Transcript 08. Physical Basics of Telecommunications

Lecture #8: Physical Basics of
Telecommunications.
Contents
 Mathematical Results in Signaling
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 Physical Layer Functions
 Guided Transmission Media:
 Electric Signal Wires
 Light Transmission
2
8
13
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

Mathematical Results in
Signaling
Signals’ presentation as periodical function of time:
g(t). Period T and frequency f.
Fourier transform - a constant + endless sum of
sin and cos expressions (harmonics):
 derivation of sin coefficients
 derivation of cos-coefficients
 derivation of constant coefficient



Power losses in data transmission.
Selective harmonics’ amplitude deminission


transmission subsiding
filtering
Bandwidth - frequency interval of harmonics
(frequency components) in which the signal is
transmitted
2
Mathematical Results in
Signaling

Impact of the number of harmonics on the
transmitted signal shape.
2/1

Boud rate and bit rate

Data rates and number of harmonics
– 9.6 kb/s  2 harmonics
2/2
– 38.4 kb/s  0 harmonics (no transmission of binary
signal by phone twisted pair, that has a cut-off at 3 kHz)
3
Mathematical Results in
Signaling

Maximum data rate of a noiseless channel
– Nyquist’s theorem: arbitrary signal passing filter
with frequency bandwidth H can be reconstructed
by 2H observations per second. Faster observations
are pointless, because higher frequencies (>H) are
filtered. And vice-versa:
MAX(Data Rate) = 2H b/S for two-level (i.e. binary)
signal.
MAX(Data Rate) = 2H log2V for V-level discrete
signal.
4
Mathematical Results in
Signaling

Maximum data rate of a noisy channel
– Signal/Noise Ratio RSN=S/N (S - signal power; N noise power) or usually
RSN = 10 log10 S /N dB
S/N=10
RSN =10dB
S/N=100
RSN =20dB
S/N=1000
RSN =30dB
– Shannon’s theorem: arbitrary signal passing
filter with frequency bandwidth H and signal-noiseratio RSN has
MAX(Data Rate) = H log2(1+S/N) b/S
Phone lines: H=3000, RSN =30 dB 
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MAX(Data Rate)  30 kb/S
Physical Layer
The physical layer provides mechanical,
electrical, functional and procedural means to
activate, maintain and deactivate physicalconnections for bit transmission between data
link entities.
 Physical layer entities are interconnected by
means of a physical medium.
 Data-circuit
A communication path in the physical media for
OSI between two physical entities, together with
the facilities necessary in the physical layer for
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the transmission of bits on it.

Physical layer

Services provided to the data link layer
–physical-connections;
–physical-service-data-units;
–physical-connection-endpoints;
–data-circuit identification;
–sequencing;
–fault condition notification; and
–quality of service parameters.
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
Guided Transmission Media
Parameters of the transmission media:
 bandwidth
cost
delay
carry distance
support devices
general support
 durability, noise protection
 spread and popularity







Guided media:
 conductor wires
 fiber optics
Unguided media:


radio waves
LASER rays
Light Amplification
by Stimulated Emission of Radiation
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Twisted Pair


Pair of conductor wires that are helical twisted
Reduction of the interference and induction
between neighbor pairs
2/3a
 bandwidth: up to 1 Gb/S (not in phone lines)
cost: low
delay:
carry distance: 102 -104 m without amplification
support devices: analog and digital transmission
general support:
 durability, noise protection :
 spread and popularity: phone systems (POTS Plain Old Telephone Service)





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Baseband Coaxial Cable


Structure
2/3
Impedance 50 ; other properties:
 bandwidth: up to 10 Mb/S
 cost: low
 delay:
 carry distance: 102 -104m without
amplification
 support devices: analog and digital
transmission
 general support:
 noise protection: better than twisted pair
 spread and popularity: LAN, cable TV
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Broadband Coaxial Cable





The bandwidth up to 300-500 MHz - for analog
transmission and digital data modems on both ends.
Modems allow transmission of >1b/S for each 1Hz of
the bandwidth or
The cable bandwidth is split into multiple 6 MHz
channels
Larger areas need analog amplification that defines
transmission direction (as the amplifier has input and
output) 
Bi-directional transmission needs:
 dual cable connection between both ends
 single cable and frequency splitting
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2/4
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Broadband Coaxial Cable

