CHAPTER 2 THE PHYSICAL LAYER ( 物理层)

1. CHAPTER 2 THE PHYSICAL LAYER (物理层) • The Theoretical Basis for Data Communication • Guided Transmission Media (Terrestrial) • Wireless Transmission (Terrestrial) • Communication Satellites (Celestial) • The Public Switched Telephone Network (Stationary) • The Mobile Telephone System (Mobile) • Cable Television (Stationary)

2. THE THEORETICAL BASIS FOR DATA COMMUNICATION (数据通信的基本理论) • Fourier analysis（傅立叶分析） • Bandwidth-limited signals（有限带宽信号） • Maximum data rate of a channel（信道的最大数据传送速率）

3. The theoretical basis for data communication • Fourier Analysis • Any reasonably behaved periodic function（周期函数） can be reconstructed by summing a (possibly infinite) number of sins and cosines. Such a decomposition is called Fourier series(傅立叶级数). • A data signal that has a finite duration (which all of them do) can be handled by just imagining that it repeats the entire pattern over and over forever.

4. The theoretical basis for data communication • Bandwidth-Limited Signals • Input  System (transmission, filtering)  output • Bandwidth (带宽) for systems • The range of frequencies transmitted without being strongly attenuated • The cutoff frequency is not really sharp, is defined as the frequency at which half the power gets through. • Any signal (bandwidth-limited or unlimited)  system (filtering)  bandwidth-limited signal (in time domain, convolution; In frequency domain, multiplication)

5. The theoretical basis for data communication • Bandwidth-Limited Signals • A binary signal and its root-mean-square Fourier amplitudes. (b) – (c) Successive approximations to the original signal.

6. The theoretical basis for data communication • Bandwidth-Limited Signals • (d) – (e) Successive approximations to the original signal.

7. The theoretical basis for data communication • Bandwidth-Limited Signals • The time required to transmit the character depends on both the encoding method and the signaling speed. The number of changes per second is measured in baud(波特). • Given a bit rate b bits/sec, the time to send 1 bit is 1/b, the time required to send 8 bits 1 bit at a time is 8/b sec, so the frequency of the first harmonic (一次谐波) is b/8. • An ordinary telephone line (often called as a voice-grade line(话音级线路)) has an artificially introduced cutoff frequency near 3000Hz. This restriction means that the number of the highest harmonic passed through is 3000/(b/8) or 24000/b.

8. The theoretical basis for data communication • Bandwidth-Limited Signals Relation between data rate and harmonics for the voice-grade line Conclusion: the data rate is limited by the bandwidth of transmission system or media.

9. The theoretical basis for data communication • The maximum data rate of a channel • In 1924, Henry Nyquist (AT&T) derived an equation expressing the maximum data rate for a finite bandwidth noiseless channel. maximum number of bits/sec = 2 H log_2 V where H is the bandwidth, V is the discrete levels. • In 1948, Claude Shannon carried Nyquist‘s work further and extended it to the case of a channel subject to random noise. maximum number of bits/sec = H log_2 (1+S/N) where H is the bandwidth, S/N (信噪比) is the signal-to-noise ratio.

10. GUIDED TRANSMISSION MEDIA （有线数据传） • Magnetic media（磁介质） • Twisted pair（双绞线） • Coaxial cable（同轴电缆） • Fiber optics（光纤）

11. Guided Transmission Media: Magnetic media • One of the most common ways to transport data form one computer to another is • to write them onto magnetic tape or removable media, • To transport the tape or disks to the destination machine, • and to read them back in again. • Bandwidth: 200GB/tape * 1000 tapes/box / (24*60*60s) = 19Gbps • Cost: (200GB/tape * 1000 tapes/box) / 5000$ = 40GB/$ • Conclusion:Never underestimate the bandwidth of station wagon full of tapes hurtling down the highway • Why computer network? Delay and so on.

12. Guided Transmission Media: Twisted pair • A twisted pair consists of two insulated copper wires, typically about 1 mm thick. The wires are twisted together in a helical form, just like a DNA molecule. • Twisted pairs can run several kilometers without amplification, but for longer distances, repeaters are needed. • Twisted pairs can be used for transmitting either analog or digital signals. • Types • Category 3, 5 (16MHz, 100MHz) • Category 6,7 (250MHz, 600MHz)

13. Guided Transmission Media: Twisted pair (a) Category 3 UTP. (b) Category 5 UTP.

14. Guided Transmission Media: Coaxial cable • A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor, often as a closely-woven breaded mesh. The outer conductor is covered in a protective plastic sheath. • Two types: 50-ohm (digital), 75-ohm (analog and digital) • High bandwidth and excellent noise immunity • Used to be widely used for long-distance lines.

