Network Technology CSE3020 Week 4 - PowerPoint PPT Presentation


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Network Technology CSE3020 Week 4


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Title: Network Technology CSE3020 Week 4

Network Technology CSE3020 Week 4
Transmission Media
  • Guided Transmission
  • Twisted Pair
  • - Unshielded Twisted Pair (UTP)
  • - Shielded Twisted Pair (STP)
  • Coaxial Cable
  • Optical Fiber
  • Unguided (wireless) Transmission
  • Terrestrial Microwave
  • Satellite Microwave
  • Broadcast Radio
  • Infrared

Transmission Media
  • Guided (wire) and Unguided (wireless) use
    electromagnetic waves.
  • Characteristics and quality determined by medium
    and the transmitted signal.
  • For guided, the medium is more important.
  • For unguided, the bandwidth of signal produced by
    the antenna is more important.
  • Key concerns are data rate and distance.

Transmission Media
  • Design Factors
  • Bandwidth
  • - Higher bandwidth gives higher data rate.
  • Transmission impairments
  • - Attenuation limits the distance.
  • Interference (Noise)
  • - Overlapping frequency bands can distort or
    wipe out a
  • signal
  • - Emanations from nearby cables
  • Number of receivers
  • - More receivers (multi-point), each attachment
    introduces some attenuation in guided media.

Interference (Noise)
  • Additional signals inserted between transmitter
    and receiver.
  • Thermal - Due to thermal agitation of
    electrons. - Uniformly distributed. - White
  • Intermodulation signals that are the sum and
    difference of original frequencies
    sharing a medium.
  • Crosstalk signal from one line is picked up by
  • Impulse - Irregular pulses or spikes -
    e.g. External electromagnetic interference -
    short duration high amplitude

Electromagnetic Spectrum
Twisted Pair
  • Two separately insulated copper wires twisted
    together form a pair.
  • A wire pair acts as a single communication link
  • Twisting reduces crosstalk between adjacent pairs
  • Applications
  • Common medium for both analog and digital.
  • Telephone network - Between house and local
    exchange (local loop) 56kbps modem, ADSL modem,
  • Within buildings - Private branch exchange (PBX).
  • For local area networks (LAN) - 10Mbps,100Mbps,

Twisted Pair
  • Two categories
  • Unshielded Twisted Pair (UTP)
  • Shield Twisted Pair (STP)

  • Cat 1
  • Ordinary home telephone cable
  • Cat 3
  • Bandwidth 16 MHz.
  • Voice grade cable found in most offices.
  • Twist length of 7.5 cm to 10 cm.
  • Cat 5
  • Bandwidth 100 MHz.
  • Commonly pre-installed in new office buildings
  • 100BaseTx max 100m, then need repeater
  • Twist length 0.6 cm to 0.85 cm
  • Cat 6 250MHz
  • Cat 7 600MHz

  • Metal braid or sheathing that reduces
  • More expensive.
  • Harder to handle and work with

Near End Crosstalk
  • Coupling of signal from one pair to another.
  • Coupling takes place when transmit signal
    entering the link couples back to receiving pair.
  • i.e. near transmitted signal is picked up by near
    receiving pair.

Coaxial Cable
  • A coax cable consists of the following
  • - A center conductor usually copper.
  • - A metalic outer conductor serves as a ground.
  • - An insulator covering the center conductor.
  • - A plastic jacket.
  • Two types Thinnet and Thicknet.

Coaxial Cable
  • Applications
  • Television distribution - Cable TV
  • Long distance telephone transmission
  • - Can carry 10,000 voice calls simultaneously
    using FDM
  • - Being replaced by fiber optic.
  • Short distance computer systems links.
  • Transmission Characteristics
  • Higher frequency characteristics than twisted
  • Analog (Up to 500MHz) Amplifiers every few km,
    closer if higher frequency.
  • Digital Repeater every 1km, closer for higher
    data rates.

Optical Fiber
  • Greater capacity - Data rates of hundreds of
  • Smaller size lighter weight.
  • Lower attenuation.
  • Electromagnetic isolation.
  • Greater repeater spacing - tens of km at least.
  • Transmits a signal-encoded beam of light by means
    of total internal reflection.
  • Act as wave guide for 1014 to 1015 Hz.
  • Portions of infrared and visible spectrum.

