Overview. Chapter 4. Design Factors. Electromagnetic Spectrum

Transcription

1 Chapter 4 Transmission Media Overview Guided - wire Unguided - wireless Characteristics and quality determined by medium and signal For guided, the medium is more important For unguided, the bandwidth produced by the antenna is more important Key concerns are data rate and distance Corneliu Zaharia 2 Corneliu Zaharia Design Factors Electromagnetic Spectrum Bandwidth Higher bandwidth gives higher data rate Transmission impairments Attenuation Interference Number of receivers In guided media More receivers (multi-point) introduce more attenuation 3 Corneliu Zaharia 4 Corneliu Zaharia 1

2 Guided Transmission Media Twisted Pair Coaxial cable Optical fiber Transmission Characteristics of Guided Media Twisted pair (with loading) Frequency Range Typical Attenuation 0 to 3.5 khz khz Typical Delay 50 µs/km 2 km Repeater Spacing Twisted pairs (multi-pair cables) 0 to 1 MHz khz Coaxial cable 0 to 500 MHz 7 10 MHz Optical fiber 186 to 370 THz 0.2 to 0.5 db/km 5 µs/km 2 km 4 µs/km 1 to 9 km 5 µs/km 40 km 5 Corneliu Zaharia 6 Corneliu Zaharia Twisted Pair Twisted Pair - Applications Most common medium Telephone network Between house and local exchange (subscriber loop) Within buildings To private branch exchange (PBX) For local area networks (LAN) 10Mbps or 100Mbps 7 Corneliu Zaharia 8 Corneliu Zaharia 2

3 Twisted Pair - Pros and Cons Cheap Easy to work with Low data rate Short range Twisted Pair - Transmission Characteristics Analog Amplifiers every 5km to 6km Digital Use either analog or digital signals repeater every 2km or 3km Limited distance Limited bandwidth (1MHz) Limited data rate (100MHz) Susceptible to interference and noise 9 Corneliu Zaharia 10 Corneliu Zaharia Near End Crosstalk Coupling of signal from one pair to another Coupling takes place when transmit signal entering the link couples back to receiving pair i.e. near transmitted signal is picked up by near receiving pair Unshielded and Shielded TP Unshielded Twisted Pair (UTP) Ordinary telephone wire Cheapest Easiest to install Suffers from external EM interference Shielded Twisted Pair (STP) Metal braid or sheathing that reduces interference More expensive Harder to handle (thick, heavy) 11 Corneliu Zaharia 12 Corneliu Zaharia 3

4 UTP Categories Coaxial Cable Cat 3 up to 16MHz Voice grade found in most offices Twist length of 7.5 cm to 10 cm Cat 4 up to 20 MHz Cat 5 up to 100MHz Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm Cat 5E (Enhanced) see tables Cat 6 Cat 7 13 Corneliu Zaharia 14 Corneliu Zaharia Coaxial Cable Applications Most versatile medium Television distribution Ariel to TV Cable TV Long distance telephone transmission Can carry 10,000 voice calls simultaneously Being replaced by fiber optic Short distance computer systems links Local area networks Coaxial Cable - Transmission Characteristics Analog Amplifiers every few km Closer if higher frequency Up to 500MHz Digital Repeater every 1km Closer for higher data rates 15 Corneliu Zaharia 16 Corneliu Zaharia 4

5 Optical Fiber Optical Fiber - Benefits Greater capacity Data rates of hundreds of Gbps Smaller size & weight Lower attenuation Electromagnetic isolation Greater repeater spacing 10s of km at least 17 Corneliu Zaharia 18 Corneliu Zaharia Optical Fiber - Applications Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs Optical Fiber - Transmission Characteristics Act as wave guide for to Hz Portions of infrared and visible spectrum Light Emitting Diode (LED) Cheaper Wider operating temp range Last longer Injection Laser Diode (ILD) More efficient Greater data rate Wavelength Division Multiplexing 19 Corneliu Zaharia 20 Corneliu Zaharia 5

6 Optical Fiber Transmission Modes Wireless Transmission Frequencies 2GHz to 40GHz Microwave Highly directional Point to point Satellite 30MHz to 1GHz Omnidirectional Broadcast radio 3 x to 2 x Infrared Local 21 Corneliu Zaharia 22 Corneliu Zaharia Antennas Electrical conductor (or system of..) used to radiate electromagnetic energy or collect electromagnetic energy Transmission Radio frequency energy from transmitter Converted to electromagnetic energy By antenna Radiated into surrounding environment Reception Electromagnetic energy impinging on antenna Converted to radio frequency electrical energy Fed to receiver Same antenna often used for both Radiation Pattern Power radiated in all directions Not same performance in all directions Isotropic antenna is (theoretical) point in space Radiates in all directions equally Gives spherical radiation pattern 23 Corneliu Zaharia 24 Corneliu Zaharia 6

