CS3502,CS3502,Data and Computer Networks:Data and Computer Networks:

the physical layer-3the physical layer-3

impairments to transmission impairments to transmission three categories

1. attenuation/attenuation distortion loss of signal power through distance attenuation varies with frequency

2. delay distortion guided media only velocity varies with frequency limiting factor on bandwidth, especially with digital

signals

3. noise

impairments to transmissionimpairments to transmission

noise thermal noise - heat; electron agitation intermodulation noise

unwanted combining of signals at diff. frequencies

crosstalk common on t.p. 2 nearby paths coupled electrically

impulse

encoding techniques encoding techniques

using signals to send information: main purpose of physical layer

4 major cases

1. digital signals to send digital data

2. digital signals to send analog data

3. analog signals to send digital data

4. analog signals to send analog data

encoding techniquesencoding techniques

what needs to happen1. X and R must be synchronized2. X emits a signal3. R receives and interprets signal

factors affecting transmission data rate S/N : signal-to-noise ratio encoding technique

encoding techniquesencoding techniques

desirable properties of an encoding scheme

synchronization capability - the ability to stay synchronized, or to get re-synchronized

error detection capability

immunity to noise - the ability to separate noise from the transmitted signal

encoding techniquesencoding techniques

digital data - digital signals simple binary methods: NRZ-L, NRZ-I(M), NRZ-S

voltage level constant throughout bit time simple, but no synchronization capability most vulnerable to noise used only for low-moderate data rates, short

distances

NRZ-L: high 0, low 1NRZ-M: change on 1, not on 0NRZ-S: change on 0, not on 1 examples - diagram... what is the baud rate? -M, -S are differential methods

encoding techniquesencoding techniques

digital data - digital signals; better methods

multilevel binary, bipolar AMI these hold 0 voltage for binary 0, then

alternate between + and - for binary 1

Pseudoternary reverse of bipolar AMIbiphase methods - require at least 1

transition in each bit time increase reliability in presence of noise increased synch. capability increased ability to detect errors

encoding techniquesencoding techniques

biphase methods: always a transition in the middle of the bit time

manchester down for 1, up for 0

differential manchester change at start of bit indicates a 0

Q: what is the baud rate?

Q: can you think of a way to increase the data rate but not the baud rate?

encoding techniquesencoding techniques

digital data, analog signals analog signal: a continuously varying

electomagnetic wave

Q: why use analog signals for digital data?

what are 3 critical and widely used analog media ?

also may want to mix digital, analog data

encoding techniquesencoding techniques

digital data, analog signalscarrier signal - a constant analog signal,

transmitted from sender to receiverexample: the dial tone indicates a live

connection; a carrier wave

bits encoded by varying 1 or more of 3 properties

modulation: ASK, FDK, PSK

encoding techniquesencoding techniques

ASK: amplitude shift keying (diagram) carrier

ASK

encoding techniquesencoding techniques

ASK Summary

unaltered carrier contains no data

can be used in optical fiber

for other media, only good for lower bit rates

less efficient, relatively than other methods; more susceptible to errors, because

higher amplitudes attenuate more rapidly than lower ones

more susceptible to interference

encoding techniquesencoding techniques

FSK: frequency shift keying : diagram

FSK: summary less error prone used for high frequency (coax, microwave,etc) also used on voice lines radio

encoding techniquesencoding techniques

PSK: phase shift keying - differential binary PSK : phase shift of 1/2 period indicates 1;

no shift indicates 0; (diagram)

QPSK: use of 4 angles for higher bit rates

encoding techniquesencoding techniques

PSK - summary more efficient that ASK, FSK can be further enhanced with more signal levels number of angles media dependent

example: 2400 bauds, 9600 bps; PSK, ASK together (12 angles, 2 amplitudes)

show how to combine these techniques for higher bit rates; eg, ASK-FSK, ASK-PSK, FSK-PSK

encoding techniquesencoding techniques

analog data, digital signals 2 main techniques : pulse code modulation

(PCM), delta modulation (DM)

why? voice over optical fiber TV channels, movies, pictures over internet

principle: the sampling theorem theorem statement (see text) note: based on exact samples

encoding techniquesencoding techniques

PCM: pulse code modulation samples of the analog data taken each sample quantized samples transmited as digital signal received samples used to reconstruct analog

data

example: voice channels samples taken 8000/sec quantized to 7 bits synch. bit added -> 8 bits 8 x 8000 = 64000 bps, standard digital voice

channel

encoding techniquesencoding techniques

delta modulation similar idea (digital samples of analog data) reduction in number of bits transmitted periodically sends a sample send a “1” or “0” indicating “up” or “down” the up or down is by a fixed amount less accurate than PCM if intervals not chosen to match signal, or if

signal varies, leads to less accuracy. less widely used, but could be alternative

encoding techniquesencoding techniques

analog data, analog signals basis original telephone network; (ie, used

analog electical signals to transport analog voice signals); still in local loops to large degree

cable TV; (FDM - frequency division multiplexing)

broadcast radio

major techniques amplitude modulation frequency modulation phase modulation