Transcript : Book Slides: Chapter 3

William Stallings
Data and Computer
Communications
7th Edition
Chapter 3
Data Transmission
1
Terminology (1)
• Transmitter: converts data into transmittable signals
• Receiver: converts received signal into data
• Communication is in the form of electromagnetic
waves.
• Medium: carries data, may classified as:
—Guided medium: the waves are guided along a physical
path. e.g. twisted pair, optical fiber
—Unguided medium (wireless): provide a means for
transmitting electromagnetic waves but do not guide them.
e.g. air, water, vacuum (space)
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Terminology (2)
• Direct link: no intermediate devices (no amplifiers
or repeaters), the signal propagates directly from
transmitter to receiver.
—This term can apply to both guided an unguided media.
• Point-to-point: provides direct link between two
devices and those are the only 2 devices sharing the
link.
• Multi-point guided configuration: more than two
devices share the same link
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Terminology (3)
• Simplex transmission: signals are transmitted in only
one direction
—e.g. Television
• Half duplex transmission: both stations may transmit
but only one way at a time
—e.g. police radio
• Full duplex transmission: both station may transmit
simultaneously
—e.g. telephone
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Frequency, Spectrum and Bandwidth
• Time domain concepts: signal viewed as a function
of time, an electromagnetic signal can be either
analog or digital:
—Analog signal: the signal intensity varies in a smooth
fashion over time.
• No breaks or discontinuities in the signal.
—Digital signal: signal intensity maintains a constant level
then changes to another constant level.
• This is an idealized definition, in fact there is a small transition
period.
—Periodic signal: the same signal pattern repeats over time
—Aperiodic signal: pattern does not repeat over time
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Analogue & Digital Signals
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Periodic
Signals
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Sine Wave
• Peak Amplitude (A)
—maximum strength of signal, measured in volts.
• Frequency (f)
—Rate at which the signal repeats
—Hertz (Hz) or cycles per second
—Period = time for one repetition (T)
—T = 1/f
• Phase ()
—Relative position in time within a single period of a signal.
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Varying Sine Waves
s(t) = A sin(2ft +)
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Wavelength
• Is defined as distance occupied by one cycle or
distance between two points of corresponding phase
in two consecutive cycles ().
• Assuming signal velocity v
— = vT
—f = v
—c = 3*108 ms-1 (speed of light in free space)
—For an electromagnetic wave that travels in free space:
• f = c
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Frequency Domain Concepts
• An electromagnetic signal is usually made up of
many frequencies.
• It can be shown using, Fourier analysis, that any
signal is made up of components at various
frequencies, in which each component is a sinusoid.
• By adding together enough sinusoidal signals, each
with the appropriate amplitude, frequency, and phase,
any electromagnetic signal can be constructed.
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Addition of
Frequency
Components
(T=1/f)
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Spectrum & Bandwidth
• Spectrum
—range of frequencies contained in signal
• Absolute bandwidth
—width of spectrum
• Effective bandwidth
—Often just bandwidth
—Narrow band of frequencies containing most of the energy
in the signal.
• Data Rate and Bandwidth
—Any transmission system has a limited band of frequencies
—This limits the data rate that can be carried
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Analog and Digital Data Transmission
• Data
—Entities that convey meaning, or information.
• Signals
—Electric or electromagnetic representations of data.
• Transmission
—Communication of data by propagation and processing of
signals
• Analog data
—Continuous values within some interval (e.g. sound, video)
• Digital data
—Discrete values (e.g. text, integers)
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Acoustic Spectrum (Analog)
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Analog and Digital Signals
• Means by which data are propagated
• An analog signal is a continuously varying
electromagnetic wave that may be propagated over a
variety of media:
—Wire, fiber optic, coaxial, space (wireless)
—Speech bandwidth 100Hz to 7kHz
—Telephone bandwidth 300Hz to 3400Hz
—Video bandwidth 4MHz
• A digital signal is a sequence of voltage pulses that can
be transmitted over a wire medium:
—e.g. a constant positive level to represent binary 0 and a
constant negative level to represent binary 1.
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Advantages & Disadvantages
of Digital
• Cheaper. Provide very high bandwidth.
• Less susceptible to noise. Use distinct levels of signal
intensity to represent data.
• Suffer more from attenuation
—Pulses become rounded and smaller
—Leads to loss of information
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Attenuation of Digital Signals
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Components of Speech
• Frequency range (of hearing) 20Hz-20kHz
—Speech 100Hz-7kHz
• Easily converted into electromagnetic signal for
transmission
• Sound frequencies with varying volume converted
into electromagnetic frequencies with varying
voltage
• Limit frequency range for voice channel
—300-3400Hz
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Conversion of Voice Input into Analog
Signal
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Conversion of PC Input to Digital Signal
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Data and Signals
• Usually use digital signals for digital data and analog
signals for analog data
• Can use analog signal to carry digital data
—Modem
• Can use digital signal to carry analog data
—Compact Disc audio
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Analog Signals Carrying Analog and
Digital Data
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Digital Signals Carrying Analog and
Digital Data
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Analog Transmission
• Analog signal transmitted without regard to content
• The signals may represent analog data (e.g. voice) or
digital data (e.g. binary data pass through modem)
• The analog signal will become weaker (attenuate)
over distance
• To achieve longer distances, analog transmission use
amplifiers to boost the energy in the signal.