Optical Fiber
  • TIR Total Internal Reflection
  • 100 of light that strikes a surface is reflected
  • By comparison a mirror reflects 90
  • Optical fibers should guide light waves with
    minimal loss
  • Angle of Incidence I the angle the ray makes
    with the line perpendicular (90o) to the
    interface between two substances
  • Critical angle
  • Refraction - I lt critical angle
  • Reflection I gt critical angle

Optical Fiber - Applications
  • Long-haul trunks
  • - about 1500 km in length 20,000-60,000 voice
  • - undersea optical fiber
  • Metropolitan trunks.
  • - about 12 km in length 100,000 voice
  • - underground conduits joining telephone
  • Rural exchange trunks.
  • - about 40 160 km in length less than 5000
    voice channels.
  • Subscriber loops.
  • - handling voice, data, image and video.
  • LANs.
  • - Capacity of 100 Mbps to 1 Gbps.

Optical Fiber - Transmission Characteristics
  • Three types of transmission modes.
  • Step-index multimode
  • Rays are reflected, absorbed and propagated along
    the fiber.
  • Light pulses spread out in time.
  • Single mode
  • Single transmission path.
  • Long distance application (telephone cable TV).
  • Graded-index multimode
  • Intermediate between single mode and step-index
  • Used in LAN.

Optical Fiber Transmission Characteristics
Optical Fiber - Transmission Characteristics
  • Light sources semiconductor devices that emit a
    beam of light when a voltage is applied.
  • Light Emitting Diode (LED)
  • Cheaper.
  • Wider operating temperature range.
  • Last longer.
  • Injection Laser Diode (ILD)
  • More efficient.
  • Greater data rate.
  • Wavelength Division Multiplexing (FDM)
  • multiple beams of light at different frequencies
    (or wavelengths) are transmitted on the same
  • Commercial systems 80 channels of 10 Gbps

Wireless Transmission
  • Unguided media (Transmission and reception via
  • Directional (Higher Frequencies)
  • Focused beam.
  • Omnidirectional (Lower Frequencies)
  • Signal spreads in all directions.
  • Propagation in free space always like light
    (straight line).
  • Receiving power proportional to 1/d². (d
    distance between sender and receiver).
  • Signal can take many different paths between
    sender and receiver due to reflection,
    scattering, diffraction.

Wireless Transmission
  • Channel characteristics change over time and
  • Influences on the received signal power
  • Fading variation of signal strength with time.
  • shadowing
  • reflection at large obstacles
  • scattering at small obstacles
  • diffraction at edges

Wireless Transmission
signal at sender
signal at receiver
  • Multipath channel

Wireless Transmission
  • To reduce channel effects
  • Error correction codes remove bit errors
    introduced in transmission.
  • Equalizations remove channel attenuation delay
    (multipath effects).
  • Diversity techniques use of more than one
  • Frequencies
  • 2GHz to 40GHz
  • Microwave.
  • Highly directional beam.
  • Point-to-point transmission.
  • Satellite.
  • 30MHz to 1GHz
  • Broadcast radio.
  • Omnidirectional transmission.
  • 3 x 1011 to 2 x 1014
  • Infrared.
  • Local point-to-point and multipoint application.

Terrestrial Microwave
  • Parabolic dish antenna (typical size of 3 m in
  • Focused beam to achieve line of sight
  • Located at substantial heights above ground
  • Microwave relay towers used to achieve long
  • Applications
  • Long haul telecommunications service.
  • Short point-to-point links between buildings.
  • Higher frequencies give higher data rates.
  • Attenuation and interference.
  • Attenuation is increased with rainfall.
  • With growing applications, transmission areas
    overlap and resulting in interference.

Satellite Microwave
  • Satellite is a microwave relay station.
  • Satellite receives on one frequency band (uplink)
    and amplifies or repeats signal and transmits on
    another frequency band (downlink).
  • May requires geo-stationary orbit
  • To maintain line of sight.
  • Height of 35,784km.
  • Applications.
  • Television distribution.
  • Long-distance telephone transmission.
  • Private business networks.
  • VSAT systems.
  • High propagation delay (about 0.25 seconds).

Broadcast Radio
  • Omnidirectional.
  • Does not require dish-shaped antenna.
  • Applications
  • FM radio
  • UHF and VHF television
  • Limited by line of sight.
  • Less sensitive to attenuation from rainfall.
  • Suffers from multipath interference (reflections).

  • Modulate noncoherent infrared light.
  • Transceivers (transmitters/receivers) must be
    within the line of sight.
  • Does not penetrate walls.
  • E.g. TV remote control.
  • No frequency allocation no licensing.

Required Reading
  • W. Stallings, Data and Computer Communications
    (6th edition), Prentice-Hall, 2000.
  • gtgt Chapter 4

Asynchronous and SynchronousTransmission
  • Transmission requires cooperation and agreement
    between the two sides.
  • Fundamental requirement is Synchronization
    Receiver must know the beginning and end of a
    bit/rate at which bits are received.
  • Asynchronous transmission
  • Synchronous transmission
  • Asynchronous transmission
  • Each character treated independently and begins
    with a start bit.
  • Not good for long block of data.
  • Synchronous transmission
  • Block of data is formatted as a frame with a
    starting and an ending flag.
  • Good for block of data.

