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CS3502,CS3502,Data and Computer Data and Computer
Networks:Networks:the physical layer-1the physical layer-1
physical layer - purposephysical 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 layerideally, S = S’
Physical layer definitionsPhysical 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
SignalsSignals
examples: physical gesture, wave, hand signal flashes of light (eg, Morse code) sound: vary tone, loudness or duration flags smoke mirrors electrical voltages
transmission definitionstransmission definitions1. 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 necessary for transmission systems path for signal transfer (medium) transform signal to appropriate form (code) launch the signal (transmit) remove, receive or detect the signal (receive)
transmission propertiestransmission 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 assignal - can be modeled as f(t)f(t)
t --> f(t) ... f(t) represents some physical quantity: voltage, amplitude, frequency, etc.
signals 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 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 signalsdigital/ analog signals
digital signal1. assumed to take on finite number of values, AND2. 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 signalsdigital/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 signalsdigital/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 signalsdigital/ 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: position within a cycle/period
these quantities are varied, in order to use the signal to carry information
digital/ analog signalsdigital/ 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)
an = 2/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
digital/ analog signalsdigital/ analog signals (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)
a1=8
transmission mediatransmission 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 terminologytransmission terminology direct link
simplex
half-duplex
full duplex
spectrum - range of frequencies making up a signal
bandwidth - width of the spectrum; range of frequencies
examples
transmission terminologytransmission 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
modems, codecs, bauds, bitsmodems, 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, bitsmodems, 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, bitsmodems, 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.