Transcript Ch2 - NET 331 and net 221

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NET 221D:COMPUTER
NETWORKS FUNDAMENTALS
Networks and
Communication
Department
Lecture 2: Physical Layer
Introduction
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One of the major functions of the physical layer is to
move data in the form of electromagnetic signals
across a transmission medium.
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Thus, the data must be transformed
electromagnetic signals to be transmitted.
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Data can be analog (continuous, with many levels) or
digital (discrete, with a limited number of values).
to
Behrouz A. Forouzan” Data communications and Networking
Introduction
Both analog and digital signals can take one of
two forms: periodic or nonperiodic.
 Periodic analog signals can be classified as
simple or composite.
 A simple periodic analog signal, a sine wave,
cannot be decomposed into simpler signals.
 A composite periodic analog signal is composed
of multiple sine waves.
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Behrouz A. Forouzan” Data communications and Networking
Sin Wave Signals
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A sine wave is represented by three parameters: Peak
amplitude, Frequency, and Phase.
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Peak amplitude: it is the absolute value of the highest
intensity.
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Frequency: it refers to the number of periods in 1 s. It is
formally expressed in Hertz (Hz).
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Period is the amount of time, in seconds, a signal needs to
complete one cycle (the completion of one full pattern).
Behrouz A. Forouzan” Data communications and Networking
Sin Wave Signals
Behrouz A. Forouzan” Data communications and Networking
Sin Wave Signals
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Phase: it describes the position of the waveform
relative to time 0. It is measured in degree or radian.
Behrouz A. Forouzan” Data communications and Networking
Sin Wave Signals
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Wavelength binds the period or frequency of the
simple sine wave to the propagation speed of the
medium.
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Wavelength depends on both the frequency and the
medium.
 Wavelength
= propagation speed X period = propagation
speed/ frequency
Behrouz A. Forouzan” Data communications and Networking
Sin Wave Signals
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In a vacuum, light is propagated with a speed of 3 X 108 m/s.
(that speed is lower in air and cable.) . The frequency of red
light is 4 X 1014 .
Wavelength= c/f= (3 X 108 ) / (4 X 1014) = 0.75 X 10-6 m= 0.75
μm
Behrouz A. Forouzan” Data communications and Networking
Composite Signals
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According to Fourier analysis, any composite signal is a
combination of simple sine waves with different frequencies,
amplitudes, and phases.
Behrouz A. Forouzan” Data communications and Networking
Bandwidth
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The bandwidth of a composite signal is the difference between
the highest and the lowest frequencies contained in that signal.
Behrouz A. Forouzan” Data communications and Networking
Fourier Analysis
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Information can be transmitted on wires by varying
some physical property such as voltage or current.
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Any reasonably behaved periodic function, g(t)
with period T, can be written as:
Bandwidth-Limited Signals
An
example:
the
transmission of the
ASCII character “b”
encoded in an 8-bit
byte. The bit pattern
that is to be transmitted
is 01100010.
Bandwidth-Limited Signals
root-mean-square amplitudes:
Direct proportional with the transmitted signal energy at the
corresponding freq.
Any signal transmission occurs with power loss.
Fourier coefficients are not affected proportionally by the power
loss => signal amplitude is distorted
Frequencies : 0-Fmax =>the amplitudes are undiminished – above
they are attenuated.
Bandwidth vs Data Rate
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1924 Henri Nyquist –relation between bandwidth and data rate
in a noiseless channel (throughput):
Nyquist Theorem:
A data signal on a medium with H (Hz) bandwidth can
be reconstructed by making 2H samples/sec. For a signal
of
V
discrete
levels:
Maximum data rate=2H log2V bits/sec.
3 kHz channel (binary signals) => max_data_rate=6000 bps
throughput =2*3000 log22 = 6000 bps.
Throughput in a noisy channel
S – the signal power; N – the noise power
=> S/N the signal to noise ratio.
Signal to noise (decibels) 1 dB = 10 log10 S/N.
Ex: S/N = 10 => 10 dB; S/N =100 => 20 dB
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Shannon’s Theorem
The maximum throughput of a noisy channel of
bandwidth H with a signal to noisy ratio of S/N is:
Maximum throughput = H log2(1+S/N) bps.
