Chapter 8: Data Communication Fundamentals

1. Chapter 8:Data CommunicationFundamentals Business Data Communications, 4e

2. Three Components of Data Communication • 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

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

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

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

6. A Data D D Transmission System A A D Signal Data, Signal, and Transmission Matrix

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

8. Why Use Analog Transmission? • Already in place • Significantly less expensive • Lower attentuation rates • Fully sufficient for transmission of voice signals

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

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

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)

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

13. ASK Illustration 1 0 0 1

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

15. FSK Illustration 1 1 0 1

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)

17. PSK Illustration 0 0 1 1

18. Differential Phase Shift Keying (DPSK) 0 0 1 1

19. Analog Channel Capacity: BPS vs. Baud • Baud=# of signal changes per second • BPS=bits per second • In early modems only, baud=BPS • Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase • Increases information-carrying capacity of a channel without increasing bandwidth • Increased combinations also leads to increased likelihood of errors

20. Voice Grade Modems

21. Cable Modems

22. DSL Modems

23. Digital Encoding of Analog Data • Primarily used in retransmission devices • 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. Digital Encoding Illustration

33. 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

34. Generic CommunicationsInterface Illustration

35. DTE and DCE

36. 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)

37. EIA-232-D • new version of RS-232-C adopted in 1987 • improvements in grounding shield, test and loop-back signals • the prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace

38. RS-449 • EIA standard improving on capabilities of RS-232-C • provides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bps • covers functional/procedural portions of R-232-C • electrical/mechanical specs covered by RS-422 & RS-423

39. 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) Functional Specifications

40. 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

41. 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

42. DB-25 Female DB-25 Male RS-232 DB-25 Connectors

43. RS-232 DB-25 Pinouts

44. Limited RS-232 RS-232 DB-9 Connectors

45. Found on Macs RS-422 DIN-8 DIN-8 Male DIN-8 Female

46. 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

47. RS-232 Signals (Asynch) Odd Parity Even Parity No Parity