CS3502-Presentation
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Transcript CS3502-Presentation
CS3502,
Data and Computer Networks:
the physical layer-3
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 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
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 techniques
what
needs to happen
1. X and R must be synchronized
2. X emits a signal
3. R receives and interprets signal
factors
data
affecting transmission
rate
S/N : signal-to-noise ratio
encoding technique
encoding 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 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 1
NRZ-M: change on 1, not on 0
NRZ-S: change on 0, not on 1
examples - diagram... what is the baud rate?
-M, -S are differential methods
encoding 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 AMI
biphase 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 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:
Q:
what is the baud rate?
can you think of a way to increase the data rate
but not the baud rate?
encoding 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 techniques
digital
data, analog signals
carrier signal - a constant analog signal, transmitted
from sender to receiver
example: 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 techniques
ASK:
amplitude shift keying (diagram)
carrier
ASK
encoding techniques
ASK
Summary
unaltered
can
for
carrier contains no data
be used in optical fiber
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 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 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 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 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:
theorem
the sampling theorem
statement (see text)
note: based on exact samples
encoding 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:
samples
voice channels
taken 8000/sec
quantized to 7 bits
synch. bit added -> 8 bits
8 x 8000 = 64000 bps, standard digital voice channel
encoding 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 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