7 Parabolic Reflective Antenna Parabolic Reflective Antenna Used for terrestrial and satellite microwave Parabola is locus of point equidistant from a line and a point not on that line Fixed point is focus Line is directrix Revolve parabola about axis to get paraboloid Cross section parallel to axis gives parabola Cross section perpendicular to axis gives circle Source placed at focus will produce waves reflected from parabola in parallel to axis Creates (theoretical) parallel beam of light/sound/radio On reception, signal is concentrated at focus, where detector is placed 25 Corneliu Zaharia 26 Corneliu Zaharia Antenna Gain Measure of directionality of antenna Power output in particular direction compared with that produced by isotropic antenna Measured in decibels (db) Results in loss in power in another direction Effective area relates to size and shape Related to gain Terrestrial Microwave Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates 27 Corneliu Zaharia 28 Corneliu Zaharia 7

8 Satellite Microwave Satellite Point to Point Link Satellite is relay station Satellite receives on one frequency, amplifies or repeats signal and transmits on another frequency Requires geo-stationary orbit Height of 35,784km Television Long distance telephone Private business networks 29 Corneliu Zaharia 30 Corneliu Zaharia Satellite Broadcast Link Broadcast Radio Omnidirectional FM radio UHF and VHF television Line of sight Suffers from multipath interference Reflections 31 Corneliu Zaharia 32 Corneliu Zaharia 8

9 Infrared Modulate noncoherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port Wireless Propagation Signal travels along three routes Ground wave Follows contour of earth Up to 2MHz AM radio Sky wave Amateur radio, BBC world service, Voice of America Signal reflected from ionosphere layer of upper atmosphere (Actually refracted) Line of sight Above 30Mhz May be further than optical line of sight due to refraction More later 33 Corneliu Zaharia 34 Corneliu Zaharia Chapter 5 Signal Encoding Techniques Encoding Techniques Digital data, digital signal Analog data, digital signal Digital data, analog signal Analog data, analog signal Corneliu Zaharia 36 Corneliu Zaharia 9

10 Digital Data, Digital Signal Digital signal Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements Terms (1) Unipolar All signal elements have same sign Polar One logic state represented by positive voltage the other by negative voltage Data rate Rate of data transmission in bits per second Duration or length of a bit Time taken for transmitter to emit the bit 37 Corneliu Zaharia 38 Corneliu Zaharia Terms (2) Modulation rate Rate at which the signal level changes Measured in baud = signal elements per second Mark and Space Binary 1 and Binary 0 respectively Interpreting Signals Need to know Timing of bits - when they start and end Signal levels Factors affecting successful interpreting of signals Signal to noise ratio Data rate Bandwidth 39 Corneliu Zaharia 40 Corneliu Zaharia 10

11 Comparison of Encoding Schemes (1) Signal Spectrum Lack of high frequencies reduces required bandwidth Lack of dc component allows ac coupling via transformer, providing isolation Concentrate power in the middle of the bandwidth Clocking Synchronizing transmitter and receiver External clock Sync mechanism based on signal Comparison of Encoding Schemes (2) Error detection Can be built in to signal encoding Signal interference and noise immunity Some codes are better than others Cost and complexity Higher signal rate (& thus data rate) lead to higher costs Some codes require signal rate greater than data rate 41 Corneliu Zaharia 42 Corneliu Zaharia Encoding Schemes Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3 Nonreturn to Zero-Level (NRZ-L) Two different voltages for 0 and 1 bits Voltage constant during bit interval no transition I.e. no return to zero voltage e.g. Absence of voltage for zero, constant positive voltage for one More often, negative voltage for one value and positive for the other This is NRZ-L 43 Corneliu Zaharia 44 Corneliu Zaharia 11

12 Nonreturn to Zero Inverted NRZ Nonreturn to zero inverted on ones Constant voltage pulse for duration of bit Data encoded as presence or absence of signal transition at beginning of bit time Transition (low to high or high to low) denotes a binary 1 No transition denotes binary 0 An example of differential encoding 45 Corneliu Zaharia 46 Corneliu Zaharia Differential Encoding Data represented by changes rather than levels More reliable detection of transition rather than level In complex transmission layouts it is easy to lose sense of polarity NRZ pros and cons Pros Easy to engineer Make good use of bandwidth Cons dc component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission 47 Corneliu Zaharia 48 Corneliu Zaharia 12

13 Multilevel Binary Use more than two levels Bipolar-AMI zero represented by no line signal one represented by positive or negative pulse one pulses alternate in polarity No loss of sync if a long string of ones (zeros still a problem) No net dc component Lower bandwidth Easy error detection Pseudoternary One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-ami 49 Corneliu Zaharia 50 Corneliu Zaharia Bipolar-AMI and Pseudoternary Trade Off for Multilevel Binary Not as efficient as NRZ Each signal element only represents one bit In a 3 level system could represent log 2 3 = 1.58 bits Receiver must distinguish between three levels (+A, -A, 0) Requires approx. 3dB more signal power for same probability of bit error 51 Corneliu Zaharia 52 Corneliu Zaharia 13

