1Fundamentals of Data & Signals (Part II)School of BusinessEastern Illinois University © Abdou Illia, Spring 2007(Week 5, Thursday 2/8/2007)2Learning Objectives Identify the basic characteristics of a signal Understand signal strength and attenuation Understand Principles of transmitting data using signals3Analog versus Digital Analog and Digital signals– Could be affected by Interference– Could be affected by Noise Interference = External signals Noise = Random electrical energy generated in the line when the signal is propagating (traveling)– Happens unless the line is at absolute zero temperature24Noise and Signals Noise appears as an analog waveformDifficult to distinguish Noise from the original waveformEasy to distinguish Noise from the original waveform Can use filtering devices (Filters) to remove/reduce NoiseFigure 2-1 in textbookFigure 2-2 in textbookFigure 2-3 in textbookFigure 2-4 in textbook5AttenuationOverall gain or loss = -10 dB + 20dB - 15dB = -5dB Attenuation = Loss of Signal Strength Function of Distance and Friction within the Medium If high signals get too weak, the receiver will not be able to detected it.  Decibel (dB) is a relative measure of signal loss or gain of strength. dB = 10 log10(P2 / P1), Where P2 & P1 are ending and beginning power levels (in watt)(Figure 2-10 in textbook)6Attenuation A signal starts at a transmitter with 10 watts of power and arrived at a receiver with 5 watts of power. Calculate the loss of power in dB.1. dB = 10 log10(P2 / P1) 2. dB = 10 log10(5/10)3. dB = 10 log10(0.5)4. dB = 10 (-0.3)5. dB = -3Q: What is the decibel loss of a signal that starts at 50 watts and experiences a 10-watt loss over a given section of cable ?Q: What is the decibel loss of a signal that loses half its power during the course of transmission ?Q: Do Week 6 Exercise 2available in the Notes section of the course website.37Three Main Characteristics of Signals Amplitude Frequency Phase8Amplitude The amplitude of a signal is the height of the wave above or below a given reference point. Height can denote– Voltage level (measured in volts)– Power level of the signal (in watts)– Current level of signal (in amps)(Figure 2-6 in textbook)9FrequencyQ: What is the frequency (in Hz) of a signal that repeats 80000 times within one minute ? Frequency is the number of times a signal makes a complete cycle within a given time frame Frequency, or Cycles per second, measured in Hertz (Hz)(Figure 2-7 in textbook)410Frequency and related concepts Human voice, as well as all signals, composed of multiple frequencies Multiple frequencies allow to distinguish one person’s voice from another Average human voice frequency: From 300 Hz to 3100Hz Telephone system transmits signals in the range of 300 Hz to 3100 Hz Spectrum = The range of frequencies that a signal spans from minimum to maximum Bandwidth = The absolute value of the difference between the lowest and highest frequencies of a signal Example: │3100 Hz – 300 Hz│= 2800 Hz Effective Bandwidth versus Theoretical Bandwidth (Noise, interference)Q: What is the bandwidth of a signal composed of frequencies from -50 Hz to 500 Hz ?Note: Negative frequency used here to help understand the meaning of Absolute value11Phase The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero. Phase changes often occur on common angles, such as 45, 90, 180, etc(Figure 2-8 in textbook)12Converting Digital Data into Digital Signals There are numerous techniques (Encoding schemes) for converting digital data into digital signals Those techniques are implemented in Hardware and Software Most common techniques:– Non-Return to Zero Level (NRZ-L)– Differential Manchester513NRZ-L Encoding Scheme Transmits 1s as low level voltage and 0s as high level voltage Problem associated to NRZ-L: Long sequences of 0s in data produce signal that never changes. The receiver can have problem to know when one bit ends and the next bit begins (since Receiver and Sender use different clock systems)NRZ-LFigure 2-11 (a) in textbook14Differential Manchester Used in most transceivers Ensure that each bit has some type of signal change (solve the synchronization problem) Disadvantage: For transmitting a series of 0s, the signal has to change at the beginning of each bit, as well as in the middle of each bit.Baud rate = 10Bit Rate = 5=> To send 100 0s, the signal changes 200 times => Devices more elaborate.1 secFigure 2-11 (d) in textbookFigure 2-12 in textbook15Converting Digital Data into Analog Signals Technique for converting digital data to Analog signal is called modulation or Shift Keying. Three current techniques:– Amplitude Modulation– Frequency Modulation– Phase Modulation616Amplitude Modulation A data value of 1 and a data value of 0 represented by two different amplitudes of the signal During each bit period the amplitude is constant. Some systems use multiple amplitudes Noise can increase or decrease the amplitude• Each amplitude level represents 2 bits• Every time the signal changes 2 bits are transmitted• => Bit Rate = Twice the Baud RateFigure 2-14 in textbookFigure 2-15 in textbook17Frequency Modulation A data value of 1 and a data value of 0 represented by two different Frequency range During each bit period the frequency is constant. More robust than Amplitude Modulation, but subject to intermodulation distortion Figure 2-16 in textbook18Phase Modulation A data value of 1 and a data value of 0 represented by two different Phase changes More robust than Amplitude Modulation and Frequency Modulation• 0 represented by No Phase change• 1 represented by a Phase change719Summary Questions1. What is the main advantage of digital signals over analog signals with regard to noise?2. What are the three main components (characteristics) of signals ? 3. What is the bandwidth of signals? The spectrum?4. (a) Name one technique for converting digital data into digital signals. (b) Name 3 techniques for converting digital data into analog