2. The Physical Layer Read Chapter 2

3. CSC 3352 Computer Communications Chapter 22 Putting the bits on the wire…

4. CSC 3352 Computer Communications Chapter 23 Terminology (1) Transmitter Receiver Medium Guided medium  e.g. twisted pair, optical fiber Unguided medium  e.g. air, water, vacuum

5. CSC 3352 Computer Communications Chapter 24 Terminology (2) Point-to-point Direct link Only 2 devices share link Multi-point More than two devices share the link

6. CSC 3352 Computer Communications Chapter 25 Terminology (3) Simplex One direction  e.g. Television Half duplex Either direction, but only one way at a time  e.g. police radio Full duplex Both directions at the same time  e.g. telephone

7. CSC 3352 Computer Communications Chapter 26 Continuous Signals Continuous signal: Intensity Varies in a smooth way over time, no breaks...

8. CSC 3352 Computer Communications Chapter 27 Discrete signal: Maintains a constant level then changes to another constant level Discrete Signals

9. CSC 3352 Computer Communications Chapter 28 Periodic Signals Periodic signal: The same pattern is repeated over time...

10. CSC 3352 Computer Communications Chapter 29 Amplitude (A) maximum strength of signal…Volts Frequency (f) Rate of change of signal, Hertz (Hz) or cycles per second The period Period = time for one repetition (T) T = 1/f Phase (  ) Relative position in time Amplitude, Frequency and Period...

11. CSC 3352 Computer Communications Chapter 210 Amplitude, Frequency and Period...

12. CSC 3352 Computer Communications Chapter 211 Wavelength ( ) : Distance occupied by one cycle. It is also the distance between two points of corresponding phase in two consecutive cycles In a vacuum space: c = 3*10 8 ms (speed of light in free space) = cT f = c

13. CSC 3352 Computer Communications Chapter 212 Baud Definition: the unit baud measures the number of changes per second in the signal. OR, how many times the signal changes its amplitude per second...

14. CSC 3352 Computer Communications Chapter 213 Spectrum & Bandwidth Spectrum range of frequencies contained in signal Absolute bandwidth width of spectrum Effective bandwidth Often just bandwidth Narrow band of frequencies containing most of the energy

15. CSC 3352 Computer Communications Chapter 214 Data Rate and Bandwidth Any transmission system has a limited band of frequencies This limits the data rate that can be carried

16. CSC 3352 Computer Communications Chapter 215 The Maximum data rate of a channel Nyquist: maximum data rate = 2 H log 2 V bits/s Shannon (Nyquist is an optimist) maximum data rate = H log 2 (1+S/N) S/N…..

17. CSC 3352 Computer Communications Chapter 216 Analog and Digital Data Transmission Data Entities that convey meaning Signals Electric or electromagnetic representations of data Transmission Communication of data by propagation and processing of signals

18. CSC 3352 Computer Communications Chapter 217 Data Analog Continuous values within some interval e.g. sound, video Digital Discrete values e.g. text, integers

19. CSC 3352 Computer Communications Chapter 218 Signals Means by which data are propagated Analog Continuously variable Various media: wire, fiber optic, space Speech bandwidth 100Hz to 7kHz Telephone bandwidth 300Hz to 3400Hz Video bandwidth 4MHz Digital Use only two levels of voltage

20. CSC 3352 Computer Communications Chapter 219 Data and Signals Usually use digital signals for digital data and analog signals for analog data Can use analog signal to carry digital data Modem Can use digital signal to carry analog data Compact Disc audio

21. CSC 3352 Computer Communications Chapter 220 Transmission Impairments Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by: Attenuation Distortion Noise

22. CSC 3352 Computer Communications Chapter 221 Attenuation Signal strength falls off with distance Loss of energy due to the propagation of bits in long distance… Depends on medium Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error

23. CSC 3352 Computer Communications Chapter 222 Distortion Only in guided media Propagation velocity varies with frequency Bits do not arrive in the initial order...

