1 Chapter 2 Fundamentals of Data and Signals Data Communications and Computer Networks: A Business User’s Approach

2 Data Communications and Computer Networks Chapter 2 What we cover: Data vs signals Signals – digital and analog; aspects Conversion: A to D; D to A Data encoding

3 Data Communications and Computer Networks Chapter 2 Introduction Computer networks transmit signals Signals are the electric or electromagnetic encoding of data Data and signals can be analog or digital

4 Data Communications and Computer Networks Chapter 2 Data vs Signals Data is what we want to transmit Data is usually stored Signals are what we use to transmit the data Signals are transient

5 Data Communications and Computer Networks Chapter 2 Data and Signals Examples of data include: computer files movie on a DVD music on a compact disc collection of samples from a blood gas analysis device

6 Data Communications and Computer Networks Chapter 2 Data and Signals Examples of signals include: telephone conversation over a telephone line live television news interview from Europe web page download over your telephone line via the Internet others?

7 Data Communications and Computer Networks Chapter 2 Analog versus Digital Analog is a continuous waveform, with examples such as music and video.

8 Data Communications and Computer Networks Chapter 2 Analog versus Digital Digital is a discrete or non-continuous waveform with examples such as computer 1s and 0s.

9 Data Communications and Computer Networks Chapter 2 Analog versus Digital It is harder to separate noise from an analog signal than it is to separate noise from a digital signal.

10 Data Communications and Computer Networks Chapter 2 Analog versus Digital Noise in a digital signal. You can still discern a high voltage from a low voltage.

11 Data Communications and Computer Networks Chapter 2 Analog versus Digital Noise in a digital signal. Too much noise - you cannot discern a high voltage from a low voltage.

12 Data Communications and Computer Networks Chapter 2 All Signals Three Components Amount - Amplitude Time - Frequency Phase

13 Data Communications and Computer Networks Chapter 2 Amplitude The amplitude of a signal is the height of the wave above or below a given reference point.

14 Frequency The frequency is the number of times a signal makes a complete cycle within a given time frame. Measured in Hz (hertz = cycles/second) Period is the length of one cycle = 1/frequency Spectrum - The range of frequencies that a signal spans from minimum to maximum. Human speech: 300 Hz to 3100 Hz Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal. Human speech: 2800 Hz

15 Data Communications and Computer Networks Chapter 2

16 Data Communications and Computer Networks Chapter 2 Phase The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero. A change in phase can be any number of angles between 0 and 360 degrees. Phase changes often occur on common angles, such as 45, 90, 135, etc.

17 Data Communications and Computer Networks Chapter 2

18 Data Communications and Computer Networks Chapter 2 Loss or Gain of Signal Strength All signals experience loss (attenuation). Attenuation and gain is denoted as a decibel (dB) loss or gain. Decibel losses (and gains) are additive.

19 Data Communications and Computer Networks Chapter 2 Loss of Signal Strength Decibel is a relative loss or gain of signal dB = 10 log 10 (output power/input power) dB = 10 log 10 (P o /P i ) = 10 log 10 P o – 10 log 10 P i dB = dB o - dB i Attenuation is denoted as a decibel (dB) loss. Decibel losses (and gains) are additive. dB is RELATIVE; cannot be calculated for a single power level

20 Data Communications and Computer Networks Chapter 2 Loss or gain of Signal Strength Signal runs from point A to point C through point B Both lines and nodes can have a lose or gain. Rarely do lines have gains -10 dB + 20 dB – 15 dB = -5 dB

21 Data Communications and Computer Networks Chapter 2 Signal Strength So if a signal loses 3 dB, is that a lot? A 3 dB loss indicates the signal lost half of its power. dB = 10 log 10 (P2 / P1) -3 dB = 10 log 10 (X / 100) -0.3 = log 10 (X / 100) = X / = X / 100 X = 50

22 Data Communications and Computer Networks Chapter 2 Converting data into signals - types Transmitting Digital data to digital signals Digital data with analog signals Analog data with digital signals Analog data with analog signals

23 Data Communications and Computer Networks Chapter 2 Converting Digital Data (0,1)’s into Digital Signals There are numerous techniques available to convert digital data into digital signals. Many systems are designed only to carry analog signals Let’s examine four techniques: NRZ-L (non return to zero level) NRZ-I (non return to zero – inverted) Manchester Differential Manchester

24 Data Communications and Computer Networks Chapter 2 Converting Digital Data NRZ-L & NRZ-I have synchronization problems No signal change at the beginning of each bit Requires to systems to have clocks in synch Manchester & Differential Manchester Each bit has signal change Encoding schemes are self-clocking

25 Data Communications and Computer Networks Chapter 2

26 Data Communications and Computer Networks Chapter 2 Bits per second (bps) = number of bits transmitted across a medium in a given second Baud rate: number of times a signal changes value per second bps and baud are not always the same

27 Data Communications and Computer Networks Chapter 2 Note how with a Differential Manchester code, every bit has at least one signal change. Some bits have two signal changes per bit (baud rate is twice the bps). Presence of transition at beginning of bit time means a zero

28 Data Communications and Computer Networks Chapter 2 4B/5B Digital Encoding Yet another encoding technique that converts four bits of data into five-bit quantities. The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes. The five-bit code is then transmitted using an NRZ-I encoded signal.

