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Lectures 8 -10 2019

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Lectures 8 -10 2019 Digital Transmission Digital Transmission Media Error Detection Digital Transmission Transmitted Received Transmitter Signal Signal Receiver Communication channel Transmitter Key questions l Accepts bits from data link l How many bits per second can layer be transmitted reliably across the l Converts bits into digital channel? modulated signal l How much power and Receiver bandwidth is consumed in the l Recovers bits from received process? signal l How do we control error rate in l Transfers bits to its data link delivered bits? layer Transmission Impairments Transmitted Received Transmitter Signal Signal Receiver Communication channel Communication Channels Transmission Impairments l Pair of copper wires l Signal attenuation l Coaxial cable l Signal distortion l Radio l Spurious noise l Light in optical fiber l Interference from other l Light in air signals l Infrared Digital Binary Signal 1 0 1 1 0 1 +A 0 T 2T 3T 4T 5T 6T -A Bit rate = 1 bit / T seconds For a given communications medium: l How do we increase transmission speed? l How do we achieve reliable communications? l Are there limits to speed and reliability? Pulse Transmission Rate l Objective: Maximize pulse rate through a channel, that is, make T as small as possible ??? Channel T t t l If input is a narrow pulse, then typical output is a spread-out pulse with ringing l Question: How frequently can these pulses be transmitted without interfering with each other? l Answer: 2 x Wc pulses/second where Wc is the bandwidth of the channel Multilevel Pulse Transmission l If channel bandwidth Wc, we can transmit 2Wc pulses/sec (without interference) l If pulses amplitudes are either -A or +A, then each pulse conveys 1 bit, so Bit Rate = 1 bit/pulse x 2Wc pulses/sec = 2Wc bps l If amplitudes are from {-A, -A/3, +A/3, +A}, then bit rate is 2 x 2Wc bps l By going to M = 2m amplitude levels, we achieve Bit Rate = m bits/pulse x 2Wc pulses/sec = 2mWc bps In the absence of noise, the bit rate can be increased without limit by increasing m Noise & Reliable Communications l All physical systems have noise l Electrons always vibrate randomly at non-zero temperature l Motion of electrons induces noise l Presence of noise limits accuracy of measurement of received signal amplitude l Errors occur if signal separation is comparable to noise level l Bit Error Rate (BER) increases with decreasing signal-to-noise ratio l Noise places a limit on how many amplitude levels can be used in pulse transmission Signal-to-Noise Ratio Signal Noise Signal + noise High SNR t t t No errors Signal Noise Signal + noise Low SNR t t t Average signal power error SNR = Average noise power SNR (dB) = 10 log10 SNR Shannon Channel Capacity C = Wc log2 (1 + SNR) bps l Arbitrarily reliable communications is possible if the transmission rate R < C. l If R > C, then arbitrarily reliable communications is not possible. l Arbitrarily reliable means the BER can be made arbitrarily small through sufficiently complex coding. l C can be used as a measure of how close a system design is to the best achievable performance. l Bandwidth Wc & SNR determine C Example l Find the Shannon channel capacity for a telephone channel with Wc = 3400 Hz and SNR = 10000 C = 3400 log2 (1 + 10000) = 3400 log10 (10001)/log102 = 45200 bps Note that SNR = 10000 corresponds to SNR (dB) = 10 log10(10000) = 40 dB Examples of Channels Channel Bandwidth Bit Rates Telephone voice 3-4 kHz 33 kbps channel Copper pair 1 MHz 1-6 Mbps (~1 mile) Coaxial cable 500 MHz 38 Mbps/ channel (6 MHz channels) 5 GHz radio 300 MHz 54 Mbps / channel (IEEE 802.11) (11 channels) Optical fiber Many TeraHertz 40-100 Gbps / wavelength Bit Rates of Digital Transmission Systems System Bit Rate Observations Telephone 33.6-56 kbps 4 kHz telephone channel copper pair Ethernet twisted 10 Mbps, 100 Mbps 100 meters of unshielded twisted pair copper wire pair Cable modem .5Mbps-38-343Mbps Shared CATV return channel; 6Mz TV channel; 8 combined channels ADSL twisted 64-640 kbps in, 1.536- Coexists with analog telephone pair 6.144 Mbps out signal FTTC+copper 100Mbps, 1 Gbps Fiber to the curb 2.4 GHz radio 2-11-54 Mbps IEEE 802.11 wireless LAN 28 GHz radio Multi-Gbps mmwave radio in 5G Optical fiber 10-40-100-200-400 Gbps Ethernet Optical fiber >400Gbps, >petabit/s 1 wavelength; multiple wavelengths Telephone Local Loop Local Loop: Last Mile l Copper pair from telephone to CO l Pedestal to SAI to Main Distribution Frame (MDF) l 2700 cable pairs in a feeder cable l MDF connects Pedestal l voice signal to telephone switch l DSL signal to routers Serving area Local telephone office interface Distribution cable Switch Serving Distribution frame area Feeder interface cable lFor interesting pictures of switches & MDF, see lweb.mit.edu/is/is/delivery/5ess/photos.html