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CS3502, Data and Computer Networks: the physical layer-1 physical layer - purpose To transmit bits, by encoding them onto signals; and to receive the signals, interpreting them as bits input: sequence of bit S, from DL or MAC layer output: sequence of bit S’, to DL or MAC layer ideally, S = S’ Physical layer definitions signal 1. a mechanism used to carry information over time OR distance 2. a sign or gesture giving information 3. a sequence of electrical impulses or waves 4. electromagnetic encoding of data Signals examples: physical gesture, wave, hand signal flashes of light (eg, Morse code) sound: vary tone, loudness or duration flags smoke mirrors electrical voltages transmission definitions 1. the action of conveying electrical or optical signals from 1 point to 1 or more other points in space 2. the process of sending information from 1 point to another functions path necessary for transmission systems for signal transfer (medium) transform signal to appropriate form (code) launch the signal (transmit) remove, receive or detect the signal (receive) transmission properties functions present in some transmission systems (often helpful) compensate for loss in media compensate for distortion introduced by media control access to the media (ie, protect from interference) monitor and control quality of transmission signal - can be modeled as f(t) t --> f(t) ... f(t) represents some physical quantity: voltage, amplitude, frequency, etc. signals functions/signals may be either --- continous no breaks in the graph or discrete takes only a finite or countable number of values Q: can a function be both? Q: must a function be one or the other? examples of functions f(t) = 1 f(t) = c f(t) = Roof(t) f(t) = Floor(t) f(t) = t - Floor(t) f(t) = sin(t) you should be able to graph all of these digital/ analog signals digital signal 1. assumed to take on finite number of values, AND 2. has meaning only at discrete points in time. digital signals represented by discrete functions. (however this is an idealized and somewhat unrealistic picture). digital/analog signals analogy: a partial likeness between 2 things that are compared (Oxford Dict.) analog signal: 1. a signal that is an analog of the quantity being represented; eg, signal voltage proportional to volume of sound 2. continuous range of values 3. continuous write time; always valued. digital/analog signals digital data: text, bits; discrete valued. analog data: sound, vision; music, etc. continuous valued. Note: digital (analog) signals can transport both digital and analog data, so 4 combinations (DD,DA,AD,AA) possible some media only propagate analog signals efficiently, and sometimes more efficient to digitize analog data digital/ analog signals periodic function -- cyclical in values (note mathematical definition) 3 key properties of periodic signals: amplitude: instantaneous value frequency: cycles per second (hertz) phase: these position within a cycle/period quantities are varied, in order to use the signal to carry information digital/ analog signals key fact: any signal can be represented as a sum (possibly infinite) of periodic functions. (Fourier analysis mathematical picture) f(t) = (1/2)·k0 +n=1.inf an·sin(2··n·f·t) + n=1.inf bn·cos(2··n·f·t) t=0…Tf(t) ·sin(2··n·f·t) ·dt bn = 2/T t=0…Tf(t) ·cos(2··n·f·t) ·dt k0 = 2/T t=0…Tf(t) ·dt ; the average amplitude an = 2/T digital/ analog signals (Fourier analysis graphical picture) .5·sin(2··7·f·t) Tuned to 7·f 1·sin(2··6·f·t) Tuned to 6·f 5·sin(2··5·f·t) Tuned to 5·f 1·sin(2··4·f·t) Tuned to 4·f 2·sin(2··3·f·t) Tuned to 3·f f(t) 4·sin(2··2·f·t) Tuned to 2·f 8·sin(2··1·f·t) Tuned to 1·f a1=8 transmission media transmission medium: the physical element through which signals must pass, from transmitter to receiver examples: air, water, (outer) space, copper wires, optical fiber two main categories: guided and unguided propagation delays of signals in media transmission terminology direct link simplex half-duplex full duplex spectrum a signal - range of frequencies making up bandwidth frequencies examples - width of the spectrum; range of transmission terminology note 1: bandwidth key factor in determining data rate; note 2: however do not confuse bandwidth (hertz) and and data rate (bps) attenuation amplifier boosts energy of analog signal; increases amplitude makes no distinction between noise and signal repeater receives, interprets and repeats a digital signal adds little or no noise/distortion transmission terminology repeater-amplifier diagram comparison modems, codecs, bauds, bits modem (modulator-demodulator) translates a digital signal (bit) into an analog signal, for transmission as an analog signal; receives the corresponding analog signal, and translates back into digital (bit) purpose: use analog medium for digital data/signals example: PC modem, phone lines; TV cable modems techniques: PSK, ASK, FSK and combinations. modems, codecs, bauds, bits codec (codec/decoder) converts analog data into digital form (bits), and the reverse. two main techniques: PCM, DM PCM (pulse code modulation) absolute values, based on sampling theorem; (nearly) total information DM (delta modulation) based on differences; less accurate modems, codecs, bauds, bits Baud rate -maximum number of times per second that the signal element can change Baud - The unit in which the Baud rate is measured incorrect to say “9800 bauds per second.” thus, the baud rate is the rate at which distinct signal elements are sent. also called “signaling rate” applies to digital signals or analog signals carrying digital data. modems, codecs, bauds, bits diagram: bauds and bits Bit rate = 3 baud rate·log2(#of signal levels) Example: Signal 2 levels 1 0 T = 1/Baud Rate measured in baud A 9600 baud modem has a baud rate of 9600 baud. If it uses two signal levels is also runs at 9600 bps.