Chapter 2 : Business Information Business Data Communications, 5 e
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: 8000 smp/sec, each sample using 8 bits – 8 bits * 8000 smp/sec = 64 kbps to transmit • CD audio quality: 44000 smp/sec, each sample using 16 bits – 16 bits * 44000 smp/sec = 1. 41 mbps 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 1 s and 0 s 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 Citi. Bank-Taipei to Citi. Bank-NY (Not just real-time, 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. . . – You. Tube • Is data communication ever “fast enough”?
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