Transcript Frequency
Chapter 2 The Physical Layer Supplementery Slides BLM431 Computer Networks Dr.Refik Samet 1 Data Transmission BLM431 Computer Networks Dr.Refik Samet 2 Terminology (1) Transmitter Receiver Medium Guided medium e.g. twisted pair, optical fiber Unguided medium e.g. air, water, vacuum BLM431 Computer Networks Dr.Refik Samet 3 Terminology (2) Direct link No intermediate devices Point-to-point Direct link Only 2 devices share link Multi-point More than two devices share the link BLM431 Computer Networks Dr.Refik Samet 4 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 BLM431 Computer Networks Dr.Refik Samet 5 Frequency, Spectrum and Bandwidth Time domain concepts Analog signal Various in a smooth way over time Digital signal Maintains a constant level then changes to another constant level Periodic signal Pattern repeated over time Aperiodic signal Pattern not repeated over time BLM431 Computer Networks Dr.Refik Samet 6 Analogue & Digital Signals BLM431 Computer Networks Dr.Refik Samet 7 Periodic Signals BLM431 Computer Networks Dr.Refik Samet 8 Sine Wave Peak Amplitude (A) maximum strength of signal volts Frequency (f) Rate of change of signal Hertz (Hz) or cycles per second Period = time for one repetition (T) T = 1/f Phase () Relative position in time BLM431 Computer Networks Dr.Refik Samet 9 Varying Sine Waves s(t) = A sin(2ft +) BLM431 Computer Networks Dr.Refik Samet 10 Wavelength Distance occupied by one cycle Distance between two points of corresponding phase in two consecutive cycles Assuming signal velocity v = vT f = v c = 3*108 ms-1 (speed of light in free space) BLM431 Computer Networks Dr.Refik Samet 11 Frequency Domain Concepts Signal usually made up of many frequencies Components are sine waves Can be shown (Fourier analysis) that any signal is made up of component sine waves Can plot frequency domain functions BLM431 Computer Networks Dr.Refik Samet 12 Addition of Frequency Components (T=1/f) BLM431 Computer Networks Dr.Refik Samet 13 Frequency Domain Representations BLM431 Computer Networks Dr.Refik Samet 14 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 DC Component Component of zero frequency BLM431 Computer Networks Dr.Refik Samet 15 Signal with DC Component BLM431 Computer Networks Dr.Refik Samet 16 Data Rate and Bandwidth Any transmission system has a limited band of frequencies This limits the data rate that can be carried BLM431 Computer Networks Dr.Refik Samet 17 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 BLM431 Computer Networks Dr.Refik Samet 18 Analog and Digital Data Analog Continuous values within some interval e.g. sound, video Digital Discrete values e.g. text, integers BLM431 Computer Networks Dr.Refik Samet 19 Acoustic Spectrum (Analog) BLM431 Computer Networks Dr.Refik Samet 20 Analog and Digital 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 two DC components BLM431 Computer Networks Dr.Refik Samet 21 Advantages & Disadvantages of Digital Cheaper Less susceptible to noise Greater attenuation Pulses become rounded and smaller Leads to loss of information BLM431 Computer Networks Dr.Refik Samet 22 Attenuation of Digital Signals BLM431 Computer Networks Dr.Refik Samet 23 Components of Speech Frequency range (of hearing) 20Hz-20kHz Speech 100Hz-7kHz Easily converted into electromagnetic signal for transmission Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage Limit frequency range for voice channel 300-3400Hz BLM431 Computer Networks Dr.Refik Samet 24 Conversion of Voice Input into Analog Signal BLM431 Computer Networks Dr.Refik Samet 25 Video Components USA - 483 lines scanned per frame at 30 frames per second 525 lines but 42 lost during vertical retrace So 525 lines x 30 scans = 15750 lines per second 63.5s per line 11s for retrace, so 52.5 s per video line Max frequency if line alternates black and white Horizontal resolution is about 450 lines giving 225 cycles of wave in 52.5 s Max frequency of 4.2MHz BLM431 Computer Networks Dr.Refik Samet 26 Binary Digital Data From computer terminals etc. Two dc components Bandwidth depends on data rate BLM431 Computer Networks Dr.Refik Samet 27 Conversion of PC Input to Digital Signal BLM431 Computer Networks Dr.Refik Samet 28 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 BLM431 Computer Networks Dr.Refik Samet 29 Analog Signals Carrying Analog and Digital Data BLM431 Computer Networks Dr.Refik Samet 30 Digital Signals Carrying Analog and Digital Data BLM431 Computer Networks Dr.Refik Samet 31 