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Berkeley ELENG 122 - Physical Layer

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Physical LayerA digital communication linkLink FunctionsLink ComponentsLink PropertiesExample: Optical LinksLink rate and DistanceNoiseNoise limits the link rateBandwidth affects the data rateFundamental Result: CapacityThe Frequency Spectrum is crowded…Sampling Result (Nyquist)Sampling ContinuedEncodingGoalsAssumptionsNon-Return to Zero (NRZ)Non-Return to Zero Inverted (NRZI)Manchester4-bit/5-bit (100Mb/s Ethernet)ModulationFramingGoalsByte-Oriented Protocols: Sentinel ApproachByte-Oriented Protocols: Byte Counting ApproachBit-Oriented ProtocolsClock-Based Framing (SONET)DescriptionSONET MultiplexingSTS-1 FrameDetailsSummaryPhysical LayerA digital communication link Link Functions Link Components Link Properties Example: Optical Links Link Rate and Distance Noise  Noise Limits the Link Rate  Bandwidth Affects Data Rate Fundamental Result: Capacity Spectrum Sampling: Converting Analog Signals into Bits Encoding Modulation Framing SummaryTOC –PhysicalLink FunctionsAdaptorAdaptorAdaptorAdaptorSignalAdaptor: convert bits into physical signal and physical signal back into bits Functions1. Construct Frame with Error Detection Code2. Encode bit sequence into analog signal3. Transmit bit sequence on a physical medium (Modulation)4. Receive analog signal5. Convert Analog Signal to Bit Sequence6. Recover errors through error correction and/or ARQ TOC – Physical – Link FunctionsLink ComponentsNRZITOC – Physical – Link ComponentsLink Properties Function Duplex/Half Duplex One stream, multiple streams Characteristics Bit Error Rate Data Rate (this sometimes mistakenly called bandwidth!) Degradation with distance Cables and Fibers CAT 5 twisted pair: 10-100Mbps, 100m Coax: 10-100Mbps, 200-500m Multimode Fiber: 100Mbps, 2km Single Mode Fiber: 100-2400Mbps, 40km WirelessTOC – Physical – Link PropertiesExample: Optical LinksTOC – Physical –OpticalLink rate and Distance Links become slower with distance because of attenuation of the signalAmplifiers and repeaters can help TOC – Physical – Rate/DistanceNoise A signal s(t) sent over a link is generally Distorted by the physical nature of the medium This distortion may be known and reversible at the receiver Affected by random physical effects Shot noise Fading Multipath Effects Also interference from other links Wireless Crosstalk Dealing with noise is what communications engineers doTOC – Physical –NoiseNoise limits the link rate  Suppose there were no noise E.g., assume that if send s(t) = V then receive aV after T seconds Take a message of N bits say b1b2….bN, and send a pulse of amplitude of size 0.b1b2….bN Can send at an arbitrarily high rate This is true even if the link distorts the signal but in a known way In practice the signal always gets distorted in an unpredictable(random) way Receiver tries to estimate the effects but this lowers the effective rate One way to mitigate noise is to jack up the power of the signal Signal to Noise ratio (SNR) measures the extent of the distortion effects TOC – Physical – Noise Limits RateBandwidth affects the data rate There is usually a fixed range of frequencies at which the analog wave can traverse a link The physical characteristics of the link might govern this Example: Voice Grade Telephone line 300Hz – 3300Hz The bandwidth is 3000Hz For the same SNR, a higher bandwidth gives a higher rate TOC – Physical – Bandwidth Affects RateFundamental Result: Capacity The affect of noise on the data is modeled probabilistically.  It turns out that there is a maximum possible reliable rate for most channels called the capacity C: There is a scheme to transmit at C with almost no errors Finding this scheme is tricky but it exists For a commonly observed kind of noise called Additive White Gaussian Noise (AWGN) the capacity is given by: C = Wlog2(1 + S/N) bits/sec (Shannon) Example: Voice grade line: S/N = 1000, W=3000, C=30Kbps Technology has improved S/N and W to yield higher speeds such as 56Kb/s or even more than 1Mbps (DSL)TOC – Physical – CapacityThe Frequency Spectrum is crowded…TOC – Physical –SpectrumSampling Result (Nyquist) Suppose a signal s(t) has a bandwidth B. Sampling Result: Suppose we sample it (accurately) every T seconds. If T≤ 1/2B then it is possible to reconstruct the s(t) correctly Only one signal with bandwidth B has these sample points There are multiple signals with these sample points for signals with bandwidth greater than B Increasing the bandwidth results in a richer signal space  No noise allowed in the sampling resultTOC – Physical – SamplingSampling Continued But now assume noise that is distributed uniformly over the frequency band.  Then the richer signal space will enable more information to be transmitted in the same amount of time. Higher bandwidth Æ Higher rate (for the same SNR)TOC – Physical – SamplingEncoding Goal Assumptions NRZ NRZI Manchester 4b/5bTOC – Physical – EncodingGoals Objective: send bits from one node to another node on the same physical media This service is provided by the physical layer Problem: specify a robust and efficientencoding scheme to achieve this goalTOC – Physical – Encoding –GoalsAssumptions We use two discrete signals, high and low, to encode 0 and 1 The transmission is synchronous, i.e., there is a clock used to sample the signal In general, the duration of one bit is equal to one or two clock ticks If the amplitude and duration of the signals is large enough, the receiver can do a reasonable job of looking at the distorted signal and estimating what was sent. TOC – Physical – Encoding – AssumptionsNon-Return to Zero (NRZ) 1 Æ high signal; 0 Æ low signal Disadvantages: when there is a long sequence of 1’s or 0’s Sensitive to clock skew, i.e., difficult to do clock recovery  Difficult to interpret 0’s and 1’s (baseline wander)001010 110NRZ(non-return to zero)ClockTOC – Physical – Encoding – NRZNon-Return to Zero Inverted (NRZI) 1 Æ make transition; 0 Æ stay at the same level Solve previous problems for long sequences of 1’s, but not for 0’s001010 110ClockNRZI(non-return to zero intverted)TOC – Physical – Encoding – NRZIManchester 1 Æ high-to-low transition;


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Berkeley ELENG 122 - Physical Layer

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