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)

• Audio • Data • Image • Video Types of Information

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 8 th 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

Vector Graphics

• 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

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