Analog Transmission Analog signal transmitted without regard to content May be analog or digital data Attenuated over distance Use amplifiers to boost signal Also amplifies noise BLM431 Computer Networks Dr.Refik Samet 32 Digital Transmission Concerned with content Integrity endangered by noise, attenuation etc. Repeaters used Repeater receives signal Extracts bit pattern Retransmits Attenuation is overcome Noise is not amplified BLM431 Computer Networks Dr.Refik Samet 33 Advantages of Digital Transmission Digital technology Low cost LSI/VLSI technology Data integrity Longer distances over lower quality lines Capacity utilization High bandwidth links economical High degree of multiplexing easier with digital techniques Security & Privacy Encryption Integration Can treat analog and digital data similarly BLM431 Computer Networks Dr.Refik Samet 34 Transmission Impairments Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by Attenuation and attenuation distortion Delay distortion Noise BLM431 Computer Networks Dr.Refik Samet 35 Attenuation Signal strength falls off with distance Depends on medium Received signal strength: must be enough to be detected must be sufficiently higher than noise to be received without error Attenuation is an increasing function of frequency BLM431 Computer Networks Dr.Refik Samet 36 Delay Distortion Only in guided media Propagation velocity varies with frequency BLM431 Computer Networks Dr.Refik Samet 37 Noise (1) Additional signals inserted between transmitter and receiver Thermal Due to thermal agitation of electrons Uniformly distributed White noise Intermodulation Signals that are the sum and difference of original frequencies sharing a medium BLM431 Computer Networks Dr.Refik Samet 38 Noise (2) Crosstalk A signal from one line is picked up by another Impulse Irregular pulses or spikes e.g. External electromagnetic interference Short duration High amplitude BLM431 Computer Networks Dr.Refik Samet 39 Channel Capacity Data rate In bits per second Rate at which data can be communicated Bandwidth In cycles per second of Hertz Constrained by transmitter and medium BLM431 Computer Networks Dr.Refik Samet 40 Nyquist Bandwidth If rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rate Given bandwidth B, highest signal rate is 2B Given binary signal, data rate supported by B Hz is 2B bps Can be increased by using M signal levels C= 2B log2M BLM431 Computer Networks Dr.Refik Samet 41 Shannon Capacity Formula Consider data rate,noise and error rate Faster data rate shortens each bit so burst of noise affects more bits At given noise level, high data rate means higher error rate Signal to noise ration (in decibels) SNRdb=10 log10 (signal/noise) Capacity C=B log2(1+SNR) This is error free capacity BLM431 Computer Networks Dr.Refik Samet 42 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 BLM431 Computer Networks Dr.Refik Samet 43 Design Factors Bandwidth Higher bandwidth gives higher data rate Transmission impairments Attenuation Interference Number of receivers In guided media More receivers (multi-point) introduce more attenuation BLM431 Computer Networks Dr.Refik Samet 44 Electromagnetic Spectrum BLM431 Computer Networks Dr.Refik Samet 45 Guided Transmission Media Twisted Pair Coaxial cable Optical fiber BLM431 Computer Networks Dr.Refik Samet 46 Transmission Characteristics of Guided Media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pair (with loading) 0 to 3.5 kHz 0.2 dB/km @ 1 kHz 50 µs/km 2 km Twisted pairs (multi-pair cables) Coaxial cable 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km BLM431 Computer Networks Dr.Refik Samet 47 Twisted Pair BLM431 Computer Networks Dr.Refik Samet 48 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 BLM431 Computer Networks Dr.Refik Samet 49 Twisted Pair - Pros and Cons Cheap Easy to work with Low data rate Short range BLM431 Computer Networks Dr.Refik Samet 50 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 BLM431 Computer Networks Dr.Refik Samet 51 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 BLM431 Computer Networks Dr.Refik Samet 52 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) BLM431 Computer Networks Dr.Refik Samet 53 UTP Categories 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 BLM431 Computer Networks Dr.Refik Samet 54 Comparison of Shielded and Unshielded Twisted Pair Attenuation (dB per 100 m) Frequency (MHz) Category 3 UTP Category 5 UTP 1 2.6 2.0 4 5.6 16 13.1 150-ohm STP Near-end Crosstalk (dB) Category 