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

2. CMPE 150- Introduction to Computer Networks 2 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! –When? Tue, Oct. 18 th. 9am-noon. –Where? University Center. CEFULs: CE ugrad lunch. –Meet faculty. –Talk about important topics. –Free lunch!

3. CMPE 150- Introduction to Computer Networks 3 Today PHY (cont’d). –Wireless transmission (cont’d). –Mobile telephony.

4. CMPE 150- Introduction to Computer Networks 4 Wireless Transmission

5. CMPE 150- Introduction to Computer Networks 5 Wireless Transmission Electron movement: electromagnetic waves that propagate through space. T R

6. CMPE 150- Introduction to Computer Networks 6 Propagation Maximum speed: speed of light, c, 3*10 8 m/s. In vacuum, all EM waves travel at the same speed c. Otherwise, propagation speed is function of frequency (c = * f), where f is frequency (Hz) and is wavelength (m).

7. CMPE 150- Introduction to Computer Networks 7 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

8. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 9 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

10. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 14 The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

15. CMPE 150- Introduction to Computer Networks 15 Infrared Transmission Short range (e.g., remote controls). Directional, cheap. But, do not pass through obstacles.

16. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 17 Communication Satellites Weather balloons. The moon. Artificial satellites: –Geostationary. –Medium-Earth Orbit. –Low-Earth Orbit.

18. CMPE 150- Introduction to Computer Networks 18 Satellite Communications SAT ground stations

19. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 20 Orbits Shape: circular, elliptical. Plane: equatorial, polar. Altitude: geostationary (GEO), medium earth (MEO), low earth (LEO).

21. CMPE 150- Introduction to Computer Networks 21 Communication Satellites

22. CMPE 150- Introduction to Computer Networks 22 GEOs High-flying satellites. Orbit at 35,863 Km above earth and rotates in equatorial plane. Many GEO satellites up there!

23. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 24 Principal Satellite Bands. Downlink frequencies interfere with microwave.. Internationally-agreed frequency bands.

25. CMPE 150- Introduction to Computer Networks 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 300ms for GEOs). Subject to large frequency changes and gradual orbit deterioration.

26. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 27 LEOs SAT ground stations SAT constellation

28. CMPE 150- Introduction to Computer Networks 28 Low-Earth Orbit Satellites Iridium (a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth.

29. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 32 The “Cell” Concept (a) Frequencies not reused in adjacent cells. (b) To add more users, smaller cells.

33. CMPE 150- Introduction to Computer Networks 33 Mobile Phone System Structure Hierarchy. Base station. Mobile Switching Center (MSC). MSCs connected through PSTN.

34. CMPE 150- Introduction to Computer Networks 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. CMPE 150- Introduction to Computer Networks 35 Cable Television

36. CMPE 150- Introduction to Computer Networks 36 Community Antenna Television An early cable television system.

37. CMPE 150- Introduction to Computer Networks 37 Internet over Cable Cable television

38. CMPE 150- Introduction to Computer Networks 38 DSL The fixed telephone system.

39. CMPE 150- Introduction to Computer Networks 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.