CS3502, Data and Computer Networks: the physical layer-1

physical layer - purpose u 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 u 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 u examples: u physical gesture, wave, hand signal u flashes of light (eg, Morse code) u sound: vary tone, loudness or duration u flags u smoke u mirrors u 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 u functions necessary for transmission systems u path for signal transfer (medium) u transform signal to appropriate form (code) u launch the signal (transmit) u remove, receive or detect the signal (receive)

transmission properties u functions present in some transmission systems (often helpful) u compensate for loss in media u compensate for distortion introduced by media u control access to the media (ie, protect from interference) u 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 u functions/signals may be either --- u continous u no breaks in the graph or u discrete u 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) u you should be able to graph all of these

digital/ analog signals u digital signal 1. assumed to take on finite number of values, AND 2. has meaning only at discrete points in time. u digital signals represented by discrete functions. (however this is an idealized and somewhat unrealistic picture).

digital/analog signals u analogy: a partial likeness between 2 things that are compared (Oxford Dict.) u 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 u digital data : text, bits; discrete valued. u 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 u some media only propagate analog signals efficiently, and sometimes more efficient to digitize analog data

digital/ analog signals u periodic function -- cyclical in values (note mathematical definition) u 3 key properties of periodic signals: u amplitude : instantaneous value u frequency : cycles per second ( hertz ) u phase : position within a cycle/period u these quantities are varied, in order to use the signal to carry information

digital/ analog signals u key fact: any signal can be represented as a sum (possibly infinite) of periodic functions. ( Fourier analysis mathematical picture )  f(t) = (1/2)·k 0 +  n=1 .inf a n · sin(2 ·  · n · f · t) +  n=1 .inf b n · cos(2 ·  · n · f · t) u a n = 2/T  t=0…T f(t) ·sin(2·  ·n·f·t) ·dt u b n = 2/T  t=0…T f(t) ·cos(2·  ·n·f·t) ·dt u k 0 = 2/T  t=0…T f(t) ·dt ; the average amplitude

digital/ analog signals u ( Fourier analysis graphical picture ).5·sin(2·  ·7·f·t) 1·sin(2·  ·6·f·t) 5·sin(2·  ·5·f·t) 1·sin(2·  ·4·f·t) 2·sin(2·  ·3·f·t) 4·sin(2·  ·2·f·t) 8·sin(2·  ·1·f·t) Tuned to 7·f Tuned to 6·f Tuned to 5·f Tuned to 4·f Tuned to 3·f Tuned to 2·f Tuned to 1·f f(t) a 1 =8

transmission media u transmission medium : the physical element through which signals must pass, from transmitter to receiver u examples: air, water, (outer) space, copper wires, optical fiber u two main categories: guided and unguided u propagation delays of signals in media

transmission terminology u direct link u simplex u half-duplex u full duplex u spectrum - range of frequencies making up a signal u bandwidth - width of the spectrum; range of frequencies u examples

transmission terminology u note 1: bandwidth key factor in determining data rate; u note 2: however do not confuse bandwidth (hertz) and and data rate (bps) u attenuation u amplifier u boosts energy of analog signal; increases amplitude u makes no distinction between noise and signal u repeater u receives, interprets and repeats a digital signal u adds little or no noise/distortion

transmission terminology u repeater-amplifier diagram comparison

modems, codecs, bauds, bits u modem ( modulator-demodulator ) u 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) u purpose: use analog medium for digital data/signals u example: PC modem, phone lines; TV cable modems u techniques: PSK, ASK, FSK and combinations.

modems, codecs, bauds, bits u codec (codec/decoder) u converts analog data into digital form (bits), and the reverse. u two main techniques: PCM, DM u PCM ( pulse code modulation ) u absolute values, based on sampling theorem; (nearly) total information u DM ( delta modulation ) u based on differences; less accurate

modems, codecs, bauds, bits u Baud rate -maximum number of times per second that the signal element can change u Baud - The unit in which the Baud rate is measured u incorrect to say “9800 bauds per second.” u thus, the baud rate is the rate at which distinct signal elements are sent. u also called “ signaling rate ” u applies to digital signals or analog signals carrying digital data.

modems, codecs, bauds, bits u diagram: bauds and bits   T = 1/Baud Rate measured in baud Signal levels Bit rate = baud rate·log 2 (#of signal levels) Example: A 9600 baud modem has a baud rate of 9600 baud. If it uses two signal levels is also runs at 9600 bps.