29 Data Communications and Computer Networks Chapter 2

30 Converting Digital Data into Analog Signals Modulation: change from one version to another Three basic techniques: Amplitude modulation Frequency modulation Phase modulation

31 Modem Device that converts digital data to analog signal and back again. MOdulator/DEModulator

32 Data Communications and Computer Networks Chapter 2 Amplitude Modulation One amplitude encodes a 0 while another amplitude encodes a 1.

33 Data Communications and Computer Networks Chapter 2 Amplitude Modulation Some systems use multiple amplitudes.

34 Data Communications and Computer Networks Chapter 2 Frequency Modulation One frequency encodes a 0 while another frequency encodes a 1.

35 Data Communications and Computer Networks Chapter 2 Phase Modulation One phase change encodes a 0 while another phase change encodes a 1. Here, only phase change for a 1.

36 Data Communications and Computer Networks Chapter 2 Quadrature Phase Modulation Four different phase angles are used: 45 degrees 135 degrees 225 degrees 315 degrees

37 Data Communications and Computer Networks Chapter 2

38 Data Communications and Computer Networks Chapter 2 Converting Analog Data into Digital Signals To convert analog data into a digital signal, there are two basic techniques: Pulse code modulation (used by telephone systems) Delta modulation

39 Data Communications and Computer Networks Chapter 2 Pulse Code Modulation The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values.

40 Data Communications and Computer Networks Chapter 2 Pulse Code Modulation When the binary values are later converted to an analog signal, a waveform similar to the original results.

41 Data Communications and Computer Networks Chapter 2 Pulse Code Modulation The more snapshots taken in the same amount of time, the better the resolution.

42 Data Communications and Computer Networks Chapter 2 Pulse Code Modulation Since telephone systems digitize human voice, and since the human voice has a fairly narrow bandwidth, telephone systems can digitize voice into either 128 levels or 256 levels. These levels are called quantization levels. If 128 levels, then each sample is 7 bits (2 ^ 7 = 128). If 256 levels, then each sample is 8 bits (2 ^ 8 = 256).

43 Data Communications and Computer Networks Chapter 2 Delta Modulation An analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop.

44 Data Communications and Computer Networks Chapter 2 Converting Analog Data into Analog Signals Many times it is necessary to modulate analog data onto a different set of analog frequencies. Broadcast radio and television are two very common examples of this.

45 Data Communications and Computer Networks Chapter 2

46 Data Communications and Computer Networks Chapter 2 Spread Spectrum Technology A secure encoding technique that uses multiple frequencies or codes to transmit data. Two basic spread spectrum technologies: Frequency hopping spread spectrum Direct sequence spread spectrum

47 Data Communications and Computer Networks Chapter 2 Frequency Hopping Spread Spectrum

48 Data Communications and Computer Networks Chapter 2 Direct Sequence Spread Spectrum This technology replaces each binary 0 and binary 1 with a unique pattern, or sequence, of 1s and 0s. For example, one transmitter may transmit the sequence for each binary 1, and for each binary 0. Another transmitter may transmit the sequence for each binary 1, and for each binary 0.

49 Data Code – common digital data transmitted The set of all textual characters or symbols and their corresponding binary patterns is called a data code. There are two basic data code sets: ASCII (7 bit, 1 parity bit) EBCDIC (8 bit) [IBM computers] Number of possible codeable terms 2*N

50 Data Communications and Computer Networks Chapter 2

51 Data Communications and Computer Networks Chapter 2

52 Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action Let us transmit the message “Sam, what time is the meeting with accounting? Hannah.” This message first leaves Hannah’s workstation and travels across a local area network.

53 Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action Note the order of the signals in time

54 Data Communications and Computer Networks Chapter 2 Data and Signal Conversions in Action

55 Data Communications and Computer Networks Chapter 2 Higher Data Transfer Rates How do you send data faster? 1. Use a higher frequency signal (make sure the medium can handle the higher frequency) 2. Use a higher number of signal levels In both cases, noise can be a problem.

56 What we covered Data and Signal Signal components – amplitude, frequency, phase Transmission factors – noise and attenuation Four combinations data and signal Digital encoding schemes –Bit rate vs baud Digital data on analog signals requires modulation –AM, FM, PM Analog data to digital signals –PCM, delta M

57 What we covered Analog data over analog signal – modulate to another frequency Spread spectrum – frequency hopping Data codes – transmission of data codes –ASCII (American National Standard code for Information Interchange) the most common

58 Problems Set