Business data communications 5e by stallings chapter 02

Chapter 2 : Business Information
Business Data Communications, 5e


Analog Data
• 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 (perform switching in the customer side)

– Centrex (perform switching in the telcos)


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 encoded in strings
– ASCII, IRA, UTF, etc

• Data is often redundant (why?)


Data Networking Implications
• Vary significantly based on application and data
types
– Example: Transferring financial transactions data
from CitiBank-Taipei to CitiBank-NY (Not just realtime, but also . . . )
– Daily reports vs monthly reports
– DHL parcel tracking

• Response time often a key component
• What sort of technologies are needed to support
such applications?


Understanding Images

• Vector graphics
– Collection of straight and curved line segments
– Image described as a collection of segments

• Raster graphics
– Two-dimensional array of “spots” (pixels)
– Also called “bitmap” image

• In terms of memory, which kind of graphic needs
more memory?


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


Performance Measure:
Response Time
• User response time
• System response time
• Network transfer time (throughput)


Bandwidth Requirements
• Review Figure 2.7
• What happens when bandwidth is
insufficient?
• How long does it take to become impatient?

– Let us feel about that . . .
– YouTube

• Is data communication ever “fast enough”?