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CIS-325 Data Communication

CIS-325 Data Communication. Dr. L. G. Williams, Instructor. Chapter Five. Data Communication Fundamentals. Data Communication Basics. Analog or Digital Three Components Data Signal Transmission. Analog Data Choices. Digital Data Choices. Transmission Choices. Analog transmission

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CIS-325 Data Communication

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  1. CIS-325Data Communication Dr. L. G. Williams, Instructor CIS-325: Data Communications

  2. Chapter Five Data Communication Fundamentals CIS-325: Data Communications

  3. Data Communication Basics • Analog or Digital • Three Components • Data • Signal • Transmission CIS-325: Data Communications

  4. Analog Data Choices CIS-325: Data Communications

  5. Digital Data Choices CIS-325: Data Communications

  6. Transmission Choices • Analog transmission • only transmits analog signals, without regard for data content • attenuation overcome with amplifiers • Digital transmission • transmits analog or digital signals • uses repeaters rather than amplifiers CIS-325: Data Communications

  7. Advantages of Digital Transmission • The signal is exact • Signals can be checked for errors • Noise/interference are easily filtered out • A variety of services can be offered over one line • Higher bandwidth is possible with data compression CIS-325: Data Communications

  8. Analog Encoding of Digital Data • data encoding and decoding technique to represent data using the properties of analog waves • modulation: the conversion of digital signals to analog form • demodulation: the conversion of analog data signals back to digital form CIS-325: Data Communications

  9. Modem • an acronym for modulator-demodulator • uses a constant-frequency signal known as a carrier signal • converts a series of binary voltage pulses into an analog signal by modulating the carrier signal • the receiving modem translates the analog signal back into digital data CIS-325: Data Communications

  10. Methods of Modulation • amplitude modulation (AM) or amplitude shift keying (ASK) • frequency modulation (FM) or frequency shift keying (FSK) • phase modulation or phase shift keying (PSK) CIS-325: Data Communications

  11. Amplitude Shift Keying (ASK) • In radio transmission, known as amplitude modulation (AM) • the amplitude (or height) of the sine wave varies to transmit the ones and zeros • major disadvantage • telephone lines are very susceptible to variations in transmission quality that affect amplitude CIS-325: Data Communications

  12. ASK Illustration 1 0 0 1 CIS-325: Data Communications

  13. Frequency Shift Keying (FSK) • in radio transmission, known as frequency modulation (FM) • the frequency of the carrier wave varies in accordance with the signal to be sent • signal is transmitted at constant amplitude • more immune to noise than ASK • requires more analog bandwidth than ASK CIS-325: Data Communications

  14. FSK Illustration 1 1 0 1 CIS-325: Data Communications

  15. Phase Shift Keying (PSK) • also known as phase modulation (PM) • frequency and amplitude of the carrier signal are kept constant • the carrier is shifted in phase according to the input data stream • each phase can have a constant value, or value can be based on whether or not phase changes (differential keying) CIS-325: Data Communications

  16. PSK Illustration 0 0 1 1 CIS-325: Data Communications

  17. Complex Modulations • Combining modulation techniques allows us to transmit multiple bit values per signal change (baud) • Increases information-carrying capacity of a channel without increasing bandwidth • Increased combinations also leads to increased likelihood of errors • Typically, amplitude and phase modulation are combined CIS-325: Data Communications

  18. Digital Encoding of Analog Data • Primarily used in retransmission devices • Uses pulse-code modulation (PCM) • The sampling theorem: If a signal is sampled at regular intervals of time and at a rate higher than twice the significant signal frequency, the samples contain all the information of the original signal. • 8000 samples/sec sufficient for 4000hz CIS-325: Data Communications

  19. Converting Samples to Bits • Quantizing • Similar concept to pixelization • Breaks wave into pieces, assigns a value in a particular range • 8-bit range allows for 256 possible sample levels • More bits means greater detail, fewer bits means less detail CIS-325: Data Communications

  20. Codec • Coder/Decoder • converts analog signals into a digital form and converts it back to analog signals • e.g., hi-fi music, television pictures, the output of copying machine, videoconferencing CIS-325: Data Communications

  21. Digital Encodingof Digital Data • Most common, easiest method is different voltage levels for the two binary digits • Typically, negative=1 and positive=0 • Known as NRZ-L, or nonreturn-to-zero level, because signal never returns to zero, and the voltage during a bit transmission is level CIS-325: Data Communications

  22. Differential NRZ • Differential version is NRZI (NRZ, invert on ones) • Change=1, no change=0 • Advantage of differential encoding is that it is more reliable to detect a change in polarity than it is to accurately detect a specific level CIS-325: Data Communications