14 Biphase Manchester Encoding Manchester Transition in middle of each bit period Transition serves as clock and data Low to high represents one High to low represents zero Used by IEEE Differential Manchester Midbit transition is clocking only Transition at start of a bit period represents zero No transition at start of a bit period represents one Note: this is a differential encoding scheme Used by IEEE Corneliu Zaharia 54 Corneliu Zaharia Differential Manchester Encoding Biphase Pros and Cons Con At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth Pros Synchronization on mid bit transition (self clocking) No dc component Error detection Absence of expected transition 55 Corneliu Zaharia 56 Corneliu Zaharia 14

15 Modulation Rate Scrambling Use scrambling to replace sequences that would produce constant voltage Filling sequence Must produce enough transitions to sync Must be recognized by receiver and replace with original Same length as original No dc component No long sequences of zero level line signal No reduction in data rate Error detection capability 57 Corneliu Zaharia 58 Corneliu Zaharia B8ZS Bipolar With 8 Zeros Substitution Based on bipolar-ami If octet of all zeros and last voltage pulse preceding was positive encode as If octet of all zeros and last voltage pulse preceding was negative encode as Causes two violations of AMI code Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros HDB3 High Density Bipolar 3 Zeros Based on bipolar-ami String of four zeros replaced with one or two pulses 59 Corneliu Zaharia 60 Corneliu Zaharia 15

16 B8ZS and HDB3 Digital Data, Analog Signal Public telephone system 300Hz to 3400Hz Use modem (modulator-demodulator) Amplitude shift keying (ASK) Frequency shift keying (FSK) Phase shift keying (PK) 61 Corneliu Zaharia 62 Corneliu Zaharia Modulation Techniques Amplitude Shift Keying Values represented by different amplitudes of carrier Usually, one amplitude is zero i.e. presence and absence of carrier is used Susceptible to sudden gain changes Inefficient Up to 1200bps on voice grade lines Used over optical fiber 63 Corneliu Zaharia 64 Corneliu Zaharia 16

17 Binary Frequency Shift Keying Most common form is binary FSK (BFSK) Two binary values represented by two different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using co-ax Multiple FSK More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents more than one bit 65 Corneliu Zaharia 66 Corneliu Zaharia FSK on Voice Grade Line Phase Shift Keying Phase of carrier signal is shifted to represent data Binary PSK Two phases represent two binary digits Differential PSK Phase shifted relative to previous transmission rather than some reference signal 67 Corneliu Zaharia 68 Corneliu Zaharia 17

18 Differential PSK Analog Data, Digital Signal Digitization Conversion of analog data into digital data Digital data can then be transmitted using NRZ-L Digital data can then be transmitted using code other than NRZ-L Digital data can then be converted to analog signal Analog to digital conversion done using a codec Pulse code modulation Delta modulation 69 Corneliu Zaharia 70 Corneliu Zaharia Digitizing Analog Data Pulse Code Modulation(PCM) (1) If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal (Proof - Stallings appendix 4A) Voice data limited to below 4000Hz Require 8000 sample per second Analog samples (Pulse Amplitude Modulation, PAM) Each sample assigned digital value 71 Corneliu Zaharia 72 Corneliu Zaharia 18

19 Pulse Code Modulation(PCM) (2) PCM Example 4 bit system gives 16 levels Quantized Quantizing error or noise Approximations mean it is impossible to recover original exactly 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each gives 64kbps 73 Corneliu Zaharia 74 Corneliu Zaharia PCM Block Diagram Nonlinear Encoding Quantization levels not evenly spaced Reduces overall signal distortion Can also be done by companding 75 Corneliu Zaharia 76 Corneliu Zaharia 19

20 Effect of Non-Linear Coding Typical Companding Functions 77 Corneliu Zaharia 78 Corneliu Zaharia Delta Modulation Delta Modulation - example Analog input is approximated by a staircase function Move up or down one level (δ) at each sample interval Binary behavior Function moves up or down at each sample interval 79 Corneliu Zaharia 80 Corneliu Zaharia 20

21 Delta Modulation - Operation Delta Modulation - Performance Good voice reproduction PCM levels (7 bit) Voice bandwidth 4khz Should be 8000 x 7 = 56kbps for PCM Data compression can improve on this e.g. Interframe coding techniques for video 81 Corneliu Zaharia 82 Corneliu Zaharia Analog Data, Analog Signals Analog Modulation Why modulate analog signals? Higher frequency can give more efficient transmission Permits frequency division multiplexing (chapter 8) Types of modulation Amplitude Frequency Phase 83 Corneliu Zaharia 84 Corneliu Zaharia 21