Download Business Data Communications 4e

Chapter 16:
Data Communication
Fundamentals
Business Data Communications, 5e
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 (eg 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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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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Data, Signal, and
Transmission Matrix
A
Data
D
D
A
A
D
Transmission
System
Signal
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Advantages of Digital
Transmission
•
•
•
•
The signal is exact
Signals can be checked for errors
Noise/interference are easily filtered out
A variety of services can be offered over
one line
• Higher bandwidth is possible with data
compression
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Why Use Analog Transmission?
•
•
•
•
Already in place
Significantly less expensive
Lower attenuation rates
Fully sufficient for transmission of voice
signals
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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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Amplitude Shift Keying (ASK)
• In radio transmission, known as amplitude
modulation (AM)
• The amplitude (or height) of the sine wave
varies to transmit the ones and zeros
• Major disadvantage is that telephone lines
are very susceptible to variations in
transmission quality that can affect
amplitude
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ASK Illustration
1
0
0
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13
Frequency Shift Keying (FSK)
• In radio transmission, known as frequency
modulation (FM)
• Frequency of the carrier wave varies in
accordance with the signal to be sent
• Signal transmitted at constant amplitude
• More resistant to noise than ASK
• Less attractive because it requires more
analog bandwidth than ASK
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FSK Illustration
1
1
0
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Phase Shift Keying (PSK)
• Also known as phase modulation (PM)
• Frequency and amplitude of the carrier
signal are kept constant
• The carrier signal is shifted in phase
according to the input data stream
• Each phase can have a constant value, or
value can be based on whether or not phase
changes (differential keying)
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PSK Illustration
0
0
1
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Differential Phase Shift Keying
(DPSK)
0
1
1
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Voice Grade Modems
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
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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 (see Chapter 17)
• Splitter is used to direct TV signals to a TV and the data
channel to a cable modem
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Cable Modem Layout
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Asymmetric Digital
Subscriber Line (ADSL)
• New modem technology for high-speed digital transmission over
ordinary telephone wire.
• Telephone central office can provide support for a number of ISPs,
• 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 (Chapter 17)
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DSL Modem Layout
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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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Converting Samples to Bits
• Quantizing
• Similar concept to pixelization
• Breaks wave into pieces, assigns a value in
a particular range
• 8-bit range allows for 256 possible sample
levels
• More bits means greater detail, fewer bits
means less detail
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Codec
• Coder/Decoder
• Converts analog signals into a digital form
and converts it back to analog signals
• Where do we find codecs?
–
–
–
–
Sound cards
Scanners
Voice mail
Video capture/conferencing
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Digital Encoding
of Digital Data
• Most common, easiest method is different
voltage levels for the two binary digits
• Typically, negative=1 and positive=0
• Known as NRZ-L, or nonreturn-to-zero
level, because signal never returns to zero,
and the voltage during a bit transmission is
level
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Differential NRZ
• Differential version is NRZI (NRZ, invert
on ones)
• Change=1, no change=0
• Advantage of differential encoding is that
it is more reliable to detect a change in
polarity than it is to accurately detect a
specific level
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Problems With NRZ
• Difficult to determine where one bit ends
and the next begins
• In NRZ-L, long strings of ones and zeroes
would appear as constant voltage pulses
• Timing is critical, because any drift results
in lack of synchronization and incorrect bit
values being transmitted
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Biphase Alternatives to NRZ
• Require at least one transition per bit time,
and may even have two
• Modulation rate is greater, so bandwidth
requirements are higher
• Advantages
– Synchronization due to predictable transitions
– Error detection based on absence of a
transition
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Manchester Code
•
•
•
•
Transition in the middle of each bit period
Transition provides clocking and data
Low-to-high=1 , high-to-low=0
Used in Ethernet
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Differential Manchester
•
•
•
•
Midbit transition is only for clocking
Transition at beginning of bit period=0
Transition absent at beginning=1
Has added advantage of differential
encoding
• Used in token-ring
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Digital Encoding Illustration
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Digital Interfaces
• The point at which one device connects to
another
• Standards define what signals are sent, and
how
• Some standards also define physical
connector to be used
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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 (Chapter 17)
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Asynchronous and Synchronous
Transmission
• For receiver to sample incoming bits
properly, it must know arrival time and
duration of each bit that it receives
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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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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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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
– Error correction
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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%)
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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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