Transmission Media. Beulah A L/CSE. 2 July 2008 Transmission Media Beulah A. 1

Transcription

1 Transmission Media Beulah A L/CSE 2 July 2008 Transmission Media Beulah A. 1

2 Guided Transmission Media Magnetic Media A tape can hold 7 gigabytes. A box can hold about 1000 tapes. Assume a box can be delivered in 24 hours. The effective bandwidth=7*1000*8/86400=648 Mbps Cost of 1000 tapes=5000. If a tape can be reused 10 times and the shipping cost is 200, we have a cost of 700 to ship 7000 gigabytes or 10 cents per gigabytes. No network carrier on earth can compete with that. Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway. 2 July 2008 Transmission Media Beulah A. 2

3 Twisted Pair Although the bandwidth characteristics of magnetic tape are excellent, the delay characteristics are poor. Twisted Pair: used in local loop in telephone systems The purpose of twisting the wires is to reduced electrical interference from similar pairs close by. 2 July 2008 Transmission Media Beulah A. 3

4 Twisted Pair Cont Unshielded Twisted Pair (UTP) More twists per centimeter, less crosstalk (a) Category 3 UTP. (16MHz) (b) Category 5 UTP. (100MHz) 2 July 2008 Transmission Media Beulah A. 4

5 Coaxial Cable Use digital transmission. For 1-km cables, a data rate of 1 to 2 Gbps is feasible. 2 July 2008 Transmission Media Beulah A. 5

6 Fiber Optics Computing speed: a factor of 10 improvement per decade Communication speed: a gain of more than a factor of 100 per decade In the race between computing and communication, communication won. The new conventional wisdom should be that all computers are hopelessly slow, and networks should try to avoid computation at all costs, no matter how much bandwidth that wastes. 2 July 2008 Transmission Media Beulah A. 6

7 Fiber Optics Cont An optical transmission system has three components: the light source, the transmission medium, and the detector. Light source: LED (Light Emitting Diode) or Laser (Light Amplification by Simulated Emission of Radiation) Transmission Media: ultra-thin fiber of glass Detector: using light-electricity effect, generate an electrical pulse when light falls on it 2 July 2008 Transmission Media Beulah A. 7

8 Fiber Optics Cont 2 July 2008 Transmission Media Beulah A. 8

9 refraction and reflection Fiber Optics Cont incident ray reflected ray refracted ray 2 July 2008 Transmission Media Beulah A. 9

10 Fiber Optics Cont perpendicular light partially reflected total reflection critical angle α θ 2 July 2008 Transmission Media Beulah A. 10

11 Multimode fiber cross section Fiber Optics Cont core cladding η 2 η 1 protective coating η 1 > η 2 two propagation modes 2 July 2008 Transmission Media Beulah A. 11

12 Multimode fiber Fiber Optics Cont 2 July 2008 Transmission Media Beulah A. 12

13 Unguided Transmission Media The Electromagnetic Spectrum Electromagnetic Waves one cycle speed=frequency wavelength λ f = c m/s=cycles/s m/cycles Hz(hertz) speed of light (in vacuum)= m / s 2 July 2008 Transmission Media Beulah A. 13

14 The Electromagnetic Spectrum Cont 2 July 2008 Transmission Media Beulah A. 14

15 The Electromagnetic Spectrum Cont To prevent total chaos, there are national and international agreements about who gets to use which frequencies. Since everyone wants a higher data rate, everyone wants more spectrum. Therefore, we have to share. FDMA: Frequency Division Multiple Access TDMA: Time Division Multiple Access CDMA: Code Division Multiple Access (using spread spectrum technique) 2 July 2008 Transmission Media Beulah A. 15

16 The Electromagnetic Spectrum Cont Many transmissions use a narrow frequency band to get the best reception. However, in some cases, a wide band is used with two variations. Frequency hopping spread spectrum Direct sequence spread spectrum 2 July 2008 Transmission Media Beulah A. 16

17 Radio Transmission Radio waves are easy to generate, can travel long distance, and penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves are also omnidirectional, meaning that they travel in all directions from the source, so that the transmitter and receiver do not have to be carefully aligned physically. Omnidirectional waves sometimes can have undesired side effects. 2 July 2008 Transmission Media Beulah A. 17

18 Radio Transmission Cont In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. 2 July 2008 Transmission Media Beulah A. 18

19 Radio Transmission Cont At height 100 to 500km In the HF they bounce off the ionosphere. 2 July 2008 Transmission Media Beulah A. 19

20 Microwave Transmission Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam using a parabolic antenna gives a much higher signal to noise ratio. Since the microwaves travel in a straight line, if the towers are too far apart, the earth will get in the way. Consequently, repeaters are needed periodically. 2 July 2008 Transmission Media Beulah A. 20

21 Microwave Transmission Cont Disadvantages: do not pass through buildings well multipath fading problem (the delayed waves cancel the signal) absorption by rain above 8 GHz severe shortage of spectrum Advantages: no right way is needed (compared to wired media) relatively inexpensive simple to install 2 July 2008 Transmission Media Beulah A. 21

22 Microwave Transmission ISM (Industrial/Scientific/Medical) Band The ISM bands in the United States. 2 July 2008 Transmission Media Beulah A. 22

23 Infrared and Millimeter Waves Unguided infrared and millimeter waves are widely used for short-range communication. The remote controls used on televisions, VCRs, and stereos all use infrared communication. They are relatively directional, cheap, and easy to build, but have a major drawback: they do not pass through solid objects. This property is also a plus. It means that an infrared system in one room will not interfere with a similar system in adjacent room. It is more secure against eavesdropping. 2 July 2008 Transmission Media Beulah A. 23

24 Lightwave Transmission Affected by fog or rain 2 July 2008 Transmission Media Beulah A. 24

25 Communication Satellites Contain several transponders. Properties: 1. Longer delay 2. Broadcast in nature 3. Bad security 4. Deployment is fast downlink channel uplink channel 2 July 2008 Transmission Media Beulah A. 25