1. Chapter 8: Data Communication Fundamentals Business Data Communications, 4e

2. Three Components of Data Communication 8Data 8Analog: Continuous value data (sound, light, temperature) 8Digital: Discrete value (text, integers, symbols) 8Signal 8Analog: Continuously varying electromagnetic wave 8Digital: Series of voltage pulses (square wave) 8Transmission 8Analog: Works the same for analog or digital signals 8Digital: Used only with digital signals

3. Analog Data-->Signal Options 8Analog data to analog signal 8Inexpensive, easy conversion (eg telephone) 8Data may be shifted to a different part of the available spectrum (multiplexing) 8Used in traditional analog telephony 8Analog data to digital signal 8Requires a codec (encoder/decoder) 8Allows use of digital telephony, voice mail

4. Digital Data-->Signal Options 8Digital data to analog signal 8Requires modem (modulator/demodulator) 8Allows use of PSTN to send data 8Necessary when analog transmission is used 8Digital data to digital signal 8Requires CSU/DSU (channel service unit/data service unit) 8Less expensive when large amounts of data are involved 8More reliable because no conversion is involved

5. Transmission Choices 8Analog transmission 8only transmits analog signals, without regard for data content 8attenuation overcome with amplifiers 8signal is not evaluated or regenerated 8Digital transmission 8transmits analog or digital signals 8uses repeaters rather than amplifiers 8switching equipment evaluates and regenerates signal

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

7. Advantages of Digital Transmission 8The signal is exact 8Signals can be checked for errors 8Noise/interference are easily filtered out 8A variety of services can be offered over one line 8Higher bandwidth is possible with data compression

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

9. Analog Encoding of Digital Data 8Data encoding and decoding technique to represent data using the properties of analog waves 8Modulation: the conversion of digital signals to analog form 8Demodulation: the conversion of analog data signals back to digital form

10. Modem 8An acronym for modulator-demodulator 8Uses a constant-frequency signal known as a carrier signal 8Converts a series of binary voltage pulses into an analog signal by modulating the carrier signal 8The receiving modem translates the analog signal back into digital data

11. Methods of Modulation 8Amplitude modulation (AM) or amplitude shift keying (ASK) 8Frequency modulation (FM) or frequency shift keying (FSK) 8Phase modulation or phase shift keying (PSK)

12. Amplitude Shift Keying (ASK) 8In radio transmission, known as amplitude modulation (AM) 8The amplitude (or height) of the sine wave varies to transmit the ones and zeros 8Major disadvantage is that telephone lines are very susceptible to variations in transmission quality that can affect amplitude

13. 100 1 ASK Illustration

14. Frequency Shift Keying (FSK) 8In radio transmission, known as frequency modulation (FM) 8Frequency of the carrier wave varies in accordance with the signal to be sent 8Signal transmitted at constant amplitude 8More resistant to noise than ASK 8Less attractive because it requires more analog bandwidth than ASK

15. 1 1 01 FSK Illustration

16. Phase Shift Keying (PSK) 8Also known as phase modulation (PM) 8Frequency and amplitude of the carrier signal are kept constant 8The carrier signal is shifted in phase according to the input data stream 8Each phase can have a constant value, or value can be based on whether or not phase changes (differential keying)

17. 0011 PSK Illustration

18. 011 Differential Phase Shift Keying (DPSK) 0

19. Analog Channel Capacity: BPS vs. Baud 8Baud=# of signal changes per second 8BPS=bits per second 8In early modems only, baud=BPS 8Each signal change can represent more than one bit, through complex modulation of amplitude, frequency, and/or phase 8Increases information-carrying capacity of a channel without increasing bandwidth 8Increased combinations also leads to increased likelihood of errors

20. Voice Grade Modems

21. Cable Modems

22. DSL Modems

23. Digital Encoding of Analog Data 8Primarily used in retransmission devices 8The 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. 88000 samples/sec sufficient for 4000hz

24. Converting Samples to Bits 8Quantizing 8Similar concept to pixelization 8Breaks wave into pieces, assigns a value in a particular range 88-bit range allows for 256 possible sample levels 8More bits means greater detail, fewer bits means less detail

25. Codec 8Coder/Decoder 8Converts analog signals into a digital form and converts it back to analog signals 8Where do we find codecs? 8Sound cards 8Scanners 8Voice mail 8Video capture/conferencing

26. Digital Encoding of Digital Data 8Most common, easiest method is different voltage levels for the two binary digits 8Typically, negative=1 and positive=0 8Known 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 8Differential version is NRZI (NRZ, invert on ones) 8Change=1, no change=0 8Advantage 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 8Difficult to determine where one bit ends and the next begins 8In NRZ-L, long strings of ones and zeroes would appear as constant voltage pulses 8Timing is critical, because any drift results in lack of synchronization and incorrect bit values being transmitted

29. Biphase Alternatives to NRZ 8Require at least one transition per bit time, and may even have two 8Modulation rate is greater, so bandwidth requirements are higher 8Advantages 8Synchronization due to predictable transitions 8Error detection based on absence of a transition

30. Manchester Code 8Transition in the middle of each bit period 8Transition provides clocking and data 8Low-to-high=1, high-to-low=0 8Used in Ethernet

31. Differential Manchester 8Midbit transition is only for clocking 8Transition at beginning of bit period=0 8Transition absent at beginning=1 8Has added advantage of differential encoding 8Used in token-ring

32. Digital Encoding Illustration

33. Digital Interfaces 8The point at which one device connects to another 8Standards define what signals are sent, and how 8Some standards also define physical connector to be used

34. Generic Communications Interface Illustration

35. DTE and DCE

36. RS-232C (EIA 232C) 8EIA’s “Recommended Standard” (RS) 8Specifies mechanical, electrical, functional, and procedural aspects of the interface 8Used for connections between DTEs and voice-grade modems, and many other applications

37. EIA-232-D 8new version of RS-232-C adopted in 1987 8improvements in grounding shield, test and loop-back signals 8the prevalence of RS-232-C in use made it difficult for EIA-232-D to enter into the marketplace

38. RS-449 8EIA standard improving on capabilities of RS-232-C 8provides for 37-pin connection, cable lengths up to 200 feet, and data rates up to 2 million bps 8covers functional/procedural portions of R- 232-C 8electrical/mechanical specs covered by RS-422 & RS-423

39. Functional Specifications 8Specifies the role of the individual circuits 8Data circuits in both directions allow full- duplex communication 8Timing signals allow for synchronous transmission (although asynchronous transmission is more common)

40. Procedural Specifications 8Multiple procedures are specified 8Simple example: exchange of asynchronous data on private line 8Provides means of attachment between computer and modem 8Specifies method of transmitting asynchronous data between devices 8Specifies method of cooperation for exchange of data between devices

41. Mechanical Specifications 825-pin connector with a specific arrangement of leads 8DTE devices usually have male DB25 connectors while DCE devices have female 8In practice, fewer than 25 wires are generally used in applications

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

43. RS-232 DB-25 Pinouts

44. RS-232 DB-9 Connectors 8Limited RS-232

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

46. Electrical Specifications 8Specifies signaling between DTE and DCE 8Uses NRZ-L encoding 8Voltage < -3V = binary 1 8Voltage > +3V = binary 0 8Rated for <20Kbps and <15M 8greater distances and rates are theoretically possible, but not necessarily wise

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