Chapter 2 : Business Information

Business Data Communications, 6e

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).

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)

Types of Information

• Audio• Data• Image• Video

Understanding Audio

• What makes sound? Vibration of air• How can we record that vibration?• How can we convert that to an electrical

signal?

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

Networking Implications for Voice Communication

• Requires powerful, flexible intralocation facility, and access to outside services (e.g. telcos)

• In-house alternatives– PBX– Centrex

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

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

Data Networking Implications

• Vary significantly based on application and data types

• Response time often a key component

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

•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

Image and Document Formats

• Common Raster Formats– JPEG– GIF

• Common Document Formats– PDF– Postscript– Both include text and graphics

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

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

Interlacing

• Provides flicker free images

• Odd-numbered and even-numbered scan lines are scanned separately

• Alternated on successive scans

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)

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

Network Response Time

Throughput