1 / 39

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!

claire
Download Presentation

CMPE 150 Fall 2005 Lecture 9

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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.

More Related