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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