2. CIS-325: Data Communications1 CIS-325 Data Communications Dr. L. G. Williams, Instructor

3. CIS-325: Data Communications2 Chapter Two Business Information

4. CIS-325: Data Communications3 Data Forms u Digital u Analog

5. CIS-325: Data Communications4 Digital Data u Represented as a sequence of discrete symbols from a finite “alphabet” of text and/or digits u Rate and capacity of a digital channel measured in bits per second (bps) u Digital data is binary: uses 1s and 0s to represent everything u Binary digits can be represented as voltage pulses

6. CIS-325: Data Communications5 Basic Digital Terms u Bit: digit in a binary number –Can only have a value of 1 or 0 –1 is a 1-bit number (=1 in base 10) –10 is a 2-bit number (=2 in base 10) –10011001 is an 8-bit number (=153 in base 10) u Byte: eight bits

7. CIS-325: Data Communications6 Analog Data u Continuous signal u Expressed as an oscillation (sine wave format) of frequency u Example: Analog electrical signal generated by a microphone in response to continuos changes in air pressure that make up sounds

8. CIS-325: Data Communications7 Basic Analog Terms u Wave frequency: Number of times a cycle occurs in given time period u Wave amplitude: Height of a wave cycle u Hertz: The number of times a wave cycle occurs in one second (commonly used measure of frequency)

9. CIS-325: Data Communications8 Analog Signaling u represented by sine waves time (sec) amplitude (volts) 1 cycle frequency (hertz) = cycles per second phase difference

10. CIS-325: Data Communications9 Analog Voice Communication u Primarily used for transmission of human voice (telephony) u Microphone captures voice vibrations, converts them to waves than can be expressed through variations of voltage u Examples –Telephone (3000Hz) –Hi-Fi Sound (15,000Hz; approximate range of human ear) –Compact Disc (20,000Hz for each of two channels)

11. CIS-325: Data Communications10 Digital Voice Communication u For good representation, must sample amplitude at a rate of at least twice the maximum frequency u Measured in samples per second, or smp/sec u Telephone quality: 8000smp/sec, each sample using 8 bits –8 bits * 8000smp/sec = 64kbps to transmit u CD audio quality: 44000smp/sec, each sample using 16 bits –16 bits * 44000smp/sec = 1.41mbps to transmit clearly

12. CIS-325: Data Communications11 Voice Means Telephones u Private Branch Exchange (PBX) –On-premises switch –Single connection to outside u Centrex –Offered by phone company –Each phone line runs to outside switch –Appears to work just like PBX

13. CIS-325: Data Communications12 Data Communication u In this context, we mean data stored on computers u Already digital, so no conversion necessary u Bandwidth usually affects speed, but not quality u Each character represented by a byte u Make transmission faster through compression

14. CIS-325: Data Communications13 Image Transfer u Pictures, charts, drawings u Used by fax, CAD, publishing u Images result in large files

15. CIS-325: Data Communications14 Converting Images u Break image up into small units –More units means more detail –Units called pixels

16. CIS-325: Data Communications15 Image Quality Issues u More pixels=better quality u More compression=reduced quality –“Lossy” gives from 10:1 to 20:1 compression –“Lossless” gives less than 5:1 u Less compression=reduced speed of transfer u Choices in imaging technology, conversion, and communication all affect end-user’s satisfaction

17. CIS-325: Data Communications16 Video Communication u Sequences of images over time u Same concept as image, but with the dimension of time added u Significantly higher bandwidth requirements in order to send images (frames) quickly enough u Similarity of adjacent frames allows for high compression rates

18. CIS-325: Data Communications17 Bandwidth Requirements u Review chart on page 27 u What happens when bandwidth is insufficient? u Poor quality or slow transmission u Response time u How long does it take to become impatient? u Is data communication ever “fast enough”?