www.museumofcommunications.org/coe.html Twisted Pair Twisted pair l Two insulated copper 30 26 gauge wires in regular spiral 24 gauge pattern to minimize interference 24 22 gauge l Various thicknesses, e.g. 0.016 inch (24 gauge) 18 l Low cost 19 gauge l Attenuation increases with distance 12 l Telephone loop from customer to CO Attenuation (dB/mi) 6 l Intra-building wiring closet to desktop f (kHz) 1 10 100 1000 Lower Higher attenuation rate attenuation rate analog telephone for DSL Twisted Pair Bit Rates l Twisted pairs can provide Table 3.5 Data rates of 24-gauge twisted pair high bit rates at short distances Standard Data Rate Distance l Asymmetric Digital Subscriber Loop (ADSL) T-1 1.544 Mbps 18,000 feet, 5.5 km l Internet Access l Lower 3 kHz for voice DS2 6.312 Mbps 12,000 feet, 3.7 km l Upper band for data 1/4 STS-1 12.960 4500 feet, 1.4 km l 64 kbps inbound Mbps l 640 kbps outbound 1/2 STS-1 25.920 3000 feet, 0.9 km l Much higher rates possible at Mbps shorter distances l Strategy for telephone STS-1 51.840 1000 feet, 300 m companies is to bring fiber Mbps close to home & then twisted pair l Higher-speed access + video Fiber to the x Node Curb Building Home Ethernet LANs In office or home networks l Cat 3 Unshielded twisted pair (UTP): ordinary telephone wires l l l l l l l Cat 5 UTP: tighter twisting to improve signal quality l Shielded twisted pair (STP): costly l10BASE-T Ethernet l 10 Mbps, Twisted pair l Two Cat3 pairs,100 meters l100BASE-T4 Fast Ethernet l 100 Mbps, Twisted pair l Four Cat3 pairs, 100 meters l Cat5 & STP provide other options l1000BASE-T GigE l Four Cat5 pairs, 100 meters lMany optical fiber options l 500m – many km Coaxial Cable Twisted pair l Cylindrical braided outer 35 conductor surrounds 0.7/2.9 mm insulated inner wire 30 conductor l High interference immunity 25 1.2/4.4 mm l Higher bandwidth than twisted pair 20 l Hundreds of MHz 15 l Cable TV distribution l Original Ethernet LAN 10 medium (dB/km) Attenuation 2.6/9.5 mm 5 0.1 1.0 10 100 f (MHz) Cable Modem & TV Spectrum Downstream Downstream Upstream 42 MHz 500 MHz 750 MHz 550 MHz 5 MHz 54 MHz l Cable TV network originally unidirectional l Cable plant upgraded to bidirectional l 1 analog TV channel is 6 MHz, can support very high data rates l Cable Modem: shared upstream & downstream l 5-42 MHz upstream into network; 2 MHz channels; 500 kbps to 4 Mbps l >550 MHz downstream from network; 6 MHz channels; 36 Mbps Cable Network Topology Upstream fiber Fiber Fiber Head Fiber Fiber end node node Downstream fiber Coaxial distribution plant = Bidirectional split-band amplifier Optical Fiber Properties Advantages Disadvantages l Very low attenuation l New types of optical signal impairments & dispersion l Noise immunity l Polarization dependence l Extremely high l Wavelength dependence bandwidth l Limited bend radius l Security: Very difficult to l If physical arc of cable too tap without breaking high, light lost or wont l No corrosion reflect l Will break l More compact & lighter than copper wire l Difficult to splice l Mechanical vibration becomes signal noise Very Low Attenuation 100 Water Vapor Absorption 50 (removed in new fiber designs) 10 5 1 Infrared absorption 0.5 Rayleigh scattering Loss (dB/km) Loss 0.1 0.05 0.01 0.8 1.0 1.2 1.4 1.6 1.8 Wavelength (µm) 850 nm 1300 nm Low-cost LEDs Metropolitan Area 1550 nm LANs Networks Long Distance Networks Short Haul Long Haul Huge Available Bandwidth l Optical range from λ1 to 100 λ1 +Δλ contains Bandwidth 50 v v B = f1 – f2 = – 10 λ λ1 + Δλ 1 5 v Δλ / λ1 v Δλ = ≈ 2 λ1 1 + Δλ / λ1 λ1 1 0.5 Loss (dB/km) Loss 0.1 l Example: λ1 = 1450 nm λ1 +Δλ =1650 nm: 2(108)m/s 200nm B = ≈ 19 THz 0.8 1.0 1.2 1.4 1.6 1.8 (1450 nm)2 Fiber to the Home Fiber to the Home Evolution From CO to Multiprovider Point of Presence Cellular Communications Two basic concepts: l Frequency Reuse l A region is partitioned into cells l Each cell is covered by base station l Power transmission levels controlled to minimize inter-cell interference l Spectrum can be reused in other cells l Handoff l Procedures to ensure continuity of call as user moves from cell to another l Involves setting up call in new cell and tearing down old one Cellular Network Base station l Transmits to users on forward channels l Receives from users on reverse channels BSS BSS Mobile Switching MSC Center SS7 HLR STP l Controls connection VLR Wireline EIR terminal setup within cells & AC PSTN to telephone network AC = authentication center MSC = mobile switching center BSS = base station subsystem PSTN = public switched telephone network EIR = equipment identity register STP = signal transfer point HLR = home location register VLR = visitor location register "Gold" was 5, 20, 95, 0; changed to current 2nd column, 1st para, 5th line: interna-tionaux imageable 2nd column, 1st para, 10th line: move l'at- to the next line so that there is no hyphenation.
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