  23. 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 CIS-325: Data Communications

  24. 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 • Advantages • Synchronization due to predictable transitions • Error detection based on absence of a transition CIS-325: Data Communications

  25. Manchester Code • Transition in the middle of each bit period • Transition provides clocking and data • Low-to-high=1 , high-to-low=0 • Used in Ethernet CIS-325: Data Communications

  26. 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 CIS-325: Data Communications

  27. Digital Encoding Schemes CIS-325: Data Communications

  28. Analog Encoding of Analog Data • Often convert data directly to signal on same bandwidth • eg. Voice • Or, modulate signal to carrier at higher frequency • unguided media CIS-325: Data Communications

  29. Asynchronous & Synchronous Transmission • Concerned with timing issues • How does the receiver know when the bit period begins and ends? • Small timing difference become more significant over time if no synchronization takes place between sender and receiver CIS-325: Data Communications

  30. Data transmitted 1 character at a time Character format is 1 start & 1+ stop bit, plus data of 5-8 bits Character may include parity bit Timing needed only within each character Resynchronization each start bit Uses simple, cheap technology Wastes 20-30% of bandwidth Asynchronous Transmission CIS-325: Data Communications

  31. Large blocks of bits transmitted without start/stop codes Synchronized by clock signal or clocking data Data framed by preamble and postamble bit patterns More efficient than asynchronous Overhead typically below 5% Used at higher speeds than asynchronous Requires error checking Synchronous Transmission CIS-325: Data Communications

  32. Data Flow: Simplex • only transmit in one direction • rarely used in data communications • e.g., receiving signals from the radio station or CATV • the sending station has only one transmitter the receiving station has only one receiver CIS-325: Data Communications

  33. Simplex Illustration CIS-325: Data Communications

  34. Data Flow: Half Duplex • data may travel in both directions, but only in one direction at a time • provides non-simultaneous two-way communication • computers use control signals to negotiate when to send and when to receive • the time it takes to switch between sending and receiving is called turnaround time CIS-325: Data Communications

  35. Half Duplex Illustration CIS-325: Data Communications

  36. Data Flow: Full Duplex • complete two-way simultaneous transmission • faster than half-duplex communication because no turnaround time is needed CIS-325: Data Communications

  37. Full Duplex Illustration CIS-325: Data Communications

  38. Generic Communications Interface Illustration CIS-325: Data Communications

  39. DTE and DCE CIS-325: Data Communications

  40. Telecommunications Standards • Where do they come from? • Standard setting bodies • Governments • Two types • Market-driven and voluntary • Government-regulated and mandatory CIS-325: Data Communications

  41. Advantages • Assures a large market, which encourages mass production and often lowers costs • Encourages vendors to enter market because investment is protected • Allows products from multiple vendors to communicate, providing consumers with wider selection CIS-325: Data Communications

  42. Disadvantages • Standards process can freeze technology too early, due to the length of the standards-setting process and the speed with which technology changes • Current process allows for multiple standards for the same thing CIS-325: Data Communications

  43. Interface Standard • Four Characteristics • Mechanical • Electrical • Functional • Procedural CIS-325: Data Communications

  44. EIA’s “Recommended Standard” (RS) Specifies mechanical, electrical, functional, and procedural aspects of the interface Used for connections between DTEs and voice-grade modems, and many other applications RS-232C (EIA 232C) CIS-325: Data Communications

  45. 25-pin connector with a specific arrangement of leads DTE devices usually have male DB25 connectors while DCE devices have female In practice, fewer than 25 wires are generally used in applications Mechanical Specifications CIS-325: Data Communications

  46. RS-232 DB-25 Pinouts CIS-325: Data Communications

  47. Specifies signaling between DTE and DCE Uses NRZ-L encoding Voltage < -3V = binary 1 Voltage > +3V = binary 0 Rated for <20Kbps and <15M greater distances and rates are theoretically possible, but not necessarily wise Electrical Specifications CIS-325: Data Communications

  48. RS-232 Signals (Asynch) Odd Parity Even Parity No Parity CIS-325: Data Communications

  49. Specifies the role of the individual circuits Data circuits in both directions allow full-duplex communication Timing signals allow for synchronous transmission (although asynchronous transmission is more common) See Table 5-5 on p. 115 for this spec Functional Specifications CIS-325: Data Communications

  50. Multiple procedures are specified Simple example: exchange of asynchronous data on private line Provides means of attachment between computer and modem Specifies method of transmitting asynchronous data between devices Specifies method of cooperation for exchange of data between devices Procedural Specifications CIS-325: Data Communications

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