1. Postacademic Interuniversity Course in Information Technology – Module C1p1 Chapter 4 Communications, Theory and Media

2. Postacademic Interuniversity Course in Information Technology – Module C1p2 A three layers model. Applications Layer Internet & Transport Layer Networks Layer Connectivity Interoperability

3. Postacademic Interuniversity Course in Information Technology – Module C1p3 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

4. Postacademic Interuniversity Course in Information Technology – Module C1p4 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

5. Postacademic Interuniversity Course in Information Technology – Module C1p5 Parallel Transmission Disadvantages : Differences in propagation delay Cost of multiple channels Consequence : Restricted to very short distances Clock In computers, data is structured in bytes

6. Postacademic Interuniversity Course in Information Technology – Module C1p6 Serial Transmission Clock Serial Data b 0 b 1 b 2 b 3 b 4 b 5 b 6 b 7 b0b0 b7b7 Parallel in Serial out Serial in Parallel out Transmission rate expressed in bits/second

7. Postacademic Interuniversity Course in Information Technology – Module C1p7 Serial Transmission with clock/data multiplexing + ClockSerial Data

8. Postacademic Interuniversity Course in Information Technology – Module C1p8 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

9. Postacademic Interuniversity Course in Information Technology – Module C1p9 Digital Data Communications TX RX 011001 Analog communication channel Modem

10. Postacademic Interuniversity Course in Information Technology – Module C1p10 Encoding and Decoding Transmitter (Tx) –Input : stream of binary numbers –Output : stream of analog signals suitable for transmission over long distances Receiver (Rx) –Input : stream of analog signals generated by transmitter distorted by transmission channel –Compares each input signal with all signals which could have been transmitted and decides from which one the input is a distorted image. –Output : stream of binary numbers, preferably identical to the input of the transmitter

11. Postacademic Interuniversity Course in Information Technology – Module C1p11 Analog Transmission Channel Bandwidth –Difference between highest and lowest frequency of sine waves which can be transmitted –Number of possible state changes per second Signal to Noise ratio –S/N = (signal power) / (noise power) –S/N determines number of distinct states which can be distinguished within a given observation interval Characterized by : Frequency Received power B

12. Postacademic Interuniversity Course in Information Technology – Module C1p12 Binary vs. Multi-bit encoding Modulation rate = 1/t (in Baud) Data rate = (1/t) Log 2 n (in b/s) t 0001 t 01110010

13. Postacademic Interuniversity Course in Information Technology – Module C1p13 Shannon’s Theorem DataRate <= B.Log 2 (1+S/N) B : Channel Bandwidth (in Hertz) S/N : Signal to Noise ratio Example: Telephone channel, B = 3000 Hz, S/N = 1000 DataRate <= 30 000 b/s

14. Postacademic Interuniversity Course in Information Technology – Module C1p14 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

15. Postacademic Interuniversity Course in Information Technology – Module C1p15 Error Detection Example Belgian Bank Account Numbers Bank account number structure –Bank identification : 3 digits –Account number : 7 digits –Error detection : 2 digits The ten first digits modulo 97 are appended for error detection purposes. This algorithm allows detection of all single digit errors Example : –140-0571659-08. 1400571659 MOD 97 = 08 –140-0671659-08. 1400671659 MOD 97 = 01

16. Postacademic Interuniversity Course in Information Technology – Module C1p16 Error detection and correction Length of messages : Informative message: Redundancy: # Messages send: # Messages received: Hamming Distance (X-Y): k + r <= LMax k bits r bits, f(inf.mess.) 2 k 2 k+r  |X i -Y i | k+r i=1

17. Postacademic Interuniversity Course in Information Technology – Module C1p17 Error detecting codes k = 1; r = 1; red.bit = inf.bit. 00 01 11 10 Single bit errors are detected if hamming distance between legitimate messages > 1. No guessing is possible as erroneous messages are at equal distances from several correct ones. 1100 Hd = 2

18. Postacademic Interuniversity Course in Information Technology – Module C1p18 Error correcting codes k = 1; r = 2; red.bits = inf.bit. 001 011 111 101 000 010110 100 Hamming distance between legitimate messages > 2. This implies that each erroneous message is closer to one correct message than to any other. 111000 Hd = 3

19. Postacademic Interuniversity Course in Information Technology – Module C1p19 Error correcting codes Required Overhead for single bit error correction k+r < 2 r information redundancyOverhead 1 <= 4 <= 11 <= 26 <= 57 <= 120 <= 247 23456782345678 200 % 75 % 36 % 19 % 11 % 6 % 3 %

20. Postacademic Interuniversity Course in Information Technology – Module C1p20 Error Correction Error detecting codes –Correction by retransmission of erroneous blocks –If few errors, very low overhead –Most common approach to error correction in data communications Error correcting codes –Very high overhead with short data blocks –Longer data blocks can have multiple errors –Used when retransmission impossible or impractical –Also used when error rate rather high. –Error correcting codes for long blocks, with multiple errors exist and are used (trellis encoding)

21. Postacademic Interuniversity Course in Information Technology – Module C1p21 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

22. Postacademic Interuniversity Course in Information Technology – Module C1p22 Snell’s Law sin  1 sin  2 = n2n1n2n1 22 11 n2n2 n1n1   cc n2n2 n1n1 >c>c n2n2 n1n1 n2 < n1

23. Postacademic Interuniversity Course in Information Technology – Module C1p23 Optical Fibers (step index) n1n2 n2 < n1 Protective coating

24. Postacademic Interuniversity Course in Information Technology – Module C1p24 Multimode Fiber Diameter : > 50  Low cost but limited bandwidth * distance due to multimode dispersion Step index fiber Graded index fiber

