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CIS-325
Data Communications
Dr. L. G. Williams, Instructor
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Chapter Two
Business Information
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Data Forms
 Digital
 Analog
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Digital Data
Represented as a sequence of discrete symbols
from a finite “alphabet” of text and/or digits
 Rate and capacity of a digital channel measured in
bits per second (bps)
 Digital data is binary: uses 1s and 0s to represent
everything
 Binary digits can be represented as voltage pulses

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Basic Digital Terms
 Bit:
–
–
–
–
digit in a binary number
Can only have a value of 1 or 0
1 is a 1-bit number (=1 in base 10)
10 is a 2-bit number (=2 in base 10)
10011001 is an 8-bit number (=153 in base 10)
 Byte:
eight bits
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Analog Data
 Continuous
signal
 Expressed as an oscillation (sine wave
format) of frequency
 Example: Analog electrical signal generated
by a microphone in response to continuos
changes in air pressure that make up sounds
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Basic Analog Terms
 Wave
frequency: Number of times a cycle
occurs in given time period
 Wave amplitude: Height of a wave cycle
 Hertz: The number of times a wave cycle
occurs in one second (commonly used
measure of frequency)
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amplitude (volts)
Analog Signaling
 represented
by sine waves
1 cycle
phase
difference
time
(sec)
frequency (hertz)
= cycles per second
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Analog Voice Communication



Primarily used for transmission of human voice
(telephony)
Microphone captures voice vibrations, converts them to
waves than can be expressed through variations of voltage
Examples
– Telephone (3000Hz)
– Hi-Fi Sound (15,000Hz; approximate range of human
ear)
– Compact Disc (20,000Hz for each of two channels)
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Digital Voice Communication
For good representation, must sample amplitude at
a rate of at least twice the maximum frequency
 Measured in samples per second, or smp/sec
 Telephone quality: 8000smp/sec, each sample
using 8 bits

– 8 bits * 8000smp/sec = 64kbps to transmit

CD audio quality: 44000smp/sec, each sample
using 16 bits
– 16 bits * 44000smp/sec = 1.41mbps to transmit clearly
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Voice Means Telephones
 Private
Branch Exchange (PBX)
– On-premises switch
– Single connection to outside
 Centrex
– Offered by phone company
– Each phone line runs to outside switch
– Appears to work just like PBX
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Data Communication
 In
this context, we mean data stored on
computers
 Already digital, so no conversion necessary
 Bandwidth usually affects speed, but not
quality
 Each character represented by a byte
 Make transmission faster through
compression
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Image Transfer
 Pictures,
charts, drawings
 Used by fax, CAD, publishing
 Images result in large files
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Converting Images
 Break
image up into small units
– More units means more detail
– Units called pixels
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Image Quality Issues
 More
pixels=better quality
 More compression=reduced quality
– “Lossy” gives from 10:1 to 20:1 compression
– “Lossless” gives less than 5:1
 Less
compression=reduced speed of transfer
 Choices in imaging technology, conversion,
and communication all affect end-user’s
satisfaction
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Video Communication
 Sequences
of images over time
 Same concept as image, but with the
dimension of time added
 Significantly higher bandwidth
requirements in order to send images
(frames) quickly enough
 Similarity of adjacent frames allows for
high compression rates
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Bandwidth Requirements
 Review
chart on page 27
 What happens when bandwidth is
insufficient?
 Poor quality or slow transmission
 Response time
 How long does it take to become impatient?
 Is data communication ever “fast enough”?
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