24. CSC 3352 Computer Communications Chapter 223 Noise Additional signals inserted between transmitter and receiver Thermal: due to thermal agitation of electrons Crosstalk: a signal from one line is picked up by another Impulse: high amplitude

25. CSC 3352 Computer Communications Chapter 224 What To Do Against Attenuation, Distortion and Noise? A media adapted to the distance/bit rate Encoding schemes (we will see it later)

26. CSC 3352 Computer Communications Chapter 225 Media for Networks Twisted pair 2 twisted insulated copper wires Twesting waires reduces interferences Ex.: the telephone system Category 3: Some megabits per sec. (for only few Kms, may need repeaters for longer distances) Category 5: more twists per centimeter, less crosstalk, longer distance, more suitable for high speed computer communications…

27. CSC 3352 Computer Communications Chapter 226 Media for Networks Coaxial cable See fig. 2-3 on page 84 Has a better shielding so it can span longer distance… Higher speed… Good noise immunity… Baseband: 2Gbps for 1km, less speed for more kms (amplifiers are requiered), used in both voice and data transmission… Broadband: oriented cable TV, mainly analog transmission, accuracy is not required, long distance…

28. CSC 3352 Computer Communications Chapter 227 Media for Networks Fiber optics See page 87 for an introduction to optical transmission system… 3 main elements: the light source, the transmission medium and the light detector A pulse of light=1 bit, absence=0bit The transmission media is an ultra-thin fiber of glass How to transmit… Several Gbps for 30kms, 1 repeater per 30kms…

29. CSC 3352 Computer Communications Chapter 228 Future is With Fiber Bigger bandwidths Attenuation/distance is lower than copper Not sensitive to electromagnetic interference Lighter and thinner than copper Difficult to tap

30. CSC 3352 Computer Communications Chapter 229 Wireless Links: The spectrum of telecommunication UV 10 0 10 14 10 4 10 8 10 12 Infrared 10 16 10 22 Gamma RadioMicrowa. X-Ray 10 4 10 5 10 6 10 7 10 8 10 910 10 11 10 12 10 13 10 14 10 15 10 16 F(Hz) Band LFMFHF VHF UHF Twisted pair Coax Satellite Microwave Fiber Maritime AM Radio FM Radio TV

31. CSC 3352 Computer Communications Chapter 230 Different Bands of the Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission

32. CSC 3352 Computer Communications Chapter 231 Relation Spectrum, Frequency, Data rate... The frequency band is derived from.f = c Deriving gives the frequency band corresponding to a wavelength and wavelength interval is then :…

33. CSC 3352 Computer Communications Chapter 232 The Spectrum: A cake to Share WARC (International) FCC ( U.S. agency)

34. CSC 3352 Computer Communications Chapter 233 Radio Transmission Easy to generate Travel long distance, penetrate buildings Omnidirectional Properties depend on frequency low frequencies, penetrates buildings well but sensitive to distance (1/r 3 ). High frequencies, travel in straight lines, absorbed by rain Sensitive to interferences

35. CSC 3352 Computer Communications Chapter 234 Microwave Transmission Travel long distance, (Does not penetrate buildings) Not Omnidirectional. Travel in straight lines (transmitter and receiver must be carefully aligned) Absorbed by rain ISM band

36. CSC 3352 Computer Communications Chapter 235 Infrared and Millimiter Waves Do not go thru solid walls Good for indoor wireless LANs

37. CSC 3352 Computer Communications Chapter 236 Using the Telephone System for Computer Networks Why? What is the telephone system? How does it work? How can computers use it for communicating?

38. CSC 3352 Computer Communications Chapter 237 The Telephone System Phone Local Loop End office Toll office (We may have other levels of hierarchy…)

39. CSC 3352 Computer Communications Chapter 238 The Telephone System Local loop analog twisted pair one per user Low bandwidth Inter switches trunks Use to be analog but is now digital (see page 109) fiber High bandwidth Shared

40. CSC 3352 Computer Communications Chapter 239 Computer Modem End office Codec Digital Short digital connection RS 232 Local loop Analog Local loop for a Computer

41. CSC 3352 Computer Communications Chapter 240 Transmission impairments (Local Loop) Attenuation Delay distorsion Noise

42. CSC 3352 Computer Communications Chapter 241 Modulation Amplitude modulation Frequency modulation (FSK) Phase modulation (PSK) Constellation patterns (see ex. in page 111)