3 UTP Category 5 UTP 150-ohm STP 1.1 41 62 58 4.1 2.2 32 53 58 8.2 4.4 23 44 50.4 25 — 10.4 6.2 — 41 47.5 100 — 22.0 12.3 — 32 38.5 300 — Computer Networks — — BLM43121.4 Dr.Refik Samet — 31.3 55 Twisted Pair Categories and Classes Category 3 Class C Category 5 Class D Bandwidth 16 MHz 100 MHz Cable Type UTP Link Cost (Cat 5 =1) 0.7 Category 5E Category 6 Class E Category 7 Class F 100 MHz 200 MHz 600 MHz UTP/FTP UTP/FTP UTP/FTP SSTP 1 1.2 1.5 2.2 BLM431 Computer Networks Dr.Refik Samet 56 Coaxial Cable BLM431 Computer Networks Dr.Refik Samet 57 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 BLM431 Computer Networks Dr.Refik Samet 58 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 BLM431 Computer Networks Dr.Refik Samet 59 Optical Fiber BLM431 Computer Networks Dr.Refik Samet 60 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 BLM431 Computer Networks Dr.Refik Samet 61 Optical Fiber - Applications Long-haul trunks Metropolitan trunks Rural exchange trunks Subscriber loops LANs BLM431 Computer Networks Dr.Refik Samet 62 Optical Fiber - Transmission Characteristics Act as wave guide for 1014 to 1015 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 BLM431 Computer Networks Dr.Refik Samet 63 Optical Fiber Transmission Modes BLM431 Computer Networks Dr.Refik Samet 64 Frequency Utilization for Fiber Applications Wavelength (in vacuum) range (nm) Frequency range (THz) 820 to 900 366 to 333 1280 to 1350 234 to 222 1528 to 1561 1561 to 1620 Band label Fiber type Application Multimode LAN S Single mode Various 196 to 192 C Single mode WDM 185 to 192 L Single mode WDM BLM431 Computer Networks Dr.Refik Samet 65 Attenuation in Guided Media BLM431 Computer Networks Dr.Refik Samet 66 Wireless Transmission Frequencies 2GHz to 40GHz Microwave Highly directional Point to point Satellite 30MHz to 1GHz Omnidirectional Broadcast radio 3 x 1011 to 2 x 1014 Infrared Local BLM431 Computer Networks Dr.Refik Samet 67 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 BLM431 Computer Networks Dr.Refik Samet 68 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 BLM431 Computer Networks Dr.Refik Samet 69 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 BLM431 Computer Networks Dr.Refik Samet 70 Parabolic Reflective Antenna BLM431 Computer Networks Dr.Refik Samet 71 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 BLM431 Computer Networks Dr.Refik Samet 72 Terrestrial Microwave Parabolic dish Focused beam Line of sight Long haul telecommunications Higher frequencies give higher data rates BLM431 Computer Networks Dr.Refik Samet 73 Satellite Microwave 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 BLM431 Computer Networks Dr.Refik Samet 74 Satellite Point to Point Link BLM431 Computer Networks Dr.Refik Samet 75 Satellite Broadcast Link BLM431 Computer Networks Dr.Refik Samet 76 Broadcast Radio Omnidirectional FM radio UHF and VHF television Line of sight Suffers from multipath interference Reflections BLM431 Computer Networks Dr.Refik Samet 77 Infrared Modulate noncoherent infrared light Line of sight (or reflection) Blocked by walls e.g. TV remote control, IRD port BLM431 Computer Networks Dr.Refik Samet 78 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… BLM431 Computer Networks Dr.Refik Samet 79 Ground Wave Propagation BLM431 Computer Networks Dr.Refik Samet 80 Sky Wave Propagation BLM431 Computer Networks Dr.Refik Samet 81 Line of Sight Propagation BLM431 Computer Networks Dr.Refik Samet 82 Refraction Velocity of electromagnetic wave is a function of density of material ~3 x 108 m/s in vacuum, less in anything else As wave moves from one medium to another, its speed changes Causes bending of direction of wave at boundary Towards more dense medium Index of refraction (refractive index) is Sin(angle of incidence)/sin(angle of refraction) Varies with wavelength May cause sudden change of direction at transition between media May cause gradual bending if medium density is varying Density of atmosphere decreases with height BLM431 Computer Networks Dr.Refik Samet Results in bending towards earth of radio waves 83 Optical and Radio Horizons BLM431 Computer Networks Dr.Refik Samet 84 Line of Sight Transmission Free space loss Signal disperses with distance Greater for lower frequencies (longer wavelengths) Atmospheric Absorption Water vapour and oxygen absorb radio signals Water greatest at 22GHz, less below 15GHz Oxygen greater at 60GHz, less below 30GHz Rain and fog scatter radio waves Multipath Better to