Transcript Chapter 15 & 16 Slides

Chapter 15 & 16:
Electromagnetic Signals
• Analog Signal
– signal intensity varies in a smooth fashion
over time. In other words, there are no breaks
or discontinuities in the signal
• Digital Signal
– signal intensity maintains a constant level for
some period of time and then changes to
another constant level
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Analog and Digital Waveforms
3
Periodic Signal Characteristics
• Peak Amplitude (A)
– Maximum signal value (strength), measured in volts
• Frequency (f)
– Repetition rate
– Measured in cycles per second or Hertz (Hz)
• Period (T)
– Amount of time it takes for one repetition, T=1/f
• Phase ()
– Relative position in time, measured in degrees
4
Frequency Domain Concepts
s(t) = (4/)  (sin (2ft) + (1/3) sin (2(3f)t))
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Frequency Domain Concepts
• Spectrum of a signal is the range of frequencies
that it contains
• Absolute bandwidth of a signal is the width of
the spectrum
• Effective bandwidth contained in a relatively
narrow band of frequencies, where most of
signal’s energy is found
• The greater the bandwidth, the higher the
information-carrying capacity of the signal
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Bandwidth
• Width of the spectrum of frequencies that
can be transmitted
– if spectrum=300 to 3400Hz,
bandwidth=3100Hz
• Greater bandwidth leads to greater costs
• Limited bandwidth leads to distortion
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Analog Signaling
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Voice/Audio Analog Signals
• Easily converted from sound frequencies
(measured in loudness/db) to electromagnetic
frequencies, measured in voltage
• Human voice has frequency components ranging
from 20Hz to 20kHz
• For practical purposes, the telephone system has
a narrower bandwidth than human voice, from
300 to 3400Hz
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Voice Signals
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Image/Video: Analog Data to
Analog Signals
• Image is scanned in lines; each line is
displayed with varying levels of intensity
• Requires approximately 4Mhz of analog
bandwidth
• Since multiple signals can be sent via the
same channel, guardbands are necessary,
raising bandwidth requirements to 6Mhz
per signal
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Digital Signals
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Transmission Media
• Physical path between transmitter and
receiver (“channel”)
• Design factors affecting data rate
–
–
–
–
bandwidth
physical environment
number of receivers
impairments
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Impairments and Capacity
• Impairments exist in all forms of data
transmission
• Analog signal impairments result in
random modifications that impair signal
quality
• Digital signal impairments result in bit
errors (1s and 0s transposed)
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Transmission Impairments:
Guided Media
• Attenuation
– loss of signal strength over distance
• Attenuation Distortion
– different losses at different frequencies
• Delay Distortion
– different speeds for different frequencies
• Noise
– distortions of signal caused by interference
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Transmission Impairments:
Unguided (Wireless) Media
• Free-Space Loss
– Signals disperse with distance
• Atmospheric Absorption
– Water vapor and oxygen contribute to signal loss
• Multipath
– Obstacles reflect signal creating multiple copies
• Refraction
- Change in signal speed due to atmospheric conditions
• Thermal Noise
- White noise, arises from thermal activity of devices 16
Types of Noise
• Thermal (aka “white noise”)
– Uniformly distributed, cannot be eliminated
• Intermodulation
– When different frequencies collide (creating
“harmonics”)
• Crosstalk
– Overlap of signals
• Impulse noise
– Irregular spikes, less predictable
Business Data Communications, 5e
17
Channel Capacity
• The rate at which data can be transmitted
over a given path, under given conditions
• Four concepts
–
–
–
–
Data rate
Bandwidth
Noise
Error rate
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Data Communication
Components
• Data
– Analog: Continuous value data (sound, light,
temperature)
– Digital: Discrete value (text, integers, symbols)
• Signal
– Analog: Continuously varying electromagnetic wave
– Digital: Series of voltage pulses (square wave)
• Transmission
– Analog: Works the same for analog or digital signals
– Digital: Used only with digital signals
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Analog DataSignal Options
• Analog data to analog signal
– Inexpensive, easy conversion (e.g., telephone)
– Data may be shifted to a different part of the
available spectrum (multiplexing)
– Used in traditional analog telephony
• Analog data to digital signal
– Requires a codec (encoder/decoder)
– Allows use of digital telephony, voice mail
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Digital DataSignal Options
• Digital data to analog signal
– Requires modem (modulator/demodulator)
– Allows use of PSTN to send data
– Necessary when analog transmission is used
• Digital data to digital signal
– Requires CSU/DSU (channel service unit/data service
unit)
– Less expensive when large amounts of data are
involved
– More reliable because no conversion is involved
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Analog and Digital Signaling
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Transmission Choices
• Analog transmission
– only transmits analog signals, without regard for data
content
– attenuation overcome with amplifiers
– signal is not evaluated or regenerated
• Digital transmission
– transmits analog or digital signals
– uses repeaters rather than amplifiers
– switching equipment evaluates and regenerates signal
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Analog and Digital
Data and Signals
Analog
Signal
Digital
Signal
Analog
Data
Two alternatives:
(1) signal occupies the same
spectrum as the analog data
(2) Analog data are encoded
to occupy a different
spectrum.
Analog data are encoded
using a codec to produce a
digital bit stream.
Digital
Data
Digital data are encoded
using a modem to produce
analog signal.
Two alternatives:
(1) signal consists of two
voltage levels to represent
two binary values
(2) digital data are encoded
to produce a digital signal
with desired properties.
