Chapter Two
Fundamentals of Data and Signals
Data Communications and Computer
Networks: A Business User's Approach
Eighth Edition
Introduction
• Data are entities that convey meaning (computer
files, music on CD, results from a blood gas
analysis machine)
• Signals are the electric or electromagnetic
encoding of data (telephone conversation, web
page download)
• Computer networks and data/voice
communication systems transmit signals
• Data and signals can be analog or digital
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Introduction (continued)
Table 2-1 Four combinations of data and signals
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Data and Signals
• Data are entities that convey meaning within a
computer or computer system
• Signals are the electric or electromagnetic
impulses used to encode and transmit data
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Analog vs. Digital
• Data and signals can be either analog or digital
• Analog is a continuous waveform, with examples
such as (naturally occurring) music and voice
• It is harder to separate noise from an analog
signal than it is to separate noise from a digital
signal (see the following two slides)
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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
• Digital is a discrete or non-continuous waveform
• Something about the signal makes it obvious
that the signal can only appear in a fixed number
of forms (see next slide)
• Noise in digital signal
– You can still discern a high voltage from a low
voltage
– Too much noise – you cannot discern a high
voltage from a low voltage
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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
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Analog vs. Digital (continued)
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Fundamentals of Signals
• All signals have three components:
– Amplitude
– Frequency
– Phase
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Fundamentals of Signals – Amplitude
• Amplitude
– The height of the wave above or below a given
reference point
– Amplitude is usually measured in volts
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Fundamentals of Signals – Amplitude
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Fundamentals of Signals – Frequency
• Frequency
– The number of times a signal makes a complete cycle
within a given time frame; frequency is measured in Hertz
(Hz), or cycles per second (period = 1 / frequency)
– Spectrum – Range of frequencies that a signal spans from
minimum to maximum
– Bandwidth – Absolute value of the difference between the
lowest and highest frequencies of a signal
– For example, consider an average voice
• The average voice has a frequency range of roughly 300 Hz
to 3100 Hz
• The spectrum would be 300 – 3100 Hz
• The bandwidth would be 2800 Hz
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Fundamentals of Signals – Frequency
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Fundamentals of Signals – Phase
• Phase
– The position of the waveform relative to a given
moment of time or relative to time zero
– A change in phase can be any number of angles
between 0 and 360 degrees
– Phase changes often occur on common angles,
such as 45, 90, 135, etc.
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Fundamentals of Signals – Phase
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Fundamentals of Signals
• Phase
– If a signal can experience two different phase
angles, then 1 bit can be transmitted with each
signal change (each baud)
– If a signal can experience four different phase
angles, then 2 bits can be transmitted with each
signal change (each baud)
– Note: number of bits transmitted with each signal
change = log2 (number of different phase angles)
– (You can replace “phase angles” with “amplitude levels” or
“frequency levels”)
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Data Codes
• The set of all textual characters or symbols and
their corresponding binary patterns is called a
data code
• There are three common data code sets:
– EBCDIC
– ASCII
– Unicode
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EBCDIC
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ASCII
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Unicode
• Each character is 16 bits
• A large number of languages / character sets
• For example:
– T equals 0000 0000 0101 0100
– r equals 0000 0000 0111 0010
– a equals 0000 0000 0110 0001
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Data and Signal Conversions In Action:
Two Examples
• Let us transmit the message “Sam, what time is
the meeting with accounting? Hannah.”
• This message leaves Hannah’s workstation and
travels across a local area network
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Data and Signal Conversions In Action:
Two Examples (continued)
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Data and Signal Conversions In Action:
Two Examples (continued)
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Data and Signal Conversions In Action:
Two Examples (continued)
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Chapter 6
Error Control
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Error Control
• Once an error is detected, what is the receiver
going to do?
– Do nothing (simply toss the frame or packet)
– Return an error message to the transmitter
– Fix the error with no further help from the
transmitter
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Do Nothing (Toss the Frame/Packet)
• Seems like a strange way to control errors but
some lower-layer protocols such as frame relay
perform this type of error control
• For example, if frame relay detects an error, it
simply tosses the frame
– No message is returned
• Frame relay assumes a higher protocol (such as
TCP/IP) will detect the tossed frame and ask for
retransmission
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Return A Message
• Once an error is detected, an error message is
returned to the transmitter
• Two basic forms:
– Stop-and-wait error control
– Sliding window error control
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Stop-and-Wait Error Control
• Stop-and-wait is the simplest of the error control
protocols
• A transmitter sends a frame then stops and
waits for an acknowledgment
– If a positive acknowledgment (ACK) is received,
the next frame is sent
– If a negative acknowledgment (NAK) is received,
the same frame is transmitted again
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Stop-and-Wait Error Control (continued)
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Sliding Window Error Control
• These techniques assume that multiple frames
are in transmission at one time
• A sliding window protocol allows the transmitter
to send a number of data packets at one time
before receiving any acknowledgments
– Depends on window size
• When a receiver does acknowledge receipt, the
returned ACK contains the number of the frame
expected next
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
• Older sliding window protocols numbered each
frame or packet that was transmitted
• More modern sliding window protocols number
each byte within a frame
• An example in which the packets are numbered,
followed by an example in which the bytes are
numbered:
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
• Notice that an ACK is not always sent after each
frame is received
– It is more efficient to wait for a few received
frames before returning an ACK
• How long should you wait until you return an
ACK?
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Sliding Window Error Control (continued)
• Using TCP/IP, there are some basic rules concerning
ACKs:
– Rule 1: If a receiver just received data and wants to send
its own data, piggyback an ACK along with that data
– Rule 2: If a receiver has no data to return and has just
ACKed the last packet, receiver waits 500 ms for another
packet
• If while waiting, another packet arrives, send the ACK
immediately
– Rule 3: If a receiver has no data to return and has just
ACKed the last packet, receiver waits 500 ms
• No packet, send ACK
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
• What happens when a packet is lost?
– As shown in the next slide, if a frame is lost, the
following frame will be “out of sequence”
• The receiver will hold the out of sequence bytes in
a buffer and request the sender to retransmit the
missing frame
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
• What happens when an ACK is lost?
– As shown in the next slide, if an ACK is lost, the
sender will wait for the ACK to arrive and
eventually time out
• When the time-out occurs, the sender will resend
the last frame
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Sliding Window Error Control (continued)
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