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CMPE 150 Fall 2005 Lecture 9

CMPE 150 Fall 2005 Lecture 9. Introduction to Computer Networks. Announcements. 2 nd . lab today at 4pm BE 168. If you cannot make it to a lab session, let us know and we’ll schedule a make up slot. Hw. 2 is up on the Web page. Career Center Job and Internship Fair!

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CMPE 150 Fall 2005 Lecture 9

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  1. CMPE 150Fall 2005Lecture 9 Introduction to Computer Networks

  2. Announcements • 2nd. lab today at 4pm BE 168. • If you cannot make it to a lab session, let us know and we’ll 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!

  3. Today • PHY (cont’d). • Wireless transmission (cont’d). • Mobile telephony.

  4. Wireless Transmission

  5. Wireless Transmission • Electron movement: electromagnetic waves that propagate through space. T R

  6. Propagation • Maximum speed: speed of light, c, 3*108 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).

  7. The Electromagnetic Spectrum • The electromagnetic spectrum and its uses for communication.

  8. Radio Transmission • (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. ~1Km

  9. The Electromagnetic Spectrum • The electromagnetic spectrum and its uses for communication.

  10. 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.

  11. 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.

  12. ISM in the US . Devices with power < 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.

  13. Spread Spectrum • Narrow frequency band -> 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.

  14. The Electromagnetic Spectrum • The electromagnetic spectrum and its uses for communication.

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

  16. 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).

  17. Communication Satellites • Weather balloons. • The moon. • Artificial satellites: • Geostationary. • Medium-Earth Orbit. • Low-Earth Orbit.

  18. Satellite Communications SAT ground stations

  19. 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->satellite. • Downlink: satellite->earth. • Transponder: satellite electronics converting uplink signal to downlink.

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

  21. Communication Satellites

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

  23. GEO: Plus’s and minus’s • Plus’s: • Stationarity: no frequency changes due to movement. • Tracking by earth stations simplified. • At that altitude, provides good coverage of the earth. • Minus’s: • Weakening of signal. • Polar regions poorly served. • Delay! • Spectral waste for point-to-point communications.

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

  25. 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 < 20ms (compared to > 300ms for GEOs). • Subject to large frequency changes and gradual orbit deterioration.

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

  27. LEOs SAT constellation SAT SAT ground stations

  28. Low-Earth Orbit SatellitesIridium • (a) The Iridium satellites from six necklaces around the earth. • (b) 1628 moving cells cover the earth.

  29. 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.

  30. 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.

  31. The Mobile Telephone System • First-Generation Mobile Phones: Analog Voice • Second-Generation Mobile Phones: Digital Voice • Third-Generation Mobile Phones:Digital Voice and Data

  32. The “Cell” Concept • (a) Frequencies not reused in adjacent cells. • (b) To add more users, smaller cells.

  33. Mobile Phone System Structure • Hierarchy. • Base station. • Mobile Switching Center (MSC). • MSCs connected through PSTN.

  34. 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.

  35. Cable Television

  36. Community Antenna Television • An early cable television system.

  37. Internet over Cable • Cable television

  38. DSL • The fixed telephone system.

  39. 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. • IoC’s capacity is unpredicatble as it depends on how many users/traffic.

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