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Chapter 16: Data Communication Fundamentals

Chapter 16: Data Communication Fundamentals. Business Data Communications, 5e. Data Communication Components. Data Analog: Continuous value data (sound, light, temperature) Digital: Discrete value (text, integers, symbols) Signal Analog: Continuously varying electromagnetic wave

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Chapter 16: Data Communication Fundamentals

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  1. Chapter 16:Data Communication Fundamentals Business Data Communications, 5e

  2. Data Communication Components • Data • Analog: Continuous value data (sound, light, temperature) • Digital: Discrete value (text, integers, symbols) • Signal • Analog: Continuously varying electromagnetic wave • Digital: Series of voltage pulses (square wave) • Transmission • Analog: Works the same for analog or digital signals • Digital: Used only with digital signals Business Data Communications, 5e

  3. Analog DataSignal Options • Analog data to analog signal • Inexpensive, easy conversion (eg telephone) • Data may be shifted to a different part of the available spectrum (multiplexing) • Used in traditional analog telephony • Analog data to digital signal • Requires a codec (encoder/decoder) • Allows use of digital telephony, voice mail Business Data Communications, 5e

  4. Digital DataSignal Options • Digital data to analog signal • Requires modem (modulator/demodulator) • Allows use of PSTN to send data • Necessary when analog transmission is used • Digital data to digital signal • Requires CSU/DSU (channel service unit/data service unit) • Less expensive when large amounts of data are involved • More reliable because no conversion is involved Business Data Communications, 5e

  5. Transmission Choices • 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 Business Data Communications, 5e

  6. A Data D D Transmission System A A D Signal Data, Signal, and Transmission Matrix Business Data Communications, 5e

  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 Business Data Communications, 5e

  8. Why Use Analog Transmission? • Already in place • Significantly less expensive • Lower attenuation rates • Fully sufficient for transmission of voice signals Business Data Communications, 5e

  9. 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 Business Data Communications, 5e

  10. 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 Business Data Communications, 5e

  11. 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) Business Data Communications, 5e

  12. 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 is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude Business Data Communications, 5e

  13. ASK Illustration 1 0 0 1 Business Data Communications, 5e

  14. Frequency Shift Keying (FSK) • In radio transmission, known as frequency modulation (FM) • Frequency of the carrier wave varies in accordance with the signal to be sent • Signal transmitted at constant amplitude • More resistant to noise than ASK • Less attractive because it requires more analog bandwidth than ASK Business Data Communications, 5e

  15. FSK Illustration 1 1 0 1 Business Data Communications, 5e

  16. Phase Shift Keying (PSK) • Also known as phase modulation (PM) • Frequency and amplitude of the carrier signal are kept constant • The carrier signal 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) Business Data Communications, 5e

  17. PSK Illustration 0 0 1 1 Business Data Communications, 5e

  18. Differential Phase Shift Keying (DPSK) 0 0 1 1 Business Data Communications, 5e

  19. Voice Grade Modems Business Data Communications, 5e

  20. Cable Modems • Permits Internet access over cable television networks. • ISP is at or linked by high-speed line to central cable office • Cables used for television delivery can also be used to deliver data between subscriber and central location • Upstream and downstream channels are shared among multiple subscribers, time-division multiplexing technique (see Chapter 17) • Splitter is used to direct TV signals to a TV and the data channel to a cable modem Business Data Communications, 5e

  21. Cable Modem Layout Business Data Communications, 5e

  22. Asymmetric DigitalSubscriber Line (ADSL) • New modem technology for high-speed digital transmission over ordinary telephone wire. • Telephone central office can provide support for a number of ISPs, • At central office, a combined data/voice signal is transmitted over a subscriber line • At subscriber’s site, twisted pair is split and routed to both a PC and a telephone • At the PC, an ADSL modem demodulates the data signal for the PC. • At the telephone, a microfilter passes the 4-kHz voice signal. • The data and voice signals are combined on the twisted pair line using frequency-division-multiplexing techniques (Chapter 17) Business Data Communications, 5e

  23. DSL Modem Layout Business Data Communications, 5e

  24. Digital Encoding of Analog Data • Evolution of telecommunications networks to digital transmission and switching requires voice data in digital form • Best-known technique for voice digitization is 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. • Good-quality voice transmission can be achieved with a data rate of 8 kbps • Some videoconference products support data rates as low as 64 kbps Business Data Communications, 5e

  25. 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 Business Data Communications, 5e

  26. Codec • Coder/Decoder • Converts analog signals into a digital form and converts it back to analog signals • Where do we find codecs? • Sound cards • Scanners • Voice mail • Video capture/conferencing Business Data Communications, 5e

  27. 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 Business Data Communications, 5e

  28. 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 Business Data Communications, 5e

  29. 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 Business Data Communications, 5e

  30. 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 Business Data Communications, 5e

  31. 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 Business Data Communications, 5e

  32. 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 Business Data Communications, 5e

  33. Digital Encoding Illustration Business Data Communications, 5e

  34. Digital Interfaces • The point at which one device connects to another • Standards define what signals are sent, and how • Some standards also define physical connector to be used Business Data Communications, 5e

  35. 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 (Chapter 17) Business Data Communications, 5e

  36. Asynchronous and Synchronous Transmission • For receiver to sample incoming bits properly, it must know arrival time and duration of each bit that it receives Business Data Communications, 5e

  37. Asynchronous Transmission • Avoids timing problem by not sending long, uninterrupted streams of bits • Data transmitted one character at a time, where each character is 5 to 8 bits in length. • Timing or synchronization must only be maintained within each character; the receiver has the opportunity to resynchronize at the beginning of each new character. • Simple and cheap but requires an overhead of 2 to 3 bits per character Business Data Communications, 5e

  38. Synchronous Transmission • Block of bits transmitted in a steady stream without start and stop codes. • Clocks of transmitter and receiver must somehow be synchronized • Provide a separate clock line between transmitter and receiver; works well over short distances, • Embed the clocking information in the data signal. • Each block begins with a preamble bit pattern and generally ends with a postamble bit pattern • The data plus preamble, postamble, and control information are called a frame Business Data Communications, 5e

  39. Error Control Process • All transmission media have potential for introduction of errors • All data link layer protocols must provide method for controlling errors • Error control process has two components • Error detection • Error correction Business Data Communications, 5e

  40. Error Detection: Parity Bits • Bit added to each character to make all bits add up to an even number (even parity) or odd number (odd parity) • Good for detecting single-bit errors only • High overhead (one extra bit per 7-bit character=12.5%) Business Data Communications, 5e

  41. Error Detection: Cyclic Redundancy Check (CRC) • Data in frame treated as a single binary number, divided by a unique prime binary, and remainder is attached to frame • 17-bit divisor leaves 16-bit remainder, 33-bit divisor leaves 32-bit remainder • For a CRC of length N, errors undetected are 2-N • Overhead is low (1-3%) Business Data Communications, 5e

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