15. Guided Transmission Media: Coaxial cable • Baseband （基带）coaxial cable • Broadband （宽带）coaxial cable • Dual cable • Single cable

16. Guided Transmission Media: Fiber optics • In the race between computing and communication, communication won • IBM PC at 4.77 MHz in 1981  2GHz •  (20 per decade) and physical limits • 56kpbs (ARPANET)  1Gbps (optical communication) •  125 per decade and error rate almost zero and almost no limits • The new conventional wisdom should be that: all computers are hopeless slow and networks should try to avoid computation at all costs, no matter how much bandwidth that wastes. • An optical transmission system: the light source  the transmission medium  the detector.

17. Guided Transmission Media: Fiber optics (a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles. (b) Light trapped by total internal reflection (no refraction). • Two types of fiber optics multimode: may different rays Unimode: single rays, longer distance

18. Guided Transmission Media: Fiber optics • Transmission of light through fiber • Attenuation (衰减)of light through fiber in the infrared region. • Three wavelength bands are used for optical communication. They are centered at 0.85, 1.30, and 1.55 micros, respectively.

19. Guided Transmission Media: Fiber optics • Fiber cables • Fiber optic cables are similar to coax, except without the braid. (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

20. Guided Transmission Media: Fiber optics • Fiber cables • Deployment • Laid in the ground within a meter of the surface • buried in trenches near the shore • Lie on the bottom in deep water • Connection • Fiber sockets • Spliced mechanically • Fusion splice • Detection • Photodiode. The response time is 1ns, which limits data rates to about 1Gbps.

21. Guided Transmission Media: Fiber optics • Fiber cables • Light sources • LED • Semiconductor laser

22. Guided Transmission Media: Fiber optics • Fiber optics network • A fiber optic ring with active repeaters （有源中继器）.

23. Guided Transmission Media: Fiber optics • Fiber optics network • A passive (无源) star connection in a fiber optics network.

24. Guided Transmission Media: Fiber optics • Comparison of fiber optics and copper wire • Advantages • Higher bandwidths and Low attenuation. • Not being affected by power surges, electromagnetic interference, or power failures. • Not affected by corrosive chemicals in the air. • Thin and lightweight. • Fibers do not leak light and quite difficult to tap. • Disadvantages • Less familiar technology. • Fiber interfaces more expensive. • Conclusion: For new routes (longer ones), fiber wins!

25. WIRELESS TRANSMISSION （无线传输） • The electromagnetic spectrum（电磁波谱） • Radio transmission（无线电传输） • Microwave transmission（微波传输） • Infrared and millimeter waves（红外线和毫米波） • Lightwave transmission（光波传输）

26. Wireless Transmission: The Electromagnetic Spectrum The electromagnetic spectrum and its uses for communication.

27. Wireless Transmission: The Electromagnetic Spectrum • LF(Low), MF(Medium), HF(High), VHF(Very), UHF(Ultra), SHF(Super), EHT(Extremely), THF(Tremendously), (IHF(Incredibly), AHF(Astonishingly), PHF(Prodigiously)） • The wider the band, the higher the data rate. • To prevent total chaos, there are national and international agreements about who gets to use which frequencies. • Most transmissions use a narrow frequency band. (GSM) • Some spread its frequency over a wide frequency band (spread spectrum，散布频谱). (CDMA) • frequency hopping spread spectrum (military, 802.11, Bluetooth) • direct sequence spread spectrum (2G mobile phones)

28. Wireless Transmission: Radio transmission • Radio waves are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication • Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to carefully aligned physically. • Due to radio’s ability to travel long distances, interference between users is a problem. For this reason, all governments tightly license the use of radio transmitters.

29. Wireless Transmission: Radio transmission (a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere（电离层）.

30. Wireless Transmission: Microwave transmission • Above 100MHz, the waves travel in straight lines and can therefore be narrowly focused. • The higher the transmission towers are, the further apart they can be. For 100-m high towers, repeaters can be spaced 80km apart. • Microwave can cause multipath fading or be absorbed by rain. • Microwave communication is so widely used for long-distance telephone communication, cellular telephones, television distribution, and other uses, that a severe shortage of spectrum has developed. • Microwave has some advantages over fiber: no right of way problem, being inexpensive.

31. Wireless Transmission: Microwave transmission • Who gets to use which frequencies • ITU-R • FCC, 无线电管理委员会? • Three algorithms for deciding which company or other unit is allowed to which frequencies • Beauty contest • Holding a lottery among the interested companies • Auctioning off the band width to the highest bidder • The location of the ISM (Industrial, Scientific, Medical) band varies from country to country.

32. Wireless Transmission: Microwave transmission The ISM bands in the United States.

33. Wireless Transmission: Infrared and millimeter waves • Infrared and millimeter waves can not pass through solid objects • Infrared can be used for indoor wireless LANs, remote controllers. • Infrared can not be used outdoors for the sun shines as brightly in the infrared as in the visible spectrum. • Advantages • The infrared controller cannot control the TV of your neighbor. • More secure.

34. Wireless Transmission: Lightwave transmission Convection currents (气流) can interfere with laser communication systems. A bidirectional system with two lasers is pictured here.

