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Chapter 8

Chapter 8. Data Transmission and Modems. Agenda. Circuit Mode of transmission Digital transmission Analog transmission for digital signals - modems Methods Classification Interfaces Functions Selection Criteria. Circuit. Signal rate

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Chapter 8

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  1. Chapter 8 Data Transmission and Modems

  2. Agenda • Circuit • Mode of transmission • Digital transmission • Analog transmission for digital signals - modems • Methods • Classification • Interfaces • Functions • Selection Criteria

  3. Circuit • Signal rate • No. of signal changes (amplitude, frequency, or phase) on a circuit per second • Baud • Types: dibits, tribits, and quadbits • Speed • No. of bits that a circuit can carry in 1 second • Bits per second (bps) used for measurement

  4. Mode of Transmission • Data flow • Physical connection • Timing

  5. Data Flow - US • Simplex transmission • Television and radio • Half-duplex (HDX) transmission • CB radio, terminal • Full-duplex (FDX) transmission • Telephone, computer to computer

  6. Physical Connection • Parallel transmission • Fast • Simple • Line cost • Serial transmission • Complicated transmitter and receive • Decomposing and reconstructing

  7. Timing • Asynchronous transmission • Start/stop bits for character synchronization • Mark (1or stop) /space (0 or start ) bits for bit synchronization • Simple, inexpensive, slow speed transmission • For personal computer • Synchronous transmission • Clock circuitry • One to four synchronization characters for each block of data • Large amount of data on dedicated line

  8. Digital Signals • Unipolar (positive voltage for 1, no voltage for 0) • Bipolar, nonreturn-to zero (NRZ) • Manchester coding (low-to-high is 1, high-to- low is 0) • Differential Manchester (no transition at the beginning of the bit period is 1, second transition at the beginning of the bit period is 0) • Benefits: self-clocking, and error detection • Bipolar, return-to-zero

  9. 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)

  10. Digital Transmission of Analog Signals • Quantization • Quantizing noise or digitizing distortion • Codec (coder/decoder): analog-to-digital (A/D) converter & digital-to-analog (D/A) converter • Methods • Pulse code modulation (256 integers, 8000 per second) • Adaptive differential pulse code modulation (difference) • Delta modulation (1 for +, 0 for -)

  11. Digital Transmission of Digital Signals • Digital transmitter/receiver (data service unit/channel service unit (DSU/CSU)) • Simpler & cheaper • Transmitter for shaping & timing the signal, interface between DTE and line • Receiver for protection of excessive voltage, diagnostic and testing

  12. Analog Transmission of Digital signals (Modems – I) • Modulation and Demodulation • Receiver (modulation), control unit (auto dial & auto answer), transmitter (demodulation), & power supply • Equalizer in transmitter & adaptive equalizer in receiver (adjust to fit the characteristics of the telephone line) • Methods • Frequency shift keying (FSK) • Phase shift keying (PSK): 180 degree (1 bit), 90 degree (2 bits), 45 degree (3 bits) • Differential phase shift keying (DPSK) • Quadrature amplitude modulation (QAM): combination of 8 phases & 4 relative amplitudes

  13. Modems - II • Methods • Trellis code modulation: signal processor, error correction for error free transmission, high speed • Standards by ITU-T • V.32 for 9.6 kbps & V.32bis for 14.4 kbps • V.34 for 28.8 kbps & V.34+ or V.34bis for 33.6 kbps • V.90 for 56 kbps

  14. Modems - III • Handshaking • Exchange signals between modems • Test the characteristics of circuit and quality • determine baud rate, modulation technique and error control • Data compression • V.42bis by ITU-T • Microcom Networking Protocol (MNP 7 or 10) by Microcom, Inc.

  15. Modem Classification • 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)

  16. Modem Classification - 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 • Data Over Cable Service Interface Specification (DOCSIS) by CableLabs • For large files and not logon process

  17. Modem Classification - III • Reverse channel • Slow speed reverse channel for signaling one another • Auto dial/auto answer • Hayes Microcomputer Product, Inc. • Command AT for attention • Internet service provider usage • Dial-backup public telephone network connection for leased telephone line • Internal modem • Single circuit card • No switch to change the parameters and operations

  18. Modem Interfaces • Between modem and line • Between DTE and modem

  19. Between Modem and Line • Two- to four-wire cable • RJ-11 plug

  20. Between Modem and DTE • RS-232-C interface • V.24 interface • RS-232-D interface • RS-449 interface • RS-336 interface • X.21 interface • X.21bis interface • Current loop interface

  21. RS-232-C Interface • By EIA and ITU-T V.24 • Characteristics • 25-pin connector at both end • digital and serial • Female for DCE and male for DTE • Pins 2 and 3 to pass data, other for signaling • Limitations • 50 -100 feet cable length • 2,400 - 9,600 bps • Hard to difference 0 and 1 when speed is high and cable is long

  22. RS-232-D Interface • Specifications for the 25-pin connector • Addition of a shield on the cable • Redefinition of protective ground and some pins

  23. RS-449 Interface • 37 pins • 4,000 feet cable • Not well accepted

  24. RS-336 Interface • Automatic dialing from computer to remote terminals for data collection • Not for high speed transmission

  25. X. Interfaces • X.21 interface • Digital connection to a digital public telephone network • X.21bis interface • Terminal to packet switch network via analog line

  26. Current Loop Interface • Represent 1 and 0 by presence or absence of an electrical current • Simple and inexpensive • Nonstandard • Teletywriter system

  27. Other Modem Functions • Reverse channel for signal one another to determine the speed • Auto dial/auto answer • Modem diagnostics • Loop back for accuracy checking

  28. Modem Selection Criteria • Digital or analog signals • Asynchronous or synchronous • Speed • Distance • Type of line • Cost • Functions

  29. Spread Spectrum • Analog signal for analog or digital data • Frequency hopping ad direct sequence • Generated by Pseudo random number generator

  30. Points to Remember • Circuit • Mode of transmission • Digital transmission • Analog transmission for digital signals - modems • Methods • Classification • Interfaces • Functions • Selection Criteria

  31. Discussion • Design the data transmission and modem specification for a bank

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