CSC 335 Data Communications and Networking

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CSC 335 Data Communications and Networking

• Lecture 2 Transmission Fundamentals
• Dr. Cheer-Sun Yang

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Data Communication

• Examines how data, in the form of energy,
  travel across some medium from a source to a
  destination.

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A Simplified Communications Model
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Data Transmission

• Data
• Entities that convey meaning
• Signals
• Electric or electromagnetic representations of
  data
• Transmission
• Communication of data by propagation and
  processing of signals

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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

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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

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How can data be transmitted?

• This question will be our focus in the next
  couple of weeks. First, well introduce the
  concept of electrical signal. Then, well focus
  on the concept of communication media and how
  data can be transferred across such media.
  Finally, well explain how transmission forms the
  basis of data networking.

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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

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Continuous Discrete Signals
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PeriodicSignals
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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

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Some Examples

• If T 0.5 ms, what is the frequency?
• (NOTE 1/second Hz)
• If ƒ 1 MHz, T ?

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Some Units

• Some units
• Kilo
• Mega
• Giga

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Some Other Units

• Millisecond (ms)
• Microsecond (µs)
• Nanosecond (ns)

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Varying Sine Waves
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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.

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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

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Electromagnetic Spectrum

• James Maxwell 1865 predict the existence of
  electromagnetic waves
• Heinrich Hertz 1887 first produced and observed
  these waves. Thats why frequency is measured in
  Hertz(Hz) the number of oscillations per second
  of an electromagnetic wave.

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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.

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Sample Data Representation

• Bits can be sent as a voltage or current through
  a wire
• For example
• zero 1 volts
• one -1 volts

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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.

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Electromagnetic Spectrum
Radio
Microwave
Infrared
Visible Light
Ultra Violet Light (UV)
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Electromagnetic Spectrum
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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.

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Transmission Media

• Copper Wires
• Glass Fibers
• Radio
• Satellites
• Geosynchronous Satellites
• Low Orbit Satellites
• Low Orbit Satellite Arrays
• Microwaves
• Infrared
• Laser Lights

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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.

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Twisted Pair

• Insulated Copper wires, about 1mm thickTwisted,
  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)

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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.

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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.

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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.

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Coaxial Cable(contd)
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More about Coaxial Cable

• Coaxial cable typically transmits information in
  one of two modes baseband or broadband mode.
• Baseband mode - the cables bandwidth is devoted
  to a single stream of data.
• Broadband mode - the bandwidth is divided into
  ranges. Each range typically carries separate
  coded information.

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Optical Fiber

• 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

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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.

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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
• Dont leak light tough to tap into, thus more
  secure.

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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.

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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

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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 MMstep-index for SM

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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

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Radio (contd)

• 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.

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Microwave

• Uses electromagnetic waves in the range of
• 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

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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

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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

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Satellite Transmission System

• Geosynchronous Satellites According to Keplers
  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 earths 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)

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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.

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Satellite Frequency Bands

• L band (uplink)1.6465-1.66GHz (downlink)
  1.545-1.5585 GHz
• C band (uplink)5.925-6.425GHz(downlink)
  3.7-4.2GHz
• Ku band (uplink)14-14.5GHz (downlink) 11.7-12.2
  GHz
• Ka band (uplink)27.5-30.5GHz (downlink)
  17.7-21.7 GHz

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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?

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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.

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Reading Assignment

• Section 2.1