The Physical Layer
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defines the mechanical, electrical, and timing
interfaces to the network.
Topics in this chapter:
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theoretical analysis of data transmission
three kinds of transmission media :
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guided : copper wire and fiber optics
Wireless: terrestrial radio
satellite
This material will provide background information on the
key transmission technologies used in modern networks
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Physical layer: The Theoretical Basis for
Data Communication
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Information can be transmitted on wires by
varying some physical property such as
voltage or current.
we can model the behavior of the signal
and analyze it mathematically (f(t))
Fourier Analysis :
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periodic function, g(t) with period T can be constructed as
the sum of a (possibly infinite) number of sines and cosines:
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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A data signal that has a finite duration (which all of
them do) can be handled by just imagining that it
repeats the entire pattern over and over forever.
Computation of series coefficients :
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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Bandwidth-Limited Signals
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example: the transmission
of the ASCII character ''b''
encoded in an 8-bit byte.
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bit pattern : 01100010
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Root-mean-square:
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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all transmission facilities diminish different Fourier
components by different amounts, thus introducing
distortion.
Usually, the amplitudes are transmitted undiminished from
0 up to some frequency fc [measured in cycles/sec or Hertz
(Hz)] with all frequencies above this cutoff frequency
attenuated.
The range of frequencies transmitted without being strongly
attenuated is called the bandwidth (0 - fc )
The bandwidth is a physical property of the transmission
medium and usually depends on the construction,
thickness, and length of the medium
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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In some cases a filter is introduced into the circuit to limit
the amount of bandwidth available to each customer.
 Example:
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bandwidth of a telephone wire for short distance: 1 MHz
Restricted bandwidth by telephone company : 3100 Hz
Given a bit rate of b bits/sec
The time required to send 8 bits is 8/b sec
The frequency of the first harmonic is b/8 Hz
An ordinary telephone line, often called a voice-grade line,
has an artificially-introduced cutoff frequency just above
3000 Hz
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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The number of the highest harmonic passed through is
roughly 3000/(b/8) or 24,000/b
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Relation between data rate and harmonics :
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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The Maximum Data Rate of a pure Channel
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Nyquist’s theorem :
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an arbitrary signal has been run through a low-pass filter of
bandwidth H, the filtered signal can be completely
reconstructed by making only 2H (exact) samples per
second.
If the signal consists of V discrete levels, Nyquist's theorem
states:
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Example: A noiseless 3-kHz channel cannot transmit binary (i.e., twolevel) signals at a rate exceeding 6000 bps
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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The Maximum Data Rate of a noisy Channel
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there is always random (thermal) noise present due to
the motion of the molecules in the system.
The amount of thermal noise present is measured by
the ratio of the signal power to the noise power, called
the signal-to-noise ratio.
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If we denote the signal power by S and the noise power
by N, the signal-to-noise ratio is S/N.
Usually, the ratio itself is not quoted; instead, the
quantity 10 log10 S/N is given (decibels (dB) ).
Sharif University of Technology
Physical layer: The Theoretical Basis for
Data Communication
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Shannon's theorem:
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Example: A channel of 3000-Hz bandwidth with a signal
to thermal noise ratio of 30 dB (typical parameters of the
analog part of the telephone system) can never transmit
much more than 30,000 bps.
No matter how many or how few signal levels are used
and no matter how often or how infrequently samples are
taken.
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This is only an upper bound and real systems rarely
achieve it.
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Guided Transmission Media
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Magnetic Media
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Bandwidth characteristics is excellent
Delay characteristics is poor
Twisted Pair
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A twisted pair consists of two insulated copper wires, typically about 1 mm
thick.
Twisting is done because two parallel wires constitute a fine antenna.
Twisted pair categories:
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Category 3 UTP (Unshielded Twisted Pair) (a)
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Category 5 UTP (Unshielded Twisted Pair) (b)
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They are similar to category 3 pairs, but with more twists per centimeter,
which results in less crosstalk and a better-quality signal over longer
distances, making them more suitable for high-speed computer
communication.
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Guided Transmission Media
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Coaxial Cable
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It has better shielding than twisted pairs, so it can span longer
distances at higher speeds.
Two kinds of coaxial cable
 50-ohm cable : is commonly used when it is intended for
digital transmission.
 75-ohm cable: is commonly used for analog transmission
and cable television
 The bandwidth possible depends on the cable quality, length,
and signal-to-noise ratio of the data signal.
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Guided Transmission Media (Fiber Optics )
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Fiber Optics
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An optical transmission system has three key
components:
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Light source: Conventionally, a pulse of light indicates a 1 bit
and the absence of light indicates a 0 bit.
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Transmission medium: is an ultra-thin fiber of glass
Detector: generates an electrical pulse when light falls
on it.
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Guided Transmission Media (Fiber Optics )
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When a light ray passes from one medium to another, for
example, from fused silica to air, the ray is refracted (bent) at
the silica/air boundary
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Fiber types
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multimode fiber
single-mode fiber: diameter is reduced to a few
wavelengths of light
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Guided Transmission Media (Fiber Optics )
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Transmission of Light through Fiber
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The attenuation of light through glass depends on the
wavelength of the light
Attenuation of light through fiber in the infrared region
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Guided Transmission Media (Fiber Optics )
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Fiber Cables
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multimode fibers : The core is typically 50 microns in diameter,
about the thickness of a human hair
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single-mode fibers : The core is 8 to 10 microns.
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Guided Transmission Media (Fiber Optics )
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Fibers can be connected in
three different ways
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Two kinds of light sources
are typically used to do the
signaling
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A comparison of semiconductor diodes and LEDs as light sources.
Connectors
Mechanical
Fusion
LEDs (Light Emitting
Diodes)
Semiconductor lasers
receiving end of an
optical fiber consists of a
photodiode
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Guided Transmission Media (Fiber Optics )
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Fiber Optic Networks
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Active repeaters
Passive star
A fiber optic ring with active repeaters
A passive star connection in a fiber optics network
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Guided Transmission Media (Fiber Optics )
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Comparison of Fiber Optics and Copper Wire
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Fiber can handle much higher bandwidths than copper
Fiber not being affected by power surges, electromagnetic interference,
or power failures
Fiber is thin and lightweight
Security in fiber is high
Fiber is a less familiar technology requiring skills not all engineers have
Fibers can be damaged easily by being bent too much
Optical transmission is inherently unidirectional, two-way communication
requires either two fibers or two frequency bands on one fiber
fiber interfaces cost more than electrical interfaces
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