get line of sight if possible Signal can be reflected causing multiple copies to be received May be no direct signal at all May reinforce or cancel direct signal Refraction May result in BLM431 Computer Networks partial or total loss of signal Dr.Refik Samet at receiver 85 Free Space Loss BLM431 Computer Networks Dr.Refik Samet 86 Multipath Interference BLM431 Computer Networks Dr.Refik Samet 87 Encoding Techniques Digital data, digital signal Analog data, digital signal Digital data, analog signal Analog data, analog signal BLM431 Computer Networks Dr.Refik Samet 88 Digital Data, Digital Signal Digital signal Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements BLM431 Computer Networks Dr.Refik Samet 89 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 BLM431 Computer Networks Dr.Refik Samet 90 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 BLM431 Computer Networks Dr.Refik Samet 91 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 BLM431 Computer Networks Dr.Refik Samet 92 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 BLM431 Computer Networks Dr.Refik Samet 93 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 BLM431 Computer Networks Dr.Refik Samet 94 Encoding Schemes Nonreturn to Zero-Level (NRZ-L) Nonreturn to Zero Inverted (NRZI) Bipolar -AMI Pseudoternary Manchester Differential Manchester B8ZS HDB3 BLM431 Computer Networks Dr.Refik Samet 95 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 BLM431 Computer Networks Dr.Refik Samet 96 Nonreturn to Zero Inverted 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 BLM431 Computer Networks Dr.Refik Samet 97 NRZ BLM431 Computer Networks Dr.Refik Samet 98 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 BLM431 Computer Networks Dr.Refik Samet 99 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 BLM431 Computer Networks Dr.Refik Samet 100 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 BLM431 Computer Networks Dr.Refik Samet 101 Pseudoternary One represented by absence of line signal Zero represented by alternating positive and negative No advantage or disadvantage over bipolar-AMI BLM431 Computer Networks Dr.Refik Samet 102 Bipolar-AMI and Pseudoternary BLM431 Computer Networks Dr.Refik Samet 103 Trade Off for Multilevel Binary Not as efficient as NRZ Each signal element only represents one bit In a 3 level system could represent log23 = 1.58 bits Receiver must distinguish between three levels (+A, -A, 0) Requires approx. 3dB more signal power for same probability of bit error BLM431 Computer Networks Dr.Refik Samet 104 Biphase 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 802.3 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 802.5 BLM431 Computer Networks Dr.Refik Samet 105 Manchester Encoding BLM431 Computer Networks Dr.Refik Samet 106 Differential Manchester Encoding BLM431 Computer Networks Dr.Refik Samet 107 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 BLM431 Computer Networks Dr.Refik Samet 108 Modulation Rate BLM431 Computer Networks Dr.Refik Samet 109 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 BLM431 Computer Networks Dr.Refik Samet 110 B8ZS Bipolar With 8 Zeros Substitution Based on bipolar-AMI If octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+ If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+Causes two violations of AMI code Unlikely to occur as a result of noise Receiver detects and interprets as octet of all zeros BLM431 Computer Networks Dr.Refik Samet 111 HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or two pulses BLM431 Computer Networks Dr.Refik Samet 112 B8ZS and HDB3 BLM431 Computer Networks Dr.Refik Samet 113 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) BLM431 Computer Networks Dr.Refik Samet 114 Modulation Techniques BLM431 Computer Networks Dr.Refik Samet 115 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 BLM431 Computer Networks Dr.Refik Samet 116 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 BLM431 Computer Networks Dr.Refik Samet 117 Multiple FSK More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents more than one bit BLM431 Computer Networks Dr.Refik Samet 118 FSK on Voice Grade Line BLM431 Computer Networks Dr.Refik Samet 119 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 BLM431 Computer Networks Dr.Refik Samet 120 Differential PSK BLM431 Computer Networks Dr.Refik Samet 121 Quadrature PSK More efficient use by each signal element representing more than one bit e.g. shifts of /2 (90o) Each element represents