25. Postacademic Interuniversity Course in Information Technology – Module C1p25 Multimode Dispersion Step index fiber : < 50 MHz.Km Graded Index Fiber : < 1000 MHz.Km (1990) < 5000 MHz.Km (2000) t t

26. Postacademic Interuniversity Course in Information Technology – Module C1p26 Monomode Fiber Diameter : < 5  Only one propagation mode possible Higher cost due to end equipment but enormous bandwidth*distance product 10 Gb/s over 500 Km optical sections (1995)

27. Postacademic Interuniversity Course in Information Technology – Module C1p27 Wave Domain Multiplexing Each color can carry an independent data flow. In 2000 40 colors carrying each 10 Gb/s or 80 colors carrying each 2.5 Gb/s were commercially available

28. Postacademic Interuniversity Course in Information Technology – Module C1p28 Optical amplifiers Pump laser Erbium doped fiber Erbium atoms are pumped into a higher energy state by the light of the pump laser, they fall back in synchronism with the incoming light, amplifying it.

29. Postacademic Interuniversity Course in Information Technology – Module C1p29 Optical Switching From IEEE Com.Mag.V39,N1, Jan 2001.

30. Postacademic Interuniversity Course in Information Technology – Module C1p30 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

31. Postacademic Interuniversity Course in Information Technology – Module C1p31 Coaxial Cables Protective coating Insulator Conductor Monomode propagation for all data applications Transmission rates up to some Gb/s Distance limited by electrical attenuation

32. Postacademic Interuniversity Course in Information Technology – Module C1p32 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

33. Postacademic Interuniversity Course in Information Technology – Module C1p33 Twisted Pairs Performance highly dependant on cable quality Transmission speed up to several 100 Mb/s for distances of up to 100 m. with better cables (class 5 or 6)

34. Postacademic Interuniversity Course in Information Technology – Module C1p34 Contents Communications Theory –Parallel vs. serial transmission –Transmission Capacity (Shannon) –Error detection and correction Communications Media –Optical fibers –Coaxial cables –Twisted pairs –Wireless

35. Postacademic Interuniversity Course in Information Technology – Module C1p35 Wireless Communications Mobile terminals Cost of wiring Why Not ? Lower data rates Lower reliability Potential Lack of Security Why ?

36. Postacademic Interuniversity Course in Information Technology – Module C1p36 Wireless Communications Main restriction: Limited availability of bandwidth in the electromagnetic spectrum Some solutions: Displace some heavy users Reuse of frequencies at different locations (cellular radio, infrared LAN’s) Sharing of a set of frequencies (spread spectrum radio)

37. Postacademic Interuniversity Course in Information Technology – Module C1p37 Reuse of Frequencies P(r) = P t /r 2 r transmitter r max = PtPt P min 

38. Postacademic Interuniversity Course in Information Technology – Module C1p38 Cellular Radio Automatic handover allows continuous operation of mobiles

39. Postacademic Interuniversity Course in Information Technology – Module C1p39 Cellular Radio k = number available frequencies per cell S = Area of a cell n = Number of simultaneous calls per km 2 p t = Power of transmitter p m = Minimal power at receiver input n = k / S p t = p m * S With smaller cells, - more antenna sites are needed... - more simultaneous calls are possible - transmitted power can be reduced

40. Postacademic Interuniversity Course in Information Technology – Module C1p40 Cellular Radio in practice FlandersArdennes

41. Postacademic Interuniversity Course in Information Technology – Module C1p41 Digital Cellular Telephony Name Freq.(MHz) # rad.ch. P.max.(W) r.max.(Km) Voicerate (Kb/s) Capacity (E/Km 2 ) DECT 1880-1890 12 0.25 0.2 32 10 000 GSM 890-915 935-960 124 2 35 13 1000 DCS 1800 1710-1785 1805-1880 248 1 8 13 2000

42. Postacademic Interuniversity Course in Information Technology – Module C1p42 Infrared LAN’s Limited to line of sight, within a few meters. IR technology also widely used for data transfers between laptops, PDA’s and mobile phones.

43. Postacademic Interuniversity Course in Information Technology – Module C1p43 Bluetooth Micro-lan’s designed to eliminate desktop wires and connectors on hand-held devices Frequency : 2.4 GHz unlicensed general purpose band Power : Class 3 = 1 mW (Cl. 2 = 10 mW, Cl.1 = 100 mW) Distances : Class 3 = 5m, up to 100m with class1.

44. Postacademic Interuniversity Course in Information Technology – Module C1p44 Wireless Local Loop Radio is sometimes cheaper than digging the streets !

45. Postacademic Interuniversity Course in Information Technology – Module C1p45 Wireless Local Loop Using planes or balloons could be simpler than getting building permits for antennas !!!

46. Postacademic Interuniversity Course in Information Technology – Module C1p46 Microwave Links Cost effective for line of sight communications (30 Km) High transmission capacity (several Mb/s) transmission impaired by heavy rain

47. Postacademic Interuniversity Course in Information Technology – Module C1p47 Satellite Communications 36000 Km Geostationary Round trip Delay = 240 ms High power ground stations Used for TV and paging broadcasting and for point to point links where terrestrial lines would be unpractical or too expensive

48. Postacademic Interuniversity Course in Information Technology – Module C1p48 Satellite Communications Low Orbit Short round trip delays Low power ground stations Upcoming applications: Narrowband mobile communications (TFTS airline phones, IRIDIUM mobile phones, INMARSAT communications,...)

49. Postacademic Interuniversity Course in Information Technology – Module C1p49 Introduced concepts Serial transmission Baud rate vs. throughput An upper limit for transmission capacity Error detection and correction Optical transmission and switching Coaxial cables and Twisted pairs Cellular radio and handover Point to point radio Geostationary and low orbit satellites