Asynchronous Transmission
  • Avoid sending long, uninterrupted streams of
  • Data transmitted on character at a time (5 to 8
  • Timing only needs to be maintained within each
  • Receiver can resynchronize with each new
  • Operation
  • In idle state (binary 1), receiver looks for
    transition from 1 to 0 (start bit).
  • Then samples next seven intervals (char length).
  • May add a parity bit (odd or even).
  • The final signal element is the stop bit (binary
  • The stop bit is same as idle state.
  • Then looks for next 1 to 0 transition for next
  • Advantages Simple and Cheap.
  • Disadvantage Overhead of 2 or 3 bits per char

Asynchronous Transmission
Synchronous Transmission
  • Block of data transmitted without start or stop
  • Clocks must be synchronized.
  • Can use separate clock line.
  • Good over short distances.
  • Subject to impairments.
  • Embed clock signal in data.
  • Manchester encoding (digital).
  • Carrier frequency (analog)

Synchronous Transmission
  • Block Level
  • Need to indicate start and end of block
  • Use preamble and postamble.
  • e.g. series of SYN (hex 16) characters
  • e.g. block of 11111111 patterns ending in
  • More efficient (lower overhead) than asynchronous.

Line Configuration
  • Topology physical arrangement of
    stations on a transmission medium.
  • Point to point
  • Multi point.
  • Half duplex
  • Only one station at a time.
  • Requires one data path.
  • Full duplex
  • Simultaneous transmission.
  • Requires two data paths.

  • Data processing devices or data terminal
    equipments (DTE) do not (usually) include data
    transmission facilities.
  • Need an interface called data circuit terminating
    equipment (DCE).

  • DCE transmits bits on medium.
  • DCE communicates data and control information
    with DTE.
  • Done over interchange circuits.
  • Clear interface standards required.
  • Characteristics of Interface
  • Mechanical
  • Connection plugs.
  • Electrical
  • Voltage levels, timing of voltage changes,
  • Determines the data rate and the distance.
  • Functional
  • Data, control, timing, grounding.
  • Procedural
  • Sequence of events for transmitting data.

  • Most widely used interface (e.g. PC serial port)
  • ITU-T standard V.24
  • Only specifies functional and procedural aspects.
  • References other standards for electrical and
  • Virtually identical standard EIA-232-F (USA)
    covers all aspects.
  • Mechanical ISO 2110
  • Electrical V.28
  • Functional V.24
  • Procedural V.24
  • EIA-232 was first issued by Electronic Industries
    Alliance in 1962, as RS-232.
  • Used in modem and other interconnection

Mechanical Specification
Electrical Specification
  • Digital signals on all interchange circuits.
  • Values interpreted as data or control, depending
    on circuit.
  • Less than -3v is binary 1, more than 3v is
    binary 0 (NRZ-L).
  • Signal rate lt 20kbps.
  • Distance lt15m.
  • For control, less than -3v is off, and more than
    3v is on.

Functional Specification
  • Refer table 6.1 in Stallings chapter 6 for
    functional specifications.
  • Loopback control is a useful fault isolation
  • - Make use of functional specifications.

Local and Remote Loopback
Procedural Specification
  • Defines the sequence in which the various
    circuits are used.
  • E.g. Asynchronous private line modem
  • When turned on and ready, modem (DCE) asserts DCE
  • When DTE ready to send data, it asserts Request
    to Send.
  • Modem responds when ready by asserting Clear to
  • DTE sends data.
  • When data arrives, local modem asserts Receive
    Line Signal Detector and delivers data.

Dial Up Operation
Null Modem
ISDN Physical Interface
  • Connection between terminal equipment (correspond
    to DTE) and network terminating equipment
    (correspond to DCE).
  • Defined in ISO 8877.
  • Cables terminate in matching connectors with 8
  • Transmit/receive carry both data and control

ISDN Electrical Specification
  • Balanced transmission
  • Carried on two lines. e.g., twisted pair
  • Signals as currents down one conductor and up the
  • Differential signaling
  • Value depends on direction of voltage.
  • Tolerates more noise and generates less
    (Unbalanced, e.g. RS-232 uses single signal line
    and ground).
  • Data encoding depends on data rate.
  • Basic rate 192kbps uses pseudoternary.
  • Primary rate uses alternative mark inversion
    (AMI) and B8ZS or HDB3.

Required Reading
  • W. Stallings, Data and Computer Communications
    (6th edition), Prentice-Hall, 2000.
  • gtgt Chapter 6