Frequency splitting in single cable
–
–

Subsplit system: 5-30 MHz for input signal and
40-300 MHz output
Midsplit system: 5-116 MHz for input signal and
168-300 MHz output
Parameters:
 bandwidth: up to 10 Mb/S
 cost: lower than baseband coax
 carry distance: 104 -105 m without
amplification (analog signal)
 support devices: analog and digital transmission
 noise protection: better than twisted pair, worse
than baseband coax
 spread and popularity: cable TV (widely
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installed), perspective for MANs
Transmission Media - Fiber
Optics

Evolution of the system speed scissors:



early computer age: bottleneck is in
intercomputer communications: data processing
is faster than data transmission
mature computer age: bottleneck shifted to data
processing as the communications became faster
Example: fiber optics transmits more than 100 Tb/S
but the converters between electrical and optical
signals limit the speed to 10 Gb/S.
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Fiber Optics

Optical transmission system:
– light source
– light transmission media
– light detector



Light/electricity conversion is done by the light source
and detector: light pulse codes “1” and generates
electrical pulse in th detector
Media: glass fiber (variant “fibre”) - unidirectional
transmission (direction determined by the positions of
the source and the detector)
Physical ground of the light transmission:
total internal reflection; reflection angle, boundary
refraction; single- and multi-mode fibers.
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2/5
Fiber Optics

Glass transparency equals that of the clear air

Light attenuation A[dB] per linear km,

% of
transmission
1
10
20
30
40
50
60
A[dB]
20
10
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5.2
4
3
2.2
70
80
90
100
1.5 0.95 0.45
0
A(l) diagram, transmission bands:
– 0.85 m-6(mm): A=0.8 i.e. 85% transmission/km
– 1.3 m-6 : A=0.2 i.e. 95% transmission/km
– 1.85 m-6 : A=0.18 i.e. 96% transmission/km
– bandwidth 30 THz for the three bands.
2/6
GaAs
crystal
A single
 Light pulses’ shape: solitons (a solitary wave that
soliton
nonlinear
propagates with little loss of energy and retains its
Schrodinger
shape and speed after colliding with another such
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surface
wave)
Fiber Cables

Single core ( 50 m-6) cables and

Multiple core cables ( 10 m-6)

Cable Interconnections:
2/7
– terminating connectors with fiber plugs - 20% light losses
– mechanical junction - 5% light losses, personnel
– termofusing - less than 1% losses, special equipment

Light sources: LEDs or crystal lasers

Light sensors: photo-diodes
2/8
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Fiber Optic Networks


Networks based on fiber optic connections can
cower the range between LANs and WANs
Topology always based on point-to-point
connections e.g. ring with T-connector for each
node:
2/9
– passive interface: main light conductor (fiber) and
LED/photodiode junctions for each station; high
reliability, short distance, restricted number of computers
in the network
– active interface: main light fiber has a break at each
station and the signal is regenerated by the repeater;
repeaters are electrical (wired interface to the computer)
or optic (fiber interface to computer); reliability depends
on the junctions, unrestricted size in length and stations
number, long interstation distance (km)
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Fiber Optic Networks


Passive star topology - modified ring with
fiber interface to computer. The passive star
is the point where every light pulse of the
incoming fibers illuminates any of the
outgoing fibers to the computers. 2/10
Its properties resembles those of passive ring
topology (limited distance and number of
stations and independent reliability to the
state of each station)
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
Advantages:
Fiber vs. Wire

Drawbacks:
Lower attenuation
Unidirectional
transmission 
Wider bandwidth
doubled conductors
No interference between the lines
Power independence throughout the or occupied bands
route
More expensive
Better protection & security against interfaces
taps
Requires additional
Lighter weight, non corrosive
staff qualification
Lower installation cost for new
routers
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2/1
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Average
harmonic
frequency
Bandwidth /
Average harmonic frequency =
Number of harmonics sent
For 3kHz
bandwidth
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Twisted pair
2/3a
(a) Category 3 UTP.
(b) Category 5 UTP.
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