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

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

transmission terminologytransmission terminology repeater-amplifier diagram comparison

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.

modems, codecs, bauds, bitsmodems, codecs, bauds, bits diagram: bauds and bits

T

=

1/Baud Rate

measured in baud

Signal

levels

Bit rate =

baud rate·log2(#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.