CMPE 150 Fall 2005 Lecture 9

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CMPE 150 Fall 2005Lecture 9

• Introduction to Computer Networks

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Announcements

• 2nd. lab today at 4pm BE 168.
• If you cannot make it to a lab session, let us
  know and well schedule a make up slot.
• Hw. 2 is up on the Web page.
• Career Center Job and Internship Fair!
• When? Tue, Oct. 18th. 9am-noon.
• Where? University Center.
• CEFULs CE ugrad lunch.
• Meet faculty.
• Talk about important topics.
• Free lunch!

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Today

• PHY (contd).
• Wireless transmission (contd).
• Mobile telephony.

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Wireless Transmission
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Wireless Transmission

• Electron movement electromagnetic waves that
  propagate through space.

T
R
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Propagation

• Maximum speed speed of light, c, 3108 m/s.
• In vacuum, all EM waves travel at the same speed
  c.
• Otherwise, propagation speed is function of
  frequency (c l f), where f is frequency (Hz)
  and l is wavelength (m).

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

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Radio Transmission
1Km

• (a) In the VLF, LF, and MF bands, radio waves
  follow the curvature of the earth. E.g., AM radio
  uses MF.
• (b) In the HF and VHF bands, they bounce off the
  ionosphere. E.g., Hams and military.

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

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Microwave Transmission

• Above 100MHz.
• Waves travel in straight lines.
• Directionality.
• Better quality.
• Space Division Multiple Access.
• But, antennas need to be aligned, do not go
  through buildings, multi-path fading, etc.
• Before fiber, microwave transmission dominated
  long-distance telephone transmission.

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Politics of the Electromagnetic Spectrum

• Need agreements to regulate access.
• International and national.
• Local governments allocate spectrum for radio (AM
  and FM), TV, mobile phones, emergency services,
  etc.
• In the US, FCC.
• World-wide, ITU-R tries to coordinate allocation
  so devices work everywhere.
• Separate frequency band that is unregulated.
• ISM Industrial, Scientific, and Medical.
• Household devices, wireless phones, remote
  controls, etc.

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ISM in the US
. Devices with power lt 1W can use the ISM
bands. . 900 MHz is crowded and not available
world-wide. . At 2.4GHz, widely available but
interference prone. . Bluetooth and some 802.11
WLANs. . 5.7GHz is the next one to be populated.
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Spread Spectrum

• Narrow frequency band -gt good reception (power,
  bandwidth).
• But in some cases, wide band is used, aka, spread
  spectrum.
• Modulate signal to increase bandwidth of signal
  to be transmitted.
• 2 variations
• Frequency Hopping (FH).
• Transmitter hops frequencies
• Direct Sequence (DS).
• Use spreading code to convert each bit of the
  original signal into multiple bits.

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The Electromagnetic Spectrum

• The electromagnetic spectrum and its uses for
  communication.

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Infrared Transmission

• Short range (e.g., remote controls).
• Directional, cheap.
• But, do not pass through obstacles.

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Lightwave Transmission

• Unguided optical transmission.
• E.g., laser communication between two buildings
  for LAN interconnection.
• High bandwidth, low cost.
• Unidirectionality.
• Weather is a major problem (e.g., rain,
  convection currents).

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Communication Satellites

• Weather balloons.
• The moon.
• Artificial satellites
• Geostationary.
• Medium-Earth Orbit.
• Low-Earth Orbit.

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Satellite Communications
SAT
ground stations
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Satellite Communications

• Satellite-based antenna(e) in stable orbit above
  earth.
• Two or more (earth) stations communicate via one
  or more satellites serving as relay(s) in space.
• Uplink earth-gtsatellite.
• Downlink satellite-gtearth.
• Transponder satellite electronics converting
  uplink signal to downlink.

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Orbits

• Shape circular, elliptical.
• Plane equatorial, polar.
• Altitude geostationary (GEO), medium earth
  (MEO), low earth (LEO).

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Communication Satellites
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GEOs

• High-flying satellites.
• Orbit at 35,863 Km above earth and rotates in
  equatorial plane.
• Many GEO satellites up there!

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GEO Pluss and minuss

• Pluss
• Stationarity no frequency changes due to
  movement.
• Tracking by earth stations simplified.
• At that altitude, provides good coverage of the
  earth.
• Minuss
• Weakening of signal.
• Polar regions poorly served.
• Delay!
• Spectral waste for point-to-point communications.

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Principal Satellite Bands
. Downlink frequencies interfere with
microwave. . Internationally-agreed frequency
bands.
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LEO Satellites

• Circular or slightly eliptical orbit under 2,000
  Km.
• Orbit period 1.5 to 2 hours.
• Coverage diameter 8,000 Km.
• RTT propagation delay lt 20ms (compared to gt 300ms
  for GEOs).
• Subject to large frequency changes and gradual
  orbit deterioration.

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LEO Constellations

• Advantages over GEOs
• Lower delay, stronger signal, more localized
  coverage.
• But, for broad coverage, many satellites needed.
• Example Iridium (66 satellites).

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LEOs
SAT
constellation
SAT
SAT
ground stations
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Low-Earth Orbit SatellitesIridium

• (a) The Iridium satellites from six necklaces
  around the earth.
• (b) 1628 moving cells cover the earth.

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In Summary

• GEOs
• Long delay - 250-300 ms.
• LEOs
• Relatively low delay - 40 - 200 ms.
• Large variations in delay - multiple hops/route
  changes, relative motion of satellites, queuing.

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Satellite Data Rates

• Satellite has 12-20 transponders, each ranging
  from 36-50 Mbps.
• T1 1.54 Mbps.
• T2 6.312 Mbps.
• T3 44.736 Mbps.
• T4 274.176 Mbps.

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The Mobile Telephone System

• First-Generation Mobile Phones Analog Voice
• Second-Generation Mobile Phones Digital Voice
• Third-Generation Mobile PhonesDigital Voice and
  Data

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The Cell Concept

• (a) Frequencies not reused in adjacent cells.
• (b) To add more users, smaller cells.

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Mobile Phone System Structure

• Hierarchy.
• Base station.
• Mobile Switching Center (MSC).
• MSCs connected through PSTN.

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Handoffs

• As mobile phones move, they switch cells, and
  thus base stations.
• Soft versus hard handoffs.
• Two base stations while handoff is in progress.
• Hard handoff.
• Roaming.

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Cable Television
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Community Antenna Television

• An early cable television system.

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Internet over Cable

• Cable television

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DSL

• The fixed telephone system.

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ADSL versus Internet over Cable

• Both uses fiber in the backbone.
• ADSL uses twisted pair and IoC uses coax on the
  edge.
• Coax has higher capacity but shared with TV.
• IoCs capacity is unpredicatble as it depends on
  how many users/traffic.