• Also amplifies noise
—With cascaded amplifiers, the signal becomes more and
more distorted.
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Digital Transmission
• Concerned with content
• The integrity of the data endangered by noise,
attenuation and other impairments.
• Repeaters used
—Repeater receives signal
—Extracts bit pattern
—Retransmits (a new signal is regenerated)
—Attenuation is overcome
—Noise is not amplified
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Advantages of Digital Transmission:
reasons of preferring digital transmission
• Digital technology
— Drop in cost and size of digital circuitry because of the advent of
LSI/VLSI technology.
• Data integrity
— With the use of repeaters rather than amplifiers, it is possible to
transmit data longer distances over lower quality (less cost) lines.
• Capacity utilization
— High bandwidth links economical. Easier and more efficient
multiplexing with digital techniques (time division)
• Security & Privacy
— Encryption techniques can be easily applied to digital data.
• Integration
— Can treat analog and digital data similarly, integration of voice, video,
and digital data on the same transmission system is possible.
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Transmission Impairments
• Signal received may differ from signal transmitted
• For analog signal these impairments can degrade the
signal quality
• For digital signals bit errors may be introduced, a
binary 1 can be transformed into a binary 0 and vice
versa.
• The most significant impairments are:
—Attenuation and attenuation distortion
—Delay distortion
—Noise
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Attenuation
• Signal strength falls off with distance
• Attenuation depends on medium:
—For guided media: it is exponential and expressed as a
constant number of decibels per unit distance.
—For unguided media: it is a complex function of distance
and makeup of the atmosphere.
• The received signal strength must be:
—enough to be detected by the electronic circuitry in the
receiver.
—sufficiently higher than noise to be received without error
• It is important to know that attenuation is an
increasing function of frequency
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Delay Distortion
• Occurs because the velocity of propagation of a signal
through a guided medium varies with frequency.
—The velocity tend to be highest near the center frequency
and fall off toward the two edges of the band.
• Critical for digital data, consider that a sequence of
bits is being transmitted:
—Some of the signal components of one bit position will spill
over into other bit positions, causing intersymbol
interference.
—This is a major limitation to maximum bit rate over a
transmission channel.
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Noise (1)
•
Additional unwanted signals inserted somewhere
between transmitter and receiver.
• It is the major limiting factor in communication
system performance.
• Noise may be divided into four categories:
1. Thermal noise:
•
•
•
Is due to thermal agitation of electrons.
Uniformly distributed across the bandwidth and called
white noise.
Cannot be eliminated and therefore places an upper bound
on communications system performance.
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Noise (2)
2. Intermodulation noise
•
Signals that are the sum and difference of original
frequencies sharing a medium
3. Crosstalk
•
•
A signal from one line is picked up by another
e.g. while using telephone, hear another conversation.
4. Impulse noise
•
•
•
Irregular pulses or spikes
e.g. External electromagnetic interference (lightning)
Short duration and relatively high amplitude
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Channel Capacity
• Cannel capacity is the maximum rate at which data
can be transmitted over a given communication path,
or channel, under given conditions.
• Data rate: rate In bits per second at which data can be
communicated
• Bandwidth: constrained by transmitter and medium,
and expressed in cycles per second, or Hertz
• Noise: average level of noise over the
communication channel.
• Error rate: rate at which errors occur, limits the BW.
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Nyquist Bandwidth
• For binary signals (two voltage levels) and a
bandwidth of B, using a binary transmission system,
the carrying capacity C of the system is given by:
—C = 2B
—This is called maximum capacity of the channel (assuming
noise free channel).
• With multilevel (M level) signaling
—C = 2B log2 M
• M = number of discrete signal or voltage levels
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Signal-to-Noise Ratio
• Ratio of the power in a signal to the power contained in
the noise that’s present at a particular point in the
transmission
• Typically measured at a receiver
• Signal-to-noise ratio (SNR, or S/N)
( SNR) dB
signal power
 10 log10
noise power
• A high SNR means a high-quality signal, low number
of required intermediate repeaters
• SNR sets upper bound on achievable data rate
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Shannon Capacity Formula
• Equation:
C  B log2 1  SNR 
• Represents theoretical maximum channel capacity
that can be achieved
• In practice, only much lower rates achieved
—Formula assumes white noise (thermal noise)
—Impulse noise is not accounted for
—Attenuation distortion or delay distortion not accounted for
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Example of Nyquist and Shannon
Formulations
• Spectrum of a channel between 3 MHz and 4 MHz ;
SNRdB = 24 dB
B  4 MHz  3 MHz  1 MHz
SNR dB  24 dB  10 log10 SNR 
SNR  251
• Using Shannon’s formula
C  10  log2 1  251  10  8  8Mbps
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Example of Nyquist and Shannon
Formulations
• How many signaling levels are required?
C  2 B log2 M
 
8 106  2  106  log2 M
4  log2 M
M  16
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