Ex: tel line Bandwidth=3kHz; S/N=30 dB =>
Max throughput = 3000 * log2(1+1000) =~ 30.000 bps = 28.8 kbps
Twisted Pair
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The purpose of the physical layer is to transport bits
from one machine to another.
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Media are grouped into : guided media, such as copper
wire and fiber optics, and unguided media, such as
terrestrial wireless, satellite, and lasers through the air.
(a) Category 3 UTP.
(b) Category 5 UTP.
Twisted Pair
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A twisted pair consists of two insulated copper wires,
typically about 1 mm thick.
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When the wires are twisted, the waves from different
twists cancel out, so the wire radiates less effectively.
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The most common application of the twisted pair is the
telephone system.
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Twisted pairs can be used for transmitting either analog
or digital information.
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Twisted-pair cabling comes in several varieties.
Twisted Pair
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UTP (Unshielded Twisted Pair) consist simply of
wires and insulators.
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Links that can be used in both directions at the same
time are called full-duplex links.
Links that can be used in either direction, but only
one way at a time are called half-duplex links.
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A third category consists of links that allow traffic in
only one direction.
A category 5 twisted pair consists of two insulated
wires gently twisted together.
Coaxial Cable
A coaxial cable.
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It has better shielding and greater bandwidth than
unshielded twisted pairs.
50-ohm cable: intended for digital transmission from
the start.
75-ohm cable: analog transmission and cable
television.
Fiber Optics
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Fiber optics are used for long-haul transmission in
network backbones, high speed LANs, and high-speed
Internet access.
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An optical transmission system has three key
components: light source, transmission medium, and
the detector.
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A pulse of light indicates a 1 bit and the absence of
light indicates a 0 bit.
Fiber Optics
(a) Three examples of a light ray from inside a silica fiber impinging
on the air/silica boundary at different angles.
(b) Light trapped by total internal reflection.
Fiber Optics
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Multimode Fiber: any light ray incident on the
boundary above the critical angle will be reflected
internally, many different rays will be bouncing around
at different angles.
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Single-mode Fiber: light can propagate only in a
straight line, without bouncing.
Fiber Optics
Three wavelength bands: they are centered at 0.85, 1.30,
and 1.55 microns, respectively. All three bands are 25,000
to 30,000 GHz wide.
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0.85-micron band: has higher attenuation and so is used
for shorter distances.
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Last two bands have good attenuation properties (less
than 5% loss per kilometer).
Fiber Cables
(a) Side view of a single fiber.
(b) End view of a sheath with three fibers.
-Core: 50 microns for multi-mode, 8-10 microns for single mode
-Cladding: glass with a lower refraction index, to keep the light in the
core
-Connection:
-connectors (plug in) – about 20% attenuation
-mechanical splicing, tuned by an operator – 10% attenuation
-fused (melted together) – almost no attenuation
Fiber Optic Networks
A fiber optic ring with active repeaters.
Fiber Optic Networks
A passive star connection in a fiber optics network.
Electromagnetic Spectrum
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When electrons move, they create electromagnetic
waves that can propagate through space (even in a
vacuum).
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The number of oscillations per second of a wave is
called its frequency, f, and is measured in Hz.
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The distance between two consecutive maxima (or
minima) is called the wavelength.
The Electromagnetic Spectrum
The electromagnetic spectrum and its uses for
communication.
Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the
curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
Radio transmission
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Frequency ranges: 3 KHz to 1 GHz
Omnidirectional
Susceptible to interference by other antennas using same
frequency or band
Ideal for long-distance broadcasting
May penetrate walls
Apps: AM and FM radio, TV, maritime radio, cordless
phones, paging
Microwaves
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Frequencies between 1 and 300 GHz
Unidirectional.
Narrow focus requires sending and receiving antennas to
be aligned.
Issues:
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Line-of-sight (curvature of the Earth; obstacles)
Cannot penetrate walls
Politics of the Electromagnetic Spectrum
The ISM bands in the United States.
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Bands for unlicensed usage.
All devices in the ISM bands limit their transmit
power (e.g., to 1 watt).
Satellite Microwaves
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Similar to terrestrial microwave except the signal
travels from a ground station on earth to a satellite
and back to another ground station.
