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)
2
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
3
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
4
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
5
Analogue & Digital Signals
6
Periodic
Signals
7
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.
8
Varying Sine Waves
s(t) = A sin(2ft +)
9
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
10
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.
11
Addition of
Frequency
Components
(T=1/f)
12
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
13
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)
14
Acoustic Spectrum (Analog)
15
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.
16
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
17
Attenuation of Digital Signals
18
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
19
Conversion of Voice Input into Analog
Signal
20
Conversion of PC Input to Digital Signal
21
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
22
Analog Signals Carrying Analog and
Digital Data
23
Digital Signals Carrying Analog and
Digital Data
24
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.
25
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
26
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.
27
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
28
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
29
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.
30
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.
31
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
32
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.
33
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
34
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
35
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
36
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
6
6
37
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
38