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Analog and Digital
Treatment of Signals
Analog
Digital
Transmission Transmission
Analog
Signal
Is propagated through
amplifiers; same treatment
whether signal is used to
represent analog data or digital
data.
Assumes that the analog signal
represents digital data. Signal is
propagated through repeaters; at
each repeater, digital data are
recovered from inbound signal
and used to generate a new
analog outbound signal.
Digital
Signal
Not used.
Digital signal represents a
stream of 1s and 0s which may
represent digital data or may be
an encoding of analog data.
Signal is propagated though
repeaters; at each repeater,
stream of 1s and 0s is recovered
from inbound signal and used to
generate a new digital outbound
signal.
Advantages of Digital
Transmission
• Cost – large scale and very large scale
integration has caused continuing drop in cost
• Data Integrity – effect of noise and other
impairments is reduced
• Capacity Utilization – high capacity is more
easily and cheaply achieved with time division
rather than frequency division
• Security & Privacy – Encryption possible
• Integration – All signals (Voice. Video, image,
data) treated the same
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Analog Encoding
of Digital Data
• Data encoding and decoding technique to
represent data using the properties of
analog waves
• Modulation: the conversion of digital
signals to analog form
• Demodulation: the conversion of analog
data signals back to digital form
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Modem
• An acronym for modulator-demodulator
• Uses a constant-frequency signal known as
a carrier signal
• Converts a series of binary voltage pulses
into an analog signal by modulating the
carrier signal
• The receiving modem translates the analog
signal back into digital data
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Methods of Modulation
• Amplitude modulation (AM) or amplitude
shift keying (ASK)
• Frequency modulation (FM) or frequency
shift keying (FSK)
• Phase modulation or phase shift keying
(PSK)
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Voice Grade Modems
• Designed for digital transmission over
ordinary phone lines
• Uses 4-kHz bandwidth
• Adheres to ITU-T standards
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Cable Modems
• Permits Internet access over cable television networks.
• ISP is at or linked by high-speed line to central cable
office
• Cables used for television delivery can also be used to
deliver data between subscriber and central location
• Upstream and downstream channels are shared among
multiple subscribers, time-division multiplexing
technique
• Splitter is used to direct TV signals to a TV and the data
channel to a cable modem
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Cable Modems
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Asymmetric Digital
Subscriber Line (ADSL)
• New modem technology for high-speed digital transmission over
ordinary telephone wire.
• At central office, a combined data/voice signal is transmitted over a
subscriber line
• At subscriber’s site, twisted pair is split and routed to both a PC and a
telephone
– At the PC, an ADSL modem demodulates the data signal for the PC.
– At the telephone, a microfilter passes the 4-kHz voice signal.
• The data and voice signals are combined on the twisted pair line
using frequency-division-multiplexing techniques.
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ADSL Modem Application
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Digital Encoding
of Analog Data
• Evolution of telecommunications networks to digital
transmission and switching requires voice data in digital
form
• Best-known technique for voice digitization is pulse-code
modulation (PCM)
• The sampling theorem: If a signal is sampled at regular
intervals of time and at a rate higher than twice the
significant signal frequency, the samples contain all the
information of the original signal.
• Good-quality voice transmission can be achieved with a
data rate of 8 kbps
• Some videoconference products support data rates as low
as 64 kbps
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Pulse-Code Modulation Example
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Analog Encoding
of Analog Information
• Voice-generated sound wave can be represented
by an electromagnetic signal with the same
frequency components, and transmitted on a
voice-grade telephone line.
• Modulation can produce a new analog signal that
conveys the same information but occupies a
different frequency band
– A higher frequency may be needed for effective
transmission
– Analog-to-analog modulation permits frequencydivision multiplexing
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Analog Sine-Wave Signals
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Asynchronous Transmission
• Avoids timing problem by not sending long,
uninterrupted streams of bits
• Data transmitted one character at a time, where
each character is 5 to 8 bits in length.
• Timing or synchronization must only be
maintained within each character; the receiver
has the opportunity to resynchronize at the
beginning of each new character.
• Simple and cheap but requires an overhead of 2
to 3 bits per character
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Asynchronous Transmission
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Synchronous Transmission
• Block of bits transmitted in a steady stream
without start and stop codes.
• Clocks of transmitter and receiver must somehow
be synchronized
– Provide a separate clock line between transmitter and
receiver; works well over short distances,
– Embed the clocking information in the data signal.
• Each block begins with a preamble bit pattern
and generally ends with a postamble bit pattern
• The data plus preamble, postamble, and control
information are called a frame
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Synchronous Transmission
• More efficient than asynchronous
transmission
• Preamble, postamble and control
information are typically < 100 bits
• Introduces the need for error checking
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Error Control Process
• All transmission media have potential for
introduction of errors
• All data link layer protocols must provide
method for controlling errors
• Error control process has two components
– Error detection: redundancy introduced so that the
occurrence of an error will be detected
– Error correction: receiver and transmitter cooperate
to retransmit frames that were in error
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Error Detection: Parity Bits
• Bit added to each character to make all bits
add up to an even number (even parity) or
odd number (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit
character=12.5%)
• Noise impulses are often long enough to
destroy more than one bit
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Error Detection: Cyclic
Redundancy Check (CRC)
• Data in frame treated as a single binary
number, divided by a unique prime binary,
and remainder is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33bit divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected
are 2-N
• Overhead is low (1-3%)
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Error Detection Process
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