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Chapter 3 Basic Data Communication Technology

Chapter 3 Basic Data Communication Technology. Business Impact of Modem Technology Modems Data Compression Communication Errors. Business Impact of Modem Technology. Faster Efficient Reliable Secure. Faster. New improved model Digital transmission Benefits of Digital Transmission

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Chapter 3 Basic Data Communication Technology

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  1. Chapter 3Basic Data Communication Technology • Business Impact of Modem Technology • Modems • Data Compression • Communication Errors

  2. Business Impact of Modem Technology • Faster • Efficient • Reliable • Secure

  3. Faster • New improved model • Digital transmission • Benefits of Digital Transmission • Better data integrity (detect & correct error) • Higher capacity cables (fiber-optic) • Easier integration (voice, data, video, etc.) • Better security and privacy (encrypt data) • Lower cost (large-scale integrated circuitry)

  4. Efficient • Data compression • Backward compatibility

  5. Reliable • Error prevention • Error detection • Error correction • Error control standard • Flow control • Line-failure backup

  6. Secure • Access control • Logical: password, callback • Physical • Encryption

  7. Modems • Standard • Functions • Interfaces • Handshaking • Types • Selection Criteria

  8. Standards • V. by ITU-T (International Telecommunications Union-Telephony Sector) • V. bis & V.ter • MNP standard (Microcom networking protocols) • Modem standard: V.34 (28.8 Kbps) • Data compression standard: V.42bis & MNP 5 • Error control standard: V.42 & MNP 4

  9. Modem Functions • Convert signals • Reverse channel for signal one another • Auto dial/auto answer • Modem diagnostics for accuracy checking

  10. V.34 Technical Innovation • Multiple baud rates/carrier • Greater baud rate • Auxiliary management channel • Asymmetric transmit/receive speed • Adaptive line probing • Precoding and nonlinear encoding • Fallback/fallforward • Trellis-coded modulation • V.8 training specification

  11. Modem Interfaces • Between modem and line • Two- to four-wire cable • RJ-11 plug • Between DTE and modem • RS-232 interface • Software • Hayes AT command set

  12. Modems - Handshaking • Exchange signals between modems • Test the characteristics of circuit and quality • determine baud rate, modulation technique and error control

  13. Modem Types - I • Simplex, half-duplex, full-duplex transmission modems • Asynchronous or synchronous transmission modems • Acoustically coupled modems (portable) • Limited distance modems/short haul modems (less than 20 miles) • Modem eliminators/null modems (cable less than several thousand feet)

  14. Modem Types - II • Facsimile modems • Error correction and data compression • Modem for fiber-optics circuits • Digital-electrical to digital-optical • Cable modems • DTE to cable television system cable • Internet and cable television system problems • Cellular modems

  15. Modem Types - III • Wired/wireless modems • ISDN modems • CSU/DSU • Enhanced V.34 modems • 56-kbps modems • Digital simultaneous voice data (DSVD) • Analog simultaneous voice data (ASVD) • Asymmetric digital subscriber line (ADSL)

  16. Modem Selection Criteria • Digital or analog signals • Asynchronous or synchronous • Speed • Distance • Type of line • Cost • Use compatible modems for both end

  17. Data Compression/Compaction • Purpose: to improve transmission efficiency • Types • Character compression • Run length coding • Character stripping • Combination of the above three • Consideration • throughput • Storage and transmission cost • Hardware cost & software cost

  18. Circuit Errors • Background noise (electrical phenomenon) • Impulse noise (spike) • White noise • Attenuation • Envelope delay distortion (different propagation delay) • Phase jitter • Echo • Crosstalk • Dropouts

  19. Error Prevention • Line conditioning for leased lines reduce • Attenuation • Envelope delay distortion • Noise • Distortion • Methods • Shielding or insulation • Improving connection • Equalizer (delay distortion) • Echo suppressor • Electronic versus mechanical equipment • Repeaters or amplifiers (attenuation) • Adaptive protocols: adaptive size packet protocol & dynamic speed shift

  20. Error Detection • Echo checking every character • Vertical redundancy checking (VRC) or parity checking • Longitudinal redundancy checking (LRC) or block check character (BCC) • Checksum • Cyclic redundancy checking (CRC) • CRC-16 or CRC-32

  21. Parity & Longitudinal Redundancy Checks Example • Date bits VRC • 1 1 1 1 1 1 0 • 0 0 0 0 0 0 0 • 1 0 1 0 1 1 0 • 0 0 0 1 0 1 0 • 0 0 0 0 1 0 1 • 0 0 1 0 0 0 1 • 0 1 0 1 0 1 1 • 0 0 1 1 1 0 1 LRC

  22. Checksum Example - I • Add ASCII decimal face value of the 128 characters in the block • 7 5 6 4 3 2 1 0 • 128 64 32 16 8 4 2 1 • 1 0 0 0 0 0 1 (A) • 64+1=65 • 128 A is 128*65=8320 • Divide the number by 255 • 8320/255=32 r 160

  23. Checksum Example - I • Take the remainder and express in binary format • 7 5 6 4 3 2 1 0 • 128 64 32 16 8 4 2 1 • 1 0 1 0 0 0 0 0 • Transmit 10100000 (checksum)

  24. Error Correction • Equipment and circuit requirement • Buffer • Reverse channel or D channel • Retransmission - automatic repeat request (ARQ) • Methods • Discrete ARQ - ACK/NAK: long data block & low error rate • Continuous ARQ - sliding window protocol with block sequence number: long propagation time • Selective ARQ • Forward error correction (FEC): simplex transmission • Trillis-Coded modulation • High cost (extra bits & codes)

  25. Communication Software Analysis • Easy to use • Efficient • Cost-effective • Compatibility • Support overall business objectives

  26. File Transfer Protocols - I • XMODEM (simple, less reliable error checking) • XMODEM-CRC (more reliable) • XMODEM-1K (more efficient) • XMODEM-G (G for streaming protocol) • YMODEM-CRC-16 (Reliable, multiple files transfer) • YMODEM-Batch • ZMODEM (fast, failure recovery)

  27. File Transfer Protocols - II • X.PC (pack switching network, multiple sessions on one circuit) • KERMIT (reliable, not very fast, PC & mainframe) • Sliding window Kermit (fast) • Point-to-point Protocol (PPP) • PC to a TCP/IP network • Authentication, compression, error correction, & packet sequencing

  28. File Transfer Protocols - III • Serial Line Internet Protocol (SLIP) • Dial-up or leased lines • No error correction

  29. Selection Criteria for File Transfer Protocol • Error checking • Error Correction • Block size • Batch capability • Multiple platforms • Auto recovery

  30. Assignment • Review chapters 1-3 • Read chapter 4 • Prepare exam 1

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