lights
Copper Wires
• Why copper? – low resistance to electrical
current; signal travels farther; low cost.
• Guided medium
• Bandwidth – depends on the thickness of
the wire and the distance traveled; typical is
several megabits/second.
• Interference – the twist helps reduce
interference.
• Two main types: twisted pair and coaxial
cable.
Twisted Pair
• Insulated Copper wires, about 1mm thick
Twisted, to avoid forming an antenna: reduces
interference
• Two major kinds
– Cat 3 (1988 and earlier)
• four pairs: (allows four telephone lines)
– Cat 5: (new installations)
• more twists per centimeter, and Teflon insulation
• more suitable for high speed networks.
• Shielded vs. Unshielded:
– shielded twisted pair (STP)
• (shield serves as ground, some applications in business use this,
but becoming more rare)
– unshielded twisted pair (telco local loop to home is
usually UTP)
More about Twisted Pair
• Bandwidth = 250 kHz for analog signals
• Bandwidth varies for carrying digital
signals. For example, a local area network
can use twisted pair to operate at 100 Mbps
over a segment length of 100 meters.
• Twisted pair can also support a 2400 bps
rate for up to 10 miles.
Signal Distortion
• attenuation - when a signal is transmitted
over a copper wire, it will distort and lose
strength. This situation is called attenuation.
• Repeater – the device connecting two
sections of twisted pairs. A repeater
removes distortion, amplifies and receives
received signal.
Coaxial Cable
• Provides more protection from interference
• Single wire – surrounded by a heavier metal shield
which protects from incoming electromagnetic waves
• Bandwidth – depends on cable length; typical data
rate of 1 to 2 Gbps for 1 Km cable.
• Cost higher than TP
• Installation – heavy and unwieldy.
Copper Wire
Insulation
Copper Mesh
Outside Insulation
Coaxial Cable(cont’d)
More about Coaxial Cable
Coaxial cable typically transmits information
in one of two modes: baseband or
broadband mode.
• Baseband mode - the cable’s bandwidth is
devoted to a single stream of data.
• Broadband mode - the bandwidth is divided
into ranges. Each range typically carries
separate coded information.
Optical Fiber
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•
•
•
Very prevalent
Medium – glass fiber
Energy – light pulses
Three components of a fiber system:
– Light source: Light-emitting diode(LED) or laser (Light
Amplification by Simulated Emission of Radiation)
– Glass fiber
– Detector: transforms the light to electrical pulses at the
receiving end.
• Cost – higher than copper
• Installation – requires specialized technicians.
• Bandwidth - huge
Laser
• Unguided medium (fiber was guided).
• Technology uses a laser beam of light to
carry data through the air
• 2 sites: transmitter and receiver
• Equipment is fixed
• Beam is unidirectional, traveling in a
straight line.
Advantages of Fiber over Copper
• Interference – does not cause interference; is not
susceptible to interference.
• Bandwidth – handles much higher bandwidth than
copper
• Low attenuation – requires fewer repeaters and
amplifiers (every 30 km vs. 5 km, or 20 miles vs.
4 miles)
• Immune to power surges, failures, and other
electromagnetic interference
• Thin and lightweight
• Don’t leak light; tough to tap into, thus more
secure.
Why no leakage?
• Property of refraction – a light ray reflects
when passing from one medium to another.
Some will cross the boundary into the other.
It is called refraction. When  is less than a
certain angle, there is no refrected light.
(Fig 2.6)
• This is what makes fiber optics work.
Fiber Cables
• Similar to coax
• 3 parts: core (glass), cladding (glass), and jacket
(plastic). The cladding has a lower index of
refraction than the core to keep the light in.
• Where are they?
– Terrestrial – within 1 meter of surface
– Transoceanic fibers – buried in trenches by sea plows
– Deep water – just lie on the bottom
Fiber Optics - Disadvantages
• Inherently unidirectional – For two-way
communication, two fibers are required.
• Costly – fiber interfaces are more expansive
than copper or coax.
• Modal Dispersion - as distance increases,
the difference between modes of the lights
becomes bigger. (mode: path of light)
Solution: graded-index for MM;step-index
for SM
Radio
• Electromagnetic spectrum: 102 – 1010 Hz.
• Using radio waves(RF) of the spectrum to
transmit computer data
• Radio waves are omni directional
• No physical connection required – unguided
• Each computer attaches to an antenna
which both transmits and receives
Radio (cont’d)
• Antennas
• Sizes of antenna depends on distance of
communication to be performed
• Communication of several miles: antenna should
be about two meters high mounted on building top
• Communication within same building: antenna
can be small enough to fit inside a portable
computer.
Microwave
• Uses electromagnetic waves in the range of
1012  1014 Hz
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Used for localized, small areas
Transmitter pointed directly at receiver
No antenna needed
Repeaters may be needed.(Fig. 2.13, 2.14)
Others: infrared
See Fig. 2.10
Satellites
• Satellite – an object launched to orbit a celestial
body
• Orbit – the path of a satellite as it revolves around
another body
• Geostationary orbit – a path of a satellite that
coincides with the revolution of the earth such that
the satellite remains seemingly fixed at the same
point above the equator from the perspective of a
person standing on earth
• Geostationary orbit for earth – 22300 miles
(36000km) above the equator
Satellite Transmission System
• Uplink earth station –
– Takes baseband signals as inputs
– Modulates a high frequency radio frequency
• Satellite (receiver, transponder, transmitter)
– Receives the radiated signal
– Shifts its frequency using a transdponder to avoid
interference
– Amplifies the signal
– Retransmits the signal back to earth where it can be
received by downlink earth stations in the coverage
area
• Downlink earth station –
– Receives and demodulates the radiated signal
– Transmits the information to local receivers
Satellite Transmission System
• Geosynchronous Satellites – According to
Kepler’s Law, at the orbit height of 22,300 miles
above the equator, a satellite can appear stationary
to a ground observer.
• Low Earth Orbit Satellites – Military surveillance
require that a satellite not remain in a fixed
position. LEO allows the satellite to move relative
to the earth’s surface and scan different areas..
LEO requires less powerful rocket. However,
since it keeps moving, eventually it may move out
of the range of a ground station. A row of LEO
may be required. (Fig 2.21)
Satellite Transmission System
• Transponder - a device that accepts a signal within
a specified frequency range and rebroadcast it
over a different frequency
• Each satellite has several transponders.
• A ground based transmitter sends a signal (uplink)
to a satellite, where one of the transponders relays
the signal back down to earth (downlink) to a
different location.
• Satellite communications are now commonly used
to transmit telephone and television signals.
• Satellite dish - a private receiver for cable
television reception.
Satellite Frequency Bands
• L band: (uplink)1.6465-1.66GHz; (downlink)
1.545-1.5585 GHz
• C band: (uplink)5.925-6.425GHz;(downlink) 3.74.2GHz
• Ku band: (uplink)14-14.5GHz; (downlink) 11.712.2 GHz
• Ka band: (uplink)27.5-30.5GHz; (downlink) 17.721.7 GHz
Problems
• How can a satellite discriminate signals that were
not meant for it? - FCC defines US satellite
positions.
• How do you prevent unauthorized reception of
signals?
• How do you prevent unauthorized transmission
via satellite?
Wireless LAN
• Allows PC and other Local Area Network (LAN)
to communicate without physical link.
• Many applications can take advantage of this kind
of systems. For example, medical personnel can
use notebook computer to connect to a wireless
LAN.
• Fiber optic and microwave systems installed at
Edwards Air Force Base is another example.
• Disadvantage: data rate is low.
Reading Assignment
• Section 2.1