43. CSC 3352 Computer Communications Chapter 242 Full/Half duplex Other problems with telephone lines: Echo effects: a person hears its own words after a short delay… Echo suppressors: a device that detects human voice from one end and suppresses signals from the other end… switching… half duplex connection… Echo cancelers… Estimates the echo and subtracts it from the original signal…Full duplex connection

44. CSC 3352 Computer Communications Chapter 243 Interface Computer (DTE)- Modem (DCE) The RS 232 is a good example of physical layer protocol Data Terminal Equipment Data Circuit-Term. Equipment Protective ground (1) Transmit (2) Receive(3) Request To Send (4) Clear To Send(5) Data Set Ready(6) Common Ground (7) Carrier Detect(8) Data terminal Ready(20)

45. CSC 3352 Computer Communications Chapter 244 Interface Computer (DTE)-Computer (DTE) With a null modem device we can use RS 232 to connect 2 DTE… DTE Shield (1) Transmit (2) Receive(3) Request To Send (4) Clear To Send(5) Data Set Ready(6) Common Ground (7) Carrier Detect(8) Data terminal Ready(20)

46. CSC 3352 Computer Communications Chapter 245 Computer Modem End office Codec Digital RS 232 Local loop Analog Review We looked in detail into this We focus now on trunks

47. CSC 3352 Computer Communications Chapter 246 Trunks: transporting many conversations on the same “wire” Frequency Division Multiplexing Wavelength Division Multiplexing (Fiber) Time Division Multiplexing

48. CSC 3352 Computer Communications Chapter 247 Frequency Division Multiplexing Let the signals we need to transmit working in  frequency. All signals are raised in frequency by n.  ’ such that  ’ > . The extra is called guard band. All signals are added

49. CSC 3352 Computer Communications Chapter 248 Frequency Division Multiplexing (Example) The phone voice is limited to  Hz frequency. All signals are raised in frequency by n.  ’ with  ’=4000 Hz (two 500Hz guard band) All signals are added

50. CSC 3352 Computer Communications Chapter 249 Frequency Division Multiplexing 12 4000Hz 1 group (60 to 108KHz) 5 1 super group 5 1 mastergroup

51. CSC 3352 Computer Communications Chapter 250 Time Division Multiplexing Works only for digital signals Allocate time slots to every signal Each signal has the exclusive use of the line for a given time slot

52. CSC 3352 Computer Communications Chapter 251 TDM (Example) 1200 bits/s 2400 bits/s

53. CSC 3352 Computer Communications Chapter 252 PCM : Transporting Digital Voice A codec digitalizes 24 voice channels, sends-receives 8 bits per channel, If a standard voice sampling ratio (8000bps) is used, the codec will perform 192bits every 125 M sec In fact it is 192bits+1checkbit=193bits/125 M sec What should be the data rate to transport the samples of the 24 channel? 1.544Mbps (this is the capacity of a T1 career)

54. CSC 3352 Computer Communications Chapter 253 A word on ISDN Read section 2.5 Idea : integrated voice and digital service Voice channel 2 and more digital channels in multiples of 8, 16, or 64 Kbps

55. CSC 3352 Computer Communications Chapter 254 Cellular Radio :the American system Mobile phones: Two way system The AMPS was invented by Bell Labs in 1982 Based on cell architecture ( see figure in next slide ) Moving in the space… Security issues

56. CSC 3352 Computer Communications Chapter 255 Cellular Radio :the European system The Global Systems for Mobile communications (GSM). The standard of digital mobile systems in Europe (and north africa) Incompatible with USA and Japan A new frequency (1.8GHz) Uses FDM and TDM Uses a smart card to store the serial number and phone number — » less vandalsime…

57. CSC 3352 Computer Communications Chapter 256 Cellular Radio : Architecture Public Switching Telephone System A cellulaire area MSC

58. CSC 3352 Computer Communications Chapter 257 Conclusion The Physical layer is the basis of all networks… 2 fundamental limits: the Nyquist limit and the Shannon limit… Guided media and unguided media… The telephone system is a major playor in this layer… ISDN and mobile links are the future…