Satellite receives on one frequency, amplifies or
repeats signal and transmits on another frequency
Satellite is relay station
Television
Long distance telephone
Private business networks
Satellite Microwaves
Communication Satellites
The principal satellite bands.
Infrared
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Frequencies between 300 GHz and 400 THz.
Short-range communication.
High frequencies cannot penetrate walls.
Requires line-of-sight propagation.
Advantage: prevents interference between systems in
adjacent rooms.
Disadvantage: cannot use for long-range communication
or outside a building due to sun’s rays.
Baseband Transmission
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Baseband transmission: the signal occupies
frequencies from zero up to a maximum that depends
on the signaling rate.
It is common for wires.
Passband transmission: the signal occupies a band
of frequencies around the frequency of the carrier
signal.
It is common for wireless and optical channels.
Multiplexing: Channels are often shared by multiple
signals.
Baseband Transmission
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NRZ: a positive voltage to represent a 1 and a
negative voltage to represent a 0.
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The signal will be attenuated and distorted by the
channel and noise at the receiver.
NRZ needs a bandwidth of at least B/2 Hz when the
bit rate is B bits/sec.
By using four voltages, we can send 2 bits at once as
a single symbol.
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Baseband Transmission
Line codes: (a) Bits, (b) NRZ, (c) NRZI,
(d) Manchester, (e) Bipolar or AMI.
Clock Recovery
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The receiver must know when one symbol ends and the
next symbol begins to correctly decode the bits.
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One strategy is to send a separate clock signal to the
receiver.
Manchester: mix the clock signal with the data signal by
XORing them together.
NRZI: coding a 1 as a transition and a 0 as no transition,
or vice versa.
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4B/5B: Every 4 bits is mapped into a5-bit pattern with a
fixed translation table. The five bit patterns are chosen so
that there will never be a run of more than three
consecutive 0s.
Clock Recovery
4B/5B mapping.
Clock Recovery
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Signals that have as much positive voltage as
negative voltage even over short periods of time are
called balanced signals.
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Bipolar encoding: represent a logical 1, (say +1 V or
−1 V) with 0 V representing a logical zero. To send a
1, the transmitter alternates between the +1 V and −1
V levels.
Passband Transmission
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we can take a baseband signal that occupies 0 to B Hz
and shift it up to occupy a passband of S to S +B Hz.
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Digital modulation is accomplished with passband
transmission by regulating or modulating a carrier signal
that sits in the passband.
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In ASK (Amplitude Shift Keying), two different
amplitudes are used to represent 0 and 1.
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PSK (Phase Shift Keying): the carrier wave is
systematically shifted 0 or 180 degrees at each symbol
period.
Passband Transmission
(a) A binary signal. (b) Amplitude shift keying.
(c) Frequency shift keying. (d) Phase shift keying.
Multiplexing
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FDM: divides the spectrum into frequency bands, with
each user having exclusive possession of some band.
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Guard band: It keeps the channels well separated.
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When sending digital data, it is possible to divide the
spectrum efficiently without using guard bands.
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In
OFDM
(Orthogonal
Frequency
Division
Multiplexing), the channel bandwidth is divided into
many subcarriers that independently send data.
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The frequency response of each subcarrier is designed so
that it is zero at the center of the adjacent subcarriers.
Frequency Division Multiplexing
Frequency division multiplexing. (a) The original bandwidths.
(b) The bandwidths raised in frequency.
(c) The multiplexed channel.
Frequency Division Multiplexing
Orthogonal frequency division
multiplexing (OFDM).
Time Division Multiplexing
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The users take turns (in a round-robin fashion), each
one periodically getting the entire bandwidth for a
little burst of time.
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Bits from each input stream are taken in a fixed time
slot and output to the aggregate stream.
Code Division Multiplexing
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CDM (Code Division Multiplexing) is a form of spread
spectrum communication.
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CDMA allows each station to transmit over the entire
frequency spectrum all the time.
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In CDMA, each bit time is subdivided into m short intervals
called chips.
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Typically, there are 64 or 128 chips per bit.
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Each station is assigned a unique m-bit code called a chip
sequence.
To transmit a 1 bit, a station sends its chip sequence. To
transmit a 0 bit, it sends the negation of its chip sequence.
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Code Division Multiplexing
(a)
(b)
Chip sequences for four stations.
Signals the sequences represent