two bits Can use 8 phase angles and have more than one amplitude 9600bps modem use 12 angles , four of which have two amplitudes Offset QPSK (orthogonal QPSK) Delay in Q stream BLM431 Computer Networks Dr.Refik Samet 122 QPSK and OQPSK Modulators BLM431 Computer Networks Dr.Refik Samet 123 Examples of QPSF and OQPSK Waveforms BLM431 Computer Networks Dr.Refik Samet 124 Performance of Digital to Analog Modulation Schemes Bandwidth ASK and PSK bandwidth directly related to bit rate FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies (See Stallings for math) In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK BLM431 Computer Networks Dr.Refik Samet 125 Quadrature Amplitude Modulation QAM used on asymmetric digital subscriber line (ADSL) and some wireless Combination of ASK and PSK Logical extension of QPSK Send two different signals simultaneously on same carrier frequency Use two copies of carrier, one shifted 90° Each carrier is ASK modulated Two independent signals over same medium Demodulate and combine for original binary output BLM431 Computer Networks Dr.Refik Samet 126 QAM Modulator BLM431 Computer Networks Dr.Refik Samet 127 QAM Levels Two level ASK Each of two streams in one of two states Four state system Essentially QPSK Four level ASK Combined stream in one of 16 states 64 and 256 state systems have been implemented Improved data rate for given bandwidth Increased potential error rate BLM431 Computer Networks Dr.Refik Samet 128 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 BLM431 Computer Networks Dr.Refik Samet 129 Digitizing Analog Data BLM431 Computer Networks Dr.Refik Samet 130 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 BLM431 Computer Networks Dr.Refik Samet 131 Pulse Code Modulation(PCM) (2) 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 BLM431 Computer Networks Dr.Refik Samet 132 PCM Example BLM431 Computer Networks Dr.Refik Samet 133 PCM Block Diagram BLM431 Computer Networks Dr.Refik Samet 134 Nonlinear Encoding Quantization levels not evenly spaced Reduces overall signal distortion Can also be done by companding BLM431 Computer Networks Dr.Refik Samet 135 Effect of Non-Linear Coding BLM431 Computer Networks Dr.Refik Samet 136 Typical Companding Functions BLM431 Computer Networks Dr.Refik Samet 137 Delta Modulation Analog input is approximated by a staircase function Move up or down one level () at each sample interval Binary behavior Function moves up or down at each sample interval BLM431 Computer Networks Dr.Refik Samet 138 Delta Modulation - example BLM431 Computer Networks Dr.Refik Samet 139 Delta Modulation - Operation BLM431 Computer Networks Dr.Refik Samet 140 Delta Modulation - Performance Good voice reproduction PCM - 128 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 BLM431 Computer Networks Dr.Refik Samet 141 Analog Data, Analog Signals Why modulate analog signals? Higher frequency can give more efficient transmission Permits frequency division multiplexing (chapter 8) Types of modulation Amplitude Frequency Phase BLM431 Computer Networks Dr.Refik Samet 142 Analog Modulation BLM431 Computer Networks Dr.Refik Samet 143 Digital Data Communications Techniques BLM431 Computer Networks Dr.Refik Samet 144 Asynchronous and Synchronous Transmission Timing problems require a mechanism to synchronize the transmitter and receiver Two solutions Asynchronous Synchronous BLM431 Computer Networks Dr.Refik Samet 145 Asynchronous Data transmitted on character at a time 5 to 8 bits Timing only needs maintaining within each character Resynchronize with each character BLM431 Computer Networks Dr.Refik Samet 146 Asynchronous (diagram) BLM431 Computer Networks Dr.Refik Samet 147 Asynchronous - Behavior In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard) BLM431 Computer Networks Dr.Refik Samet 148 Synchronous - Bit Level Block of data transmitted without start or stop bits Clocks must be synchronized Can use separate clock line Good over short distances Subject to impairments Embed clock signal in data Manchester encoding Carrier frequency (analog) BLM431 Computer Networks Dr.Refik Samet 149 Synchronous - Block Level Need to indicate start and end of block Use preamble and postamble e.g. series of SYN (hex 16) characters e.g. block of 11111111 patterns ending in 11111110 More efficient (lower overhead) than async BLM431 Computer Networks Dr.Refik Samet 150 Synchronous (diagram) BLM431 Computer Networks Dr.Refik Samet 151 Types of Error An error occurs when a bit is