Telecommunication Network

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Telecommunication Network

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1. Telecommunication Network Telecommunication Transmission Semester 1, 2006

2. KT6123 2 Electromagnetic Spectrum

3. KT6123 3 Transmission Media - Overview Transmission Medium Physical path between transmitter and receiver Guided Media Waves are guided along a solid medium e.g., copper twisted pair, copper coaxial cable, optical fiber Unguided Media Provides means of transmission but does not guide electromagnetic signals Employ an antenna for transmission e.g., atmosphere, outer space, satellites, terrestrial microwave, broadcast radio

4. KT6123 4 Transmission Media - Overview Characteristics and quality determined by medium and signal For guided Medium is more important For unguided Bandwidth produced by the antenna is more important Key concerns are Data rate and Distance Higher data rate and longer distance is better Design factor Bandwidth All other factors remaining constant, higher bandwidth gives higher data rate Transmission impairments Attenuation Interference Number of receivers In guided media More receivers (multi-point) introduce more attenuation

5. KT6123 5 Open-Wire Pairs

6. KT6123 6 Twisted Pair Most common medium Two separately insulated wires twisted together in a helical manner (like DNA) and often bundled together Advantages Cheap Easy to work with Disadvantages Low data rate Short range Applications Telephone network Between house and local exchange Within buildings To private branch exchange (PBX) For local area networks (LAN) 10 Mbps or 100 Mbps

7. KT6123 7 Twisted Pair Transmission Charateristics Analog Amplifiers every 5 km to 6 km Digital Use either analog or digital signals Repeater every 2 km or 3 km Limited in Distance Bandwidth (1 MHz) Data rate (100 Mbps) Susceptible to interference and noise

8. KT6123 8 UTP vs. STP

9. KT6123 9 Coaxial Cable Most versatile medium

10. KT6123 10 Coaxial Cable Applications Television distribution Cable TV Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Short distance computer systems links LANs Transmission characteristic Analog Amplifiers every few km Closer if higher frequency Up to 500 MHz Digital Repeater every 1 km Closer for higher data rates

11. KT6123 11 Thin Ethernet – 10 Base 2 Short for British Naval Connector or Bayonet Nut Connector or Bayonet Neill Concelman Short for British Naval Connector or Bayonet Nut Connector or Bayonet Neill Concelman

12. KT6123 12 Thick Ethernet - 10 Base 5 This type of cable is 0.5" diameter (usually supplied with a yellow outer PVC coating) and rather inflexible. It has become known in the communications industry as "Thick Ethernet". The official name for this cable is 10 Baseband5 (10B5), indicating that it is specified for baseband communications (i.e. not modulated) at 10 Mbps over distances up to 500m. This type of cable is 0.5" diameter (usually supplied with a yellow outer PVC coating) and rather inflexible. It has become known in the communications industry as "Thick Ethernet". The official name for this cable is 10 Baseband5 (10B5), indicating that it is specified for baseband communications (i.e. not modulated) at 10 Mbps over distances up to 500m.

13. KT6123 13 Optical Fiber Advantages Greater capacity - data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing - 10s of km at least System components: Transmission medium - fiber optic cable Light source LED (cheaper, wider operating temp range, last longer) Injection laser diode (ILD) (More efficient, greater data rate) Detector - photodiode

14. KT6123 14 Optical Fiber - Applications Telephone Network Applications Long-haul, metropolitan, rural, and subscriber loop circuits Local Area Networks Optical fiber networks Data rates from 100 Mbps to 1 Gbps Support hundreds (or even thousands) of stations

15. KT6123 15 Optical Fiber - Transmission Characteristics Light Sources Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) More efficient Greater data rate Wavelength Division Multiplexing

16. KT6123 16 Cost of Wired Transmission Media American Wire Gauge (AWG) A measure of the thickness of copper, aluminum and other wiring in the U.S. and elsewhere. Copper cabling typically varies from 18 to 26 AWG. The higher the number, the thinner the wire. The thicker the wire, the less susceptible it is to interference. In general, thin wire cannot carry the same amount of electrical current the same distance that thicker wire can. American Wire Gauge (AWG) A measure of the thickness of copper, aluminum and other wiring in the U.S. and elsewhere. Copper cabling typically varies from 18 to 26 AWG. The higher the number, the thinner the wire. The thicker the wire, the less susceptible it is to interference. In general, thin wire cannot carry the same amount of electrical current the same distance that thicker wire can.

17. KT6123 17 Transmission Medium

18. KT6123 18 Transmission Medium

19. KT6123 19 Transmission Medium

20. KT6123 20 Voice Communication Services Leased Line : High quality connections. High speed data transmission available known as conditioned leased line. Risky in case the line is down. No automatically re-route the call, Point-to-Point connection, Less reliability Dial-up Line : Different route available depending on dialing number. High reliability but uncertain quality connections Private Branch Exchange [PBX] : internal call never leave the customer’s premises. Only external calls are sent to the central office; therefore few central office trunk are needed. And the customer’s central office charges are far less than those associated with Centrex service. But the customer must purchase or lease the PBX to achieve these economies.

