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Chapter 5

Chapter 5. Data Communication Fundamentals. Analog and Digital Data Communications. Data Entities that convey meaning or information Transmission

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Chapter 5

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  1. Chapter 5 Data Communication Fundamentals

  2. Analog and Digital Data Communications • Data • Entities that convey meaning or information • Transmission • The communication of data across a computer network by the propagation and processing of signals The way in which the electromagnetic signals are encoded to convey data determines the efficiency and reliability of the transmission • Signal • Electric or electromagnetic representation of data • Signaling • The physical propagation of the signal along a communication medium

  3. Analog Data/Digital Data • Analog data • Continuous values on some interval • Voice and video • Data collected by sensors, such as temperature and pressure • Digital data • Discrete values • Text, integers, binary data • Signals are used to encode and transmit data

  4. Analog Signals/Digital Signals • Analog signal • Continuously varying electromagnetic wave that may be transmitted over both guided and unguided media • Digital signal • Sequence of voltage pulses • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation than analog signals • Cannot be used on optical fiber or wireless media

  5. Analog and Digital SignalingofAnalog andDigitalData

  6. Digital Data  Analog Signal Analog Data  Analog Signal Modem (modulator/demodulator) Coverts binary voltage pulses by modulating a carrier frequency Demodulates the signal to recover the original data at the other end Codec (coder/decoder) Takes an analog signal and approximates that signal by a bit stream At the other end of a line the bit stream is used to reconstruct the analog data • Transmitter technologies use modulation techniques that enable sound and/or video waveforms to be conveyed as electromagnetic waveforms over wires or airwaves

  7. Combination Benefits

  8. Table 5.1Analog and Digital Transmission

  9. Analog transmission Only transmits analog signals, without regard for data content Attenuation overcome with amplifiers Signal is not evaluated or regenerated Digital transmission Transmits analog or digital signals Uses repeaters rather than amplifiers Switching equipment evaluates and regenerates signal Transmission Choices

  10. Cost – large scale and very large scale integration has caused continuing drop in cost Data Integrity – effect of noise and other impairments is reduced Capacity Utilization – high capacity is more easily and cheaply achieved with time division rather than frequency division Security and Privacy – Encryption possible Integration – All signals (voice, video, image, data) treated the same Advantages of Digital Transmission

  11. Data Encoding Techniques • Analog or digital data must be converted into a signal for purposes of transmission • The mapping from binary digits to signal elements is the encoding scheme for transmission • The basis for analog encoding is a continuous constant frequency signal known as the carrier signal • Modulation • The conversion of digital signals to analog form • Demodulation • The conversion of analog data signals back to digital form

  12. Figure 5.2Modulation of Analog Signals for Digital Data

  13. Amplitude Shift Keying(ASK)

  14. Frequency Shift Keying (FSK)

  15. Phase Shift Keying (PSK)

  16. Multilevel Signaling • Techniques may be combined • A common combination is PSK and ASK, where some or all of the phase shifts may occur at one or two amplitudes • Is commonly used in today’s networks • Examples: • 56 Kbps dial-up modems • Digital subscriber line (DSL) modems • Gigabit Ethernet networks

  17. Modems • Continue to be one of the most widely used pieces of communications gear • Is a device that modulates an analog carrier wave to encode digital information • Also demodulates the signals it receives to decode transmitted information • Direct broadcast satellite, Wi-Fi, and mobile phones use modems to communicate • Three popular types are: • Voice-grade • Cable • ADSL

  18. Table 5.3Modem Specifications

  19. Figure 5.3Cable Modems

  20. Figure 5.4ADSL Modem Application

  21. Figure 5.5Fiber to the Home

  22. Speeds for Internet Access Methods

  23. Figure 5.6Pulse-Code Modulation Example

  24. Figure5.7Examplesof DigitalSignalEncodingSchemes

  25. Differential NRZ

  26. Problems With NRZ • Difficult to determine where one bit ends and the next begins • In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses • Timing is critical because any drift results in lack of synchronization and incorrect bit values being transmitted

  27. Biphase Alternatives to NRZ • Require at least one transition per bit time, and may even have two • Modulation rate is greater, so bandwidth requirements are higher • Maximum modulation rate is twice NRZ • Advantages • Synchronization due to predictable transitions • Error detection based on absence of a transition

  28. Manchester Code

  29. Differential Manchester • Midbit transition is only for clocking • Transition at beginning of bit period=0 • Transition absent at beginning=1 • Has added advantage of differential encoding • Used in token-ring

  30. Table 5.54B/5B Digital Encoding

  31. Analog Encoding of Analog Information • Voice-generated sound wave can be represented by an electromagnetic signal with the same frequency components and transmitted on a voice-grade telephone line • Modulation can produce a new analog signal that conveys the same information but occupies a different frequency band • A higher frequency may be needed for effective transmission • Analog-to-analog modulation permits frequency-division multiplexing

  32. Figure5.8Sine-Wave Signals

  33. Figure 5.9Asynchronous Transmission

  34. Synchronous Transmission

  35. Error Control Process • All transmission media have potential for introduction of errors • All data link layer protocols must provide a method for controlling errors • Error control process has two components • Error detection • Redundancy introduced so that the occurrence of an error will be detected • Error correction • Receiver and transmitter cooperate to retransmit frames that were in error

  36. Parity Checks

  37. Figure 5.10ErrorDetectionProcess

  38. Cyclic Redundancy Check (CRC)

  39. Table 5.6Data and Signal Combinations

  40. Summary • Asynchronous transmission • Synchronous transmission • Error detection • The need for error control • Parity checks • Cyclic redundancy check • Analog and digital data communications • Data encoding techniques • Analog encoding of digital information • Digital encoding of analog information • Digital encoding of digital data • Analog encoding of analog information • Chapter 5: Data Communication Fundamentals

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