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CSC 335 Data Communications and Networking


CSC 335 Data Communications and Networking Lecture 2: Transmission Fundamentals Dr. Cheer-Sun Yang – PowerPoint PPT presentation

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Title: CSC 335 Data Communications and Networking

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

A Simplified Communications Model
Data Transmission
  • Data
  • Entities that convey meaning
  • Signals
  • Electric or electromagnetic representations of
  • 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, 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.

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
  • Analog signals continuously varying voltage
    levels used in the communication over phone
  • 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
  • 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

Some Other Units
  • Millisecond (ms)
  • Microsecond (µs)
  • Nanosecond (ns)

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
  • 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. Thats 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
  • 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

  • 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
Visible Light
Ultra Violet Light (UV)
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
  • 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
  • Interference the twist helps reduce
  • 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.

Coaxial Cable(contd)
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.

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
  • Glass fiber
  • Detector transforms the light to electrical
    pulses at the receiving end.
  • Cost higher than copper
  • Installation requires specialized technicians.
  • Bandwidth - huge

  • 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

Advantages of Fiber over Copper
  • Interference does not cause interference is
    not susceptible to interference.
  • Bandwidth handles much higher bandwidth than
  • 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

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
  • 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 MMstep-index for SM

  • 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 (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.

  • 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

  • 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 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)

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)
  • Ku band (uplink)14-14.5GHz (downlink) 11.7-12.2
  • Ka band (uplink)27.5-30.5GHz (downlink)
    17.7-21.7 GHz

  • How can a satellite discriminate signals that
    were not meant for it? - FCC defines US satellite
  • How do you prevent unauthorized reception of
  • How do you prevent unauthorized transmission via

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
  • Fiber optic and microwave systems installed at
    Edwards Air Force Base is another example.
  • Disadvantage data rate is low.

Reading Assignment
  • Section 2.1