altered between transmission and reception Single bit errors One bit altered Adjacent bits not affected White noise Burst errors Length B Contiguous sequence of B bits in which first last and any number of intermediate bits in error Impulse noise Fading in wireless Effect greater at higher data rates BLM431 Computer Networks Dr.Refik Samet 152 Error Detection Process BLM431 Computer Networks Dr.Refik Samet 153 Error Detection Additional bits added by transmitter for error detection code Parity Value of parity bit is such that character has even (even parity) or odd (odd parity) number of ones Even number of bit errors goes undetected BLM431 Computer Networks Dr.Refik Samet 154 Cyclic Redundancy Check For a block of k bits transmitter generates n bit sequence Transmit k+n bits which is exactly divisible by some number Receive divides frame by that number If no remainder, assume no error For math, see Stallings chapter 6 BLM431 Computer Networks Dr.Refik Samet 155 Error Correction Correction of detected errors usually requires data block to be retransmitted (see chapter 7) Not appropriate for wireless applications Bit error rate is high Lots of retransmissions Propagation delay can be long (satellite) compared with frame transmission time Would result in retransmission of frame in error plus many subsequent frames Need to correct errors on basis of bits received BLM431 Computer Networks Dr.Refik Samet 156 Error Correction Process Diagram BLM431 Computer Networks Dr.Refik Samet 157 Error Correction Process Each k bit block mapped to an n bit block (n>k) Codeword Forward error correction (FEC) encoder Codeword sent Received bit string similar to transmitted but may contain errors Received code word passed to FEC decoder If no errors, original data block output Some error patterns can be detected and corrected Some error patterns can be detected but not corrected Some (rare) error patterns are not detected Results in incorrect data output from FEC BLM431 Computer Networks Dr.Refik Samet 158 Working of Error Correction Add redundancy to transmitted message Can deduce original in face of certain level of error rate E.g. block error correction code In general, add (n – k ) bits to end of block Gives n bit block (codeword) All of original k bits included in codeword Some FEC map k bit input onto n bit codeword such that original k bits do not appear Again, for math, see chapter 6 BLM431 Computer Networks Dr.Refik Samet 159 Line Configuration Topology Physical arrangement of stations on medium Point to point Multi point Computer and terminals, local area network Half duplex Only one station may transmit at a time Requires one data path Full duplex Simultaneous transmission and reception between two stations Requires two data paths (or echo canceling) BLM431 Computer Networks Dr.Refik Samet 160 Traditional Configurations BLM431 Computer Networks Dr.Refik Samet 161 Interfacing Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities Need an interface called data circuit terminating equipment (DCE) e.g. modem, NIC DCE transmits bits on medium DCE communicates data and control info with DTE Done over interchange circuits Clear interface standards required BLM431 Computer Networks Dr.Refik Samet 162 Data Communications Interfacing BLM431 Computer Networks Dr.Refik Samet 163 Characteristics of Interface Mechanical Connection plugs Electrical Voltage, timing, encoding Functional Data, control, timing, grounding Procedural Sequence of events BLM431 Computer Networks Dr.Refik Samet 164 V.24/EIA-232-F ITU-T v.24 Only specifies functional and procedural References other standards for electrical and mechanical EIA-232-F (USA) RS-232 Mechanical ISO 2110 Electrical v.28 Functional v.24 Procedural v.24 BLM431 Computer Networks Dr.Refik Samet 165 Mechanical Specification BLM431 Computer Networks Dr.Refik Samet 166 Electrical Specification Digital signals Values interpreted as data or control, depending on circuit More than -3v is binary 1, more than +3v is binary 0 (NRZ-L) Signal rate < 20kbps Distance <15m For control, more than-3v is off, +3v is on BLM431 Computer Networks Dr.Refik Samet 167 Functional Specification Circuits grouped in categories Data Control Timing Ground One circuit in each direction Full duplex Two secondary data circuits Allow halt or flow control in half duplex operation (See table in Stallings chapter 6) BLM431 Computer Networks Dr.Refik Samet 168 Local and Remote Loopback BLM431 Computer Networks Dr.Refik Samet 169 Procedural