35. COMMUNICATION SATELLITES （通讯卫星） • Geostationary Satellites （地球同步卫星） • Medium-Earth Orbit Satellites （中轨道卫星） • Low-Earth Orbit Satellites （低轨道卫星） • Satellites versus Fiber

36. Communication satellites • In the 1950s and early 1960s, people tried to set up communication systems by bouncing signals off metallized weather balloons. • The U.S. Navy built an operational system for ship-to-shore communication by bouncing signals off the moon. • Communication satellite is the real celestial communication player. • A big microwave repeater • Many transponders (异频收发机) • The higher the satellite, the longer the period. • There are two Van Allen belts, layers of highly charged particles trapped by the earth’s magnetic field.

37. Communication Satellites Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage.

38. Communication Satellites: Geostationary satellites • Orbit slots: to avoid interference, geostationary satellites should be placed 2 degrees or so in the 360 degree equatorial plane. • Principal satellite bands: • C • L,S • Ku, Ka

39. Communication Satellites: Geostationary satellites • Wide beam and spot beam: • Wide beam: covers large area ( as large as 1/3 of the earth’s surface) • Spot beam: covers small area (a few hundred km in diameter) • Satellites has many transponders. • Every transponder can use • FDM (the bandwidth is simply split up into fixed frequency bands) or • TDM (divided into time slots) techniques.

40. Communication Satellites: Geostationary satellites • VSAT (Very Small Aperture Terminals) • Many small VSATs  Satellite • Many small VSATs  VSAT hub  satellite

41. Communication Satellites: Geostationary satellites • Discussion of properties • Longer delay (270ms) (3us/km for terrestrial microwave, 5us/km for coaxial cable or fiber optic links) • Inherently broadcast • Good from broadcasting sports • Security is an issue. • The cost of transmitting a message is independent of the distance traversed. • Satellites also have excellent error rates. • Satellites can be deployed almost instantly, a major consideration for military communication.

42. Communication Satellites: Medium-earth orbit satellites • MEO satellites • Lower than GEOs. Between the two Van Allen belts • Smaller footprint on the ground. • Requires less powerful transmitters to reach them. • Examples: 24 GPS satellites. • They are not used for telecommunication.

43. Communication Satellites: Low-earth orbit satellites • LEO satellites • Lower than MEOs. • Much smaller footprint on the ground • Requires far less powerful transmitters to reach them • The round-trip delay is only a few milliseconds • Used for voice communication and Internet services.

44. Communication Satellites: Low-earth orbit satellites • Iridium project (铱星) • In 1990 Motorola applied the permission to launch 77 low-orbit satellites for the Iridium project (7766). • In 1997 Motorola and its partners launched the Iridium satellites. • In November 1998 Communication service began. • In August 1999, Iridium was not profitable and was forced into bankruptcy ($5 billion  $25million). • In March 2001, the Iridium service was restarted.

45. Communication Satellites: Low-earth orbit satellites • Iridium project (a) The Iridium satellites form six necklaces around the earth. (b) 1628 moving cells cover the earth.

46. Communication Satellites: Low-earth orbit satellites • Globalstar • 48 LEOs • Relaying on the ground • Teledesic • Conceived in 1990 by mobile phone pioneer Craig McCaw and Microsoft founder Bill gates • Scheduled to go live in 2005 if all goes as planned. • 30 LEOs

47. Communication Satellites: Satellites (celestial) versus Fiber (terrestrial) • High bandwidth: With satellites, it is practical for a user to erect an antenna on the roof of the building and completely bypass the telephone system to get high bandwidth. Teledesic is based on this idea. • Mobile communication • Good for broadcasting • For communication in places with hostile terrain or a poorly developed terrestrial infrastructure • No right of way problem • Rapid development

48. PUBLIC SWITCHED TELEPHONE SYSTEM公用交换电话系统 • Structure of the Telephone System • The Politics of Telephones • The Local Loop: Modems, ADSL, and Wireless • Trunks and Multiplexing • Switching

49. Public Switched Telephone System • How to connect computers: • For a small number of computers and a local area, LAN can be used. • For a large number of computers or a wide area, or lacking right of way, LAN can not be used and we have to rely upon the existing communication facilities such as PSTN. • The PSTN is suitable for transmitting the human voice in a more or less recognizable form. • The suitability for use in computer-computer networking is often marginal at best although the situation is rapidly changing with the introduction of fiber optics and digital technology.

50. Public Switched Telephone System • A cable running between two computers can transfer data at 10^9 bps or more. A dial-up line has maximum date date of 56 kbps, a difference of a factor of almost 20,000. That is the difference between a duck waddling leisurely though the grass and a rocket to the moon. • The combined bit rate times error performance of a computer system cable is 11 orders of magnitude better than a voice-grade telephone line. (the ratio of the cost of the entire Apollo project to the cost of a bus ride downtown is about 11 orders of magnitude (in 1965 dollars: 40 billion$ to 0.40$)) • Computer system designers have devoted much time and effort to figure out how to use it efficiently.