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

2. Three Components of Data Communication
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 Digital 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. Data, Signal, and Transmission Matrix
System A D

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 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 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 1 1

18. Differential Phase Shift Keying (DPSK)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 Encoding of 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 Communications Interface Illustration

35. DTE and DCE

36. RS-232C (EIA 232C) 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

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. Functional Specifications
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)

40. Procedural Specifications
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

41. Mechanical Specifications
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

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

43. RS-232 DB-25 Pinouts

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

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

46. Electrical Specifications
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

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