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