1	Chapter Two Fundamentals of Data and Signals
2	After reading this chapter, you should be able to: Distinguish between data and signals, and cite the advantages of digital data and signals over analog data and signals Identify the three basic components of a signal Discuss the bandwidth of a signal and how it relates to data transfer speed Identify signal strength and attenuation, and how they are related
3	After reading this chapter, you should be able to (continued): Outline the basic characteristics of transmitting analog data with analog signals, digital data with digital signals, digital data with analog signals, and analog data with digital signals List and draw diagrams of the basic digital encoding techniques, and explain the advantages and disadvantages of each Identify the different shift keying (modulation) techniques, and describe their advantages, disadvantages, and uses
4	After reading this chapter, you should be able to (continued): Identify the two most common digitization techniques, and describe their advantages and disadvantages Identify the different data codes and how they are used in communication systems
5	Introduction Data are entities that convey meaning (computer files, music on CD, results from a blood gas analysis machine) Signals are the electric or electromagnetic encoding of data (telephone conversation, web page download) Computer networks and data/voice communication systems transmit signals Data and signals can be analog or digital
6	Introduction (continued)
7	Data and Signals Data is entities that convey meaning within a computer or computer system Signals are the electric or electromagnetic impulses used to encode and transmit data
8	Analog vs. Digital Analog is a continuous waveform, with examples such as (naturally occurring) music and voice It is harder to separate noise from an analog signal than it is to separate noise from a digital signal (imagine the following waveform is a symphony with noise embedded)
9	Analog vs. Digital (continued)
10	Analog vs. Digital (continued)
11	Analog vs. Digital (continued) Digital is a discrete or non-continuous waveform with examples such as computer 1s and 0s Noise in digital signal You can still discern a high voltage from a low voltage Too much noise – you cannot discern a high voltage from a low voltage
12	Analog vs. Digital (continued)
13	Analog vs. Digital (continued)
14	Analog vs. Digital (continued)
15	Fundamentals of Signals All signals have three components: Amplitude Frequency Phase Amplitude The height of the wave above or below a given reference point
16	Fundamentals of Signals (continued)
17	Fundamentals of Signals (continued) Frequency The number of times a signal makes a complete cycle within a given time frame; frequency is measured in Hertz (Hz), or cycles per second Spectrum – Range of frequencies