Physical Layer A 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 Summary TOC Physical Link Functions Signal Adaptor Adaptor Adaptor Adaptor Adaptor convert bits into physical signal and physical signal back into bits 1 2 3 4 5 6 Functions Construct Frame with Error Detection Code Encode bit sequence into analog signal Transmit bit sequence on a physical medium Modulation Receive analog signal Convert Analog Signal to Bit Sequence Recover errors through error correction and or ARQ TOC Physical Link Functions Link Components NRZI TOC Physical Link Components Link 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 Wireless TOC Physical Link Properties Example Optical Links TOC Physical Optical Link rate and Distance Links become slower with distance because of attenuation of the signal Amplifiers and repeaters can help TOC Physical Rate Distance Noise A signal s t sent over a link is generally Distorted by the physical nature of the medium Affected by random physical effects Shot noise Fading Multipath Effects Also interference from other links This distortion may be known and reversible at the receiver Wireless Crosstalk Dealing with noise is what communications engineers do TOC Physical Noise Noise 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 Rate Bandwidth 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 Rate Fundamental 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 Capacity The Frequency Spectrum is crowded TOC Physical Spectrum Sampling 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 result TOC Physical Sampling Sampling 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 Higher rate for the same SNR TOC Physical Sampling Encoding Goal Assumptions NRZ NRZI Manchester 4b 5b TOC Physical Encoding Goals 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 efficient encoding scheme to achieve this goal TOC Physical Encoding Goals Assumptions 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 Assumptions Non Return to Zero NRZ 1 high signal 0 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 0 0 NRZ non return to zero Clock TOC Physical Encoding NRZ 1 0 1 0 1 1 0 Non Return to Zero Inverted NRZI 1 make transition 0 stay at the same level Solve previous problems for long sequences of 1 s but not for 0 s 0 0 1 NRZI non return to zero intverted Clock TOC Physical Encoding NRZI 0 1 0 1 1 0 Manchester 1 high to low transition 0 low to high transition Addresses clock recovery and baseline wander problems Disadvantage needs a clock that is twice as fast as the transmission rate Efficiency of 50 0 0 1 0 Manchester Clock TOC Physical Encoding Manchester 1 0 1 1 0 4 bit 5 bit 100Mb s Ethernet Goal address inefficiency of Manchester encoding while avoiding long periods of low signals Solution Use 5 bits to encode every sequence of four bits such that no 5 bit code has more than one leading 0 and two trailing 0 s Use NRZI to encode the 5 bit codes Efficiency is 80 4 bit 5 bit 4 bit 5 bit 0000 0001 0010 0011 0100 0101 0110 0111 11110 01001 10100 10101 01010 01011 01110 01111 1000 1001 1010 1011 1100 1101 1110 1111 10010 10011 10110 10111 11010 11011 11100 11101 TOC Physical Encoding 4b 5b Modulation The function of transmitting the encoded signal over a link often by combining it with another carrier signal E g Frequency Modulation FM Combine the signal with a carrier signal in such a way that the instantaneous frequency of the received signal contains the information of the carrier E g Frequency Hopping OFDM Signal transmitted over multiple frequencies Sequence
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