Specification E.g. Asynchronous private line modem When turned on and ready, modem (DCE) asserts DCE ready When DTE ready to send data, it asserts Request to Send Also inhibits receive mode in half duplex Modem responds when ready by asserting Clear to send DTE sends data When data arrives, local modem asserts Receive Line Signal Detector and delivers data BLM431 Computer Networks Dr.Refik Samet 170 Dial Up Operation (1) BLM431 Computer Networks Dr.Refik Samet 171 Dial Up Operation (2) BLM431 Computer Networks Dr.Refik Samet 172 Dial Up Operation (3) BLM431 Computer Networks Dr.Refik Samet 173 Null Modem BLM431 Computer Networks Dr.Refik Samet 174 ISDN Physical Interface Diagram BLM431 Computer Networks Dr.Refik Samet 175 ISDN Physical Interface Connection between terminal equipment (c.f. DTE) and network terminating equipment (c.f. DCE) ISO 8877 Cables terminate in matching connectors with 8 contacts Transmit/receive carry both data and control BLM431 Computer Networks Dr.Refik Samet 176 ISDN Electrical Specification Balanced transmission Carried on two lines, e.g. twisted pair Signals as currents down one conductor and up the other Differential signaling Value depends on direction of voltage Tolerates more noise and generates less (Unbalanced, e.g. RS-232 uses single signal line and ground) Data encoding depends on data rate Basic rate 192kbps uses pseudoternary Primary rate uses alternative mark inversion (AMI) and B8ZS or HDB3 BLM431 Computer Networks Dr.Refik Samet 177 Multiplexing BLM431 Computer Networks Dr.Refik Samet 178 Multiplexing BLM431 Computer Networks Dr.Refik Samet 179 Frequency Division Multiplexing FDM Useful bandwidth of medium exceeds required bandwidth of channel Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) e.g. broadcast radio Channel allocated even if no data BLM431 Computer Networks Dr.Refik Samet 180 Frequency Division Multiplexing Diagram BLM431 Computer Networks Dr.Refik Samet 181 FDM System BLM431 Computer Networks Dr.Refik Samet 182 FDM of Three Voiceband Signals BLM431 Computer Networks Dr.Refik Samet 183 Analog Carrier Systems AT&T (USA) Hierarchy of FDM schemes Group 12 voice channels (4kHz each) = 48kHz Range 60kHz to 108kHz Supergroup 60 channel FDM of 5 group signals on carriers between 420kHz and 612 kHz Mastergroup 10 supergroups BLM431 Computer Networks Dr.Refik Samet 184 Wavelength Division Multiplexing Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel 1997 Bell Labs 100 beams Each at 10 Gbps Giving 1 terabit per second (Tbps) Commercial systems of 160 channels of 10 Gbps now available Lab systems (Alcatel) 256 channels at 39.8 Gbps each 10.1 Tbps Over 100km BLM431 Computer Networks Dr.Refik Samet 185 WDM Operation Same general architecture as other FDM Number of sources generating laser beams at different frequencies Multiplexer consolidates sources for transmission over single fiber Optical amplifiers amplify all wavelengths Typically tens of km apart Demux separates channels at the destination Mostly 1550nm wavelength range Was 200MHz per channel Now 50GHz BLM431 Computer Networks Dr.Refik Samet 186 Dense Wavelength Division Multiplexing DWDM No official or standard definition Implies more channels more closely spaced that WDM 200GHz or less BLM431 Computer Networks Dr.Refik Samet 187 Synchronous Time Division Multiplexing Data rate of medium exceeds data rate of digital signal to be transmitted Multiple digital signals interleaved in time May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources BLM431 Computer Networks Dr.Refik Samet 188 Time Division Multiplexing BLM431 Computer Networks Dr.Refik Samet 189 TDM System BLM431 Computer Networks Dr.Refik Samet 190 TDM Link Control No headers and trailers Data link control protocols not needed Flow control Data rate of multiplexed line is fixed If one channel receiver can not receive data, the others must carry on The corresponding source must be quenched This leaves empty slots Error control Errors are detected and handled by individual channel systems BLM431 Computer Networks Dr.Refik Samet 191 Data Link Control on TDM BLM431 Computer Networks Dr.Refik Samet 192 Framing No flag or SYNC characters bracketing TDM frames Must provide synchronizing mechanism Added digit framing One control bit added to each TDM frame Looks like another channel - “control channel” Identifiable bit pattern used on control channel e.g. alternating 01010101…unlikely on a data channel Can compare incoming bit patterns on each channel with sync