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Chapter 2 : Business Information
Business Data Communications, 6e
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Analog Information
• Continuous signal• Expressed as an oscillation (sine wave
format) of frequency• Information rate and channel capacity are
measured in hertz (Hz) of bandwidth (1 Hz = 1 cycle per second).
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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 (Hz): The number of times a wave
cycle occurs in one second (commonly used measure of frequency)
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Types of Information
• Audio• Data• Image• Video
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Understanding Audio
• What makes sound? Vibration of air• How can we record that vibration?• How can we convert that to an electrical
signal?
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Digital Audio
• 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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Networking Implications for Voice Communication
• Requires powerful, flexible intralocation facility, and access to outside services (e.g. telcos)
• In-house alternatives– PBX– Centrex
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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; data is grouped for transmission
• Data encoded in strings– ASCII, IRA, UTF, etc
• Data is often redundant
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Parity
• Used to determine if a transmission error has occurred
• 7 bits are typically used to represent one character, the 8th bit represents an odd or even parity bit
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Data Networking Implications
• Vary significantly based on application and data types
• Response time often a key component
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Understanding Images
• Vector graphics– Collection of straight and curved line
segments– Image described as collection of segments
• Raster graphics– Two-dimensional array of “spots” (pixels)– Also called “bitmap” image– Used for computer and facsimile image
processing
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•Uses binary codes to represent object type, size and orientation•The most popular scheme to represent color is RGB (red-green-blue)•Colors are represented as a mixture of the proportion and intensity of red, green and blue colors
Vector Graphics
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Image and Document Formats
• Common Raster Formats– JPEG– GIF
• Common Document Formats– PDF– Postscript– Both include text and graphics
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Networking Implications for Image Data
• More pixels=better quality=larger size• More compression=reduced quality=increased
speed– “Lossy” gives from 10:1 to 20:1 compression– “Lossless” gives less than 5:1
• Format (vector vs bitmapped/raster) affects size and therefore bandwidth requirements
• 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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Interlacing
• Provides flicker free images
• Odd-numbered and even-numbered scan lines are scanned separately
• Alternated on successive scans
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Digital Video
• Refers to the capture, manipulation, and storage of video in digital formats.
• Data is either compressed or uncompressed• Images from a digital video cameras
typically are a series of digital photographs (commonly at a rate of 30 frames per second)
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Response Time
• User response time – the time between the moment a user receives a complete reply to one command and enters the next command
• System response time – the time spand between the moment the user enters a command and the moment a complete response is displayed
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Network Response Time
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Throughput