21. KT6123 21 Data Transmission An ideal data transmission system gives an output which is identical to the input. Three problems in data transmission and long-haul communication noise attenuation (amplification, line loading) distortion (equalization)

22. KT6123 22 Noise A variation in output not caused by a variation in the measurand is noise It gives an error in the measurement unless it is removed It is quantified as the signal-to-noise ratio (SNR)

23. KT6123 23 Attenuation Energy losses in the transmission medium mean that the amplitude of the signal is reduced Reduces the SNR Reduces the signal level

24. KT6123 24 Distortion Arises when the frequency response of transmission system is inadequate to deal with the frequencies in the signal.

25. KT6123 25 Analogue data transmission Generally done by using conducting wires to feed the transducer output to the signal processing, recording and/or display unit. Wires may be simple single strand conductors, or may be co-axial cables. Co-axial cables consist of inner conductor insulating layer outer earthing and screening conductor final insulating layer outside.

26. KT6123 26 Analogue data transmission Thickness and purity of both conductors and insulators vary; the cost varies accordingly. Reduction of attenuation and noise mean increased cost.

27. KT6123 27 Effect of noise on transmitted analogue data Low levels of noise enable the signal to be detected with very small errors High levels of noise may totally obscure the signal. Noise arises from external sources noise generated in the conductor itself.

28. KT6123 28 Effect of noise on transmitted analogue data Coaxial cables reduce these problems compared with single wires Internal noise is related to the size, length and quality of the conductors. Isolation from external pickup reduces with thicker, better quality insulation

29. KT6123 29 Effect of attenuation on transmitted analogue data Attenuation is determined by the thickness and quality of the conductor in the cable. Also determined by distance (length of cable)

30. KT6123 30 Effect of distortion on transmitted analogue data Distortion effects are related to frequency response The transmission system deals adequately with the low frequency A higher frequency is, however, barely transmitted. The frequency response of co-axial cable varies with cost.

31. KT6123 31 Transmission distance The greater the distance, the more serious all these problems are Simple conducting wires may be adequate to carry a signal over a distance of a metre More expensive coaxial cable will be required to carry the same signal to the same display system over a longer distance.

32. KT6123 32 Near End Crosstalk (NEXT) Coupling of signal from one pair to another The tighter the twist in the cable, the more effective the cancellation

33. KT6123 33 Echo The effect resulting from a delayed reflection of a signal. Echo in a telephone circuit manifests to talker as slightly delayed repeat of his own voice, returned from the distant end, just like a sound echo. Echo can occur at the talker and listener. An echo canceller is used in voice and data circuit to suppress telephone echo by simulating a negative version of the outgoing signal in the receive path. An echo suppressor is a device to suppress retransmission of incoming receive path signals by inserting a very large attenuation into the transmit path whenever a signal is detected in the receive path.

34. KT6123 34 Echo Listener Echo Symptom—Listener and talker echo sound similar although the signal strength of listener echo may be lower. The essential difference between them is who hears the echo and where it is produced. Listener echo is the component of the talker echo that leaks through the near-end hybrid and returns again to the listener causing a delayed softer echo. The listener hears the talker twice. Cause—Common causes are: Insufficient loss of the echo signal. Long echo tail. Echo cancellers in the gateway adjacent to the near-end hybrid not activating. Talker Echo Symptom—Talker echo is the signal which leaks in the far-end hybrid and returns to the sender (talker). The talker hears an echo of his own voice. Cause—Common causes are: Insufficient loss of the echo signal. Echo cancellers in the gateway adjacent to the far-end hybrid not activating. Acoustic echo caused by the listener's phone.

35. KT6123 35 Echo Suppressor/Cancellers Echo Suppressors : Device uses to suppress noise. Allowing only a one-way communications path. This works well for normal conversation; when we stop talking, and the other person begins, their voice take over. If they interrupt us at mid-sentence, they probably are speaking louder than we are so we can hear them. The suppressor hears the volume difference and gives them the line Echo Cancellers : Device uses to help eliminate echoes. Allowing a continuous two-way communications, but are able to remove your own echo before it returns to your telephone. A sophisticated version of acoustical ceiling tiles used to absorb noise. And detects the difference between true conversation and an echo, and selectively absorb only the echo.

36. KT6123 36 Singing If both paths of 4-wire circuit are connected directly to the 2-wire circuit at each end, a signal can circulate round the complete loop thus created. This will results in continuous oscillation, known as singing, unless the sum of the gains in the two direction were less than zero. To avoid this a transhybrid transformer (4-wire/2-wire terminating set) is used. Stability: (related to singing) -singing path loss Ls=2(B+L2) -Condition of stability Ls>0

37. KT6123 37 Sidetone

38. KT6123 38 The Maximum Data Rate of a Channel The maximum data rate of a noiseless channel with a bandwidth of H and V number of levels is = 2H log2(V) bits per second (bps) due to H. Nyquist in 1924 hence if V = 2 (binary encoding) and channel bandwidth = 3000 Hz then maximum data rate is = 2*3000 = 6000 bps

39. KT6123 39 Transmitting Signals Major problems: Attenuation weakening of signal as it propagates forward depends on frequency of signal Noise unwanted energy or signals from sources other than the transmitter

40. KT6123 40 Data transfer in the presence of noise Shannons Law: C = B * log2 (1 + S/N) where: C = achievable channel capacity B= Bandwidth of line (in Hz) S = Average signal power N = Average Noise power S/N = Signal to Noise Ratio this is usually measured in decibels (dB) where dB = 10 * log10 (S/N)

41. KT6123 41 Decibels (dB)

42. KT6123 42 Decibels (Level)

43. KT6123 43 Examples of dBw and dBm

44. KT6123 44 Decibels (Ratio)

45. KT6123 45 Examples

46. KT6123 46 Noise Factor / Figure

47. KT6123 47 Return Loss

48. KT6123 48 dBr

49. KT6123 49 2-Wire and 4-Wire

50. KT6123 50 4-Wire Circuit

51. KT6123 51 Echo and Singing

52. KT6123 52 Echo Path

53. KT6123 53 Singing Echoes: part of the signal return from the other direction talker echo and listener echo. Attenuation (2W-2W) L2 = 6 – G4 dB [G4:net gain of one side of 4W circuit = Total Ampl. Gain-Total loss] Transhybrid loss (loss in hybrid transformer)= 6+B dB [B (balance-return loss due to impedance mismatch) =20 log10 |(N+Z)/(N-Z)| [Z: impedance of two wire; N: impedance of balance network] Attenuation of talker’s echo LT = 2L2+B dB Echo delay time DT =2T4 Attenuation of listener’s echo Ll= 2L2+2B dB Singing path: Ls = 2(B + 6 – G4) = 2(B + L2) dB (singing point Ls = 0) For stability: Ls > 0, (B + L2) > 0, G2 < B where G2 = - L2 Stability margin: M = B + L2 Echo cancellers: to increase the echo attenuation.

54. KT6123 54 Telephone Channel Capacity

55. KT6123 55 CO Connectivity

56. KT6123 56 POTS Connectivity Small Cities have a CO Big Cities have CO’s Hierarchical system, add High Usage Direct Lines between CO’s Tandem (Trunk-to-Trunk) Switches Minimum of two physically separate routes out of all switches desired Best compromise of cost & reliability

57. KT6123 57 POTS Items in a typical phone: microphone & speaker hybrid dialing circuitry (DTMF) on/off hook switch ring circuitry Items in a typical CO: crosspoint switch hybrids A/D & D/A converters echo cancelers TDM

58. KT6123 58 Home Phone

59. KT6123 59 Home Phone

60. KT6123 60 Home Phone

61. KT6123 61 One Wire To get audio out of speaker, need a voltage drop across the speaker inputs Need two 'wires' to get a voltage drop across a speaker one wire can be an actual wire second 'wire' can be the earth Very Susceptible to static

62. KT6123 62 Two Wires Resistant to static Susceptible to interference over long distances Twisting the wires slashes interference

63. KT6123 63 Two Wires Hybrids allow Telco Two Wire lines to carry both outbound and inbound traffic short distances

64. KT6123 64 Four Wires Easier to amplify traffic moving one direction Telco Four Wire lines 2, one-way, 2 wire connections Long distance

65. KT6123 65 POTS Connectivity (1920)

66. KT6123 66 POTS Connectivity (1970)

67. KT6123 67 POTS Connectivity (1990)

68. KT6123 68 Simplified Central Office Switch

69. KT6123 69 Simplified CO-to-CO connectivity

70. KT6123 70 The phone system... Parts are 4 wire (headset and long haul) 4 wire = two unidirectional signals unidirectional signals make amplification a lot easier Parts are 2 wire (local loop) 2 wire = one bi-directional signal Turn-of-the-century decision to save $$$ and go 2 wire on local loops Parts are analog (phone & local loop) About 80% of U.S. Local Loops are copper all-the-way Parts are digital (long haul, many CO switches, some local loops) About 20% of U.S. Local Loops use ISDN or Digital Loop Carriers

71. KT6123 71 The phone system... 4 Wire to 2 Wire Conversion at Central Office Hybrids can cause some problems Singing (Cure: Attenuation) Echoes (Cure: Echo Canceler) Analog to Digital Conversion points also cause some problems CO Switch filters on analog voice lines, necessary to limit noise and interference on voice circuits, limit modem data speeds to about 33 Kbps Trend is to an all-digital system U.S. long haul POTS voice circuits use digital Time Division Multiplexing

72. KT6123 72 PC Modems & POTS Band Pass Filter suppresses energy outside voice bandwidth (500 - 3,500 Hz)

73. KT6123 73 PC Modems & POTS PC Bit Stream has a significant amount of energy below 0.5 KHz Modems shift the energy into the pass band of the filter

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