pattern BLM431 Computer Networks Dr.Refik Samet 193 Pulse Stuffing Problem - Synchronizing data sources Clocks in different sources drifting Data rates from different sources not related by simple rational number Solution - Pulse Stuffing Outgoing data rate (excluding framing bits) higher than sum of incoming rates Stuff extra dummy bits or pulses into each incoming signal until it matches local clock Stuffed pulses inserted at fixed locations in frame and removed at demultiplexer BLM431 Computer Networks Dr.Refik Samet 194 TDM of Analog and Digital Sources BLM431 Computer Networks Dr.Refik Samet 195 Digital Carrier Systems Hierarchy of TDM USA/Canada/Japan use one system ITU-T use a similar (but different) system US system based on DS-1 format Multiplexes 24 channels Each frame has 8 bits per channel plus one framing bit 193 bits per frame BLM431 Computer Networks Dr.Refik Samet 196 Digital Carrier Systems (2) For voice each channel contains one word of digitized data (PCM, 8000 samples per sec) Data rate 8000x193 = 1.544Mbps Five out of six frames have 8 bit PCM samples Sixth frame is 7 bit PCM word plus signaling bit Signaling bits form stream for each channel containing control and routing info Same format for digital data 23 channels of data 7 bits per frame plus indicator bit for data or systems control 24th channel is sync BLM431 Computer Networks Dr.Refik Samet 197 Mixed Data DS-1 can carry mixed voice and data signals 24 channels used No sync byte Can also interleave DS-1 channels Ds-2 is four DS-1 giving 6.312Mbps BLM431 Computer Networks Dr.Refik Samet 198 DS-1 Transmission Format BLM431 Computer Networks Dr.Refik Samet 199 SONET/SDH Synchronous Optical Network (ANSI) Synchronous Digital Hierarchy (ITU-T) Compatible Signal Hierarchy Synchronous Transport Signal level 1 (STS-1) or Optical Carrier level 1 (OC-1) 51.84Mbps Carry DS-3 or group of lower rate signals (DS1 DS1C DS2) plus ITU-T rates (e.g. 2.048Mbps) Multiple STS-1 combined into STS-N signal ITU-T lowest rate is 155.52Mbps (STM-1) BLM431 Computer Networks Dr.Refik Samet 200 SONET Frame Format BLM431 Computer Networks Dr.Refik Samet 201 SONET STS-1 Overhead Octets BLM431 Computer Networks Dr.Refik Samet 202 Statistical TDM In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines and collects data until frame full Data rate on line lower than aggregate rates of input lines BLM431 Computer Networks Dr.Refik Samet 203 Statistical TDM Frame Formats BLM431 Computer Networks Dr.Refik Samet 204 Performance Output data rate less than aggregate input rates May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay BLM431 Computer Networks Dr.Refik Samet 205 Buffer Size and Delay BLM431 Computer Networks Dr.Refik Samet 206 Cable Modem Outline Two channels from cable TV provider dedicated to data transfer One in each direction Each channel shared by number of subscribers Scheme needed to allocate capacity Statistical TDM BLM431 Computer Networks Dr.Refik Samet 207 Cable Modem Operation Downstream Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity May get 500kbps to 1.5Mbps Also used to allocate upstream time slots to subscribers Upstream User requests timeslots on shared upstream channel Dedicated slots for this Headend scheduler sends back assignment of future tme slots to subscriber BLM431 Computer Networks Dr.Refik Samet 208 Cable Modem Scheme BLM431 Computer Networks Dr.Refik Samet 209 Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network Local loop Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more BLM431 Computer Networks Dr.Refik Samet 210 ADSL Design Asymmetric Greater capacity downstream than upstream Frequency division multiplexing Lowest 25kHz for voice Plain old telephone service (POTS) Use echo cancellation or FDM to give two bands Use FDM within bands Range 5.5km BLM431 Computer Networks Dr.Refik Samet 211 ADSL Channel Configuration BLM431 Computer Networks Dr.Refik Samet 212 Discrete Multitone DMT Multiple carrier signals at different frequencies Some bits on each channel 4kHz subchannels Send test signal and use subchannels with better signal to noise ratio 256 downstream subchannels at 4kHz (60kbps) 15.36MHz Impairments bring this down to 1.5Mbps to 9Mbps BLM431 Computer Networks Dr.Refik Samet 213 DTM Bits Per Channel Allocation BLM431 Computer Networks Dr.Refik Samet 214 DMT Transmitter BLM431 Computer Networks Dr.Refik Samet 215 xDSL High data rate DSL Single line DSL Very high data rate DSL BLM431 Computer Networks Dr.Refik Samet 216