1. More on Propagation Module B Copyright 2003 Prentice Hall Panko’s Business Data Networks and Telecommunications, 4 th edition

2. Modulation

3. 3 Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line There are several forms of modulation Amplitude modulation Frequency modulation Phase modulation Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation

4. 4 Waves Frequency of a wave The number of complete cycles per second Called Hertz kHz, MHz, GHz, THz Frequency (Hz) Cycles in One Second

5. 5 Figure B.1: Frequency Modulation (FM) Low Frequency (0) High Frequency (1) Frequency Modulation (1011) Wavelength 1 0 1 1

6. 6 Amplitude Modulation (AM) Amplitude is the intensity of the signal Loud or soft Amplitude (power)

7. 7 Amplitude Modulation (AM) (from Chapter 3) Low Amplitude (0) High Amplitude (1) Amplitude Modulation (1011) Amplitude (low) Amplitude (high)

8. 8 Phase Two signals can have the same frequency and amplitude but have different phases--be at different points in their cycles at a given moment Basic Signal 180 degrees out of phase

9. 9 Figure B.2: Phase Modulation (PM) In Phase (0) 180 degrees out of phase (1) Frequency Modulation (1011)

10. 10 Phase Modulation (PM) Human hearing is largely insensitive to phase So harder to understand than FM and AM But equipment is very sensitive to phase changes PM is used in all recent forms of modulation for telephone modems

11. 11 Figure B.3: QAM Quadrature Amplitude Modulation (QAM) Uses two carrier waves: sine and cosine (out of phase), both amplitude-modulated

12. 12 Figure B.3: QAM Suppose each carrier wave has four possible amplitude levels In each clock cycle, there are 16 combined possibilities In each clock cycle, can send 4 bits (2^4=16) Sine Wave Cosine (Quadrature) Wave High/High 1111

13. 13 Figure B.4: OFDM Orthogonal Frequency Division Multiplexing Send signal is a large channel Divide the channel into many subchannels Send part of the signal in each subchannel Don’t use impaired channels or transmit more slowly Channel Bandwidth Subchannel with Part of Signal

14. Radio Propagation

15. 15 Radio Transmission Oscillating electron generates electromagnetic waves with the frequency of the oscillation Many electrons must be excited in an antenna for a strong signal

16. 16 Radio Propagation Propagation Characteristics Depend on Frequency At lower frequencies, signals bend around objects, pass through walls, and are not attenuated by rain At higher frequencies, there is more bandwidth per major band

17. 17 Major Bands (From Figure B.5) Frequency Spectrum is Divided into Major Bands Ultra High Frequency (UHF) Signals still bend around objects and pass through walls Cellular telephony

18. 18 Major Bands (From Figure B.5) Frequency Spectrum is Divided into Major Bands Super High Frequency (SHF) Needs line-of-sight view of receiver Rain attenuation is strong, especially at the higher end High channel capacity Used in microwave, satellites

19. 19 Figure B.6: Microwave Transmission Terrestrial (Earth-Bound) System Repeaters can relay signals around obstacles

20. 20 Satellite Transmission Essentially, places repeaters in sky Idea thought of by Sir Arthur C. Clarke Broadcasts to an area called its footprint Uplink is to satellite; downlink is from satellite Uplink Downlink Footprint

21. 21 Some Popular Satellite Frequency Bands BandDownlink Frequency (From Satellite) Uplink Frequency (To Satellite) C4 GHz6 GHz Ku12 GHz14 GHz Ka20 GHz30 GHz

22. 22 Infrared Transmission Uses light instead of radio for transmission Like a television remote control but diffused to reduce line-of-sight limitations Relatively low speeds Bright sunlight and light fixtures can interfere with it

23. Optical Fiber

24. 24 Optical Fiber Thin Core of Glass Surrounded by glass cladding Inject light in on-off pattern for 1s and 0s Total reflection at core-cladding boundary Little loss with distance Light Source Cladding Core Reflection

25. 25 Optical Fiber Modes Light entering at different angles will take different amounts of time to reach the other end Different ways of traveling are called modes Light modes from successive bits will begin to overlap given enough distance, making the bits unreadable Light Source Reflection

26. 26 Single Mode Fiber Single Mode Fiber is very thin Only one mode will propagate even over fairly long distances Expensive to produce Expensive to install (fragile, precise alignments needed) Used by carriers to link distant switches

27. 27 Multimode Fiber Core is thick Modes will appear even over fairly short distances Must limit distances to a few hundred meters Inexpensive to purchase and install Dominates LANs

28. 28 Graded Index Multimode Fiber Index of fraction is not constant in core Varies from center to edge Reduces time delays between different modes Can go farther than if core has only a single index of fraction (step index multimode fiber) Dominates multimode fiber today

29. 29 Frequency Signal Frequency Determines the Propagation Distance before Mode Problems Become Serious Short Wavelength (high frequency) Signals do not travel as far before mode problems occur Uses the least expensive light sources Good for LAN use within buildings

30. 30 Frequency Signal Frequency Determines the Propagation Distance before Mode Problems Become Serious Long Wavelength (low frequency) Signals travel farther but light sources cost more Within large buildings and between buildings

31. 31 Frequency There are 3 frequency windows where optical fiber attenuation is very low 850 nm: Shortest distance but lease expensive optical transceivers 1300 nm: Good balance of cost and distance 1550 nm: Longest distance and most expensive

32. 32 Fiber Quality Some fiber is higher quality in terms of internal construction Measured as bandwidth x distance product 160 MHz x km is the most common 1 km with a light signal bandwidth of 160 MHz (highest bandwidth so highest speed) 4 km with a light signal bandwidth of 40 MHz 200 MHz x km is becoming more popular

33. 33 SC and ST Connectors For UTP, there is only a single connector, RJ-45 For optical fiber, several connectors are popular SC: squarish, snaps into port, recommended in TIA/EIA-568 ST: tubular, bayonnette connection, popular Several popular small form factor connectors

34. 34 SC and ST Connectors ST Connectors (Popular) SC Connectors (Recommended) Two fiber cords for full-duplex (two- way) transmission

35. 35 SC Port Full-Duplex, So Need Two Plugs per Port

36. 36 ST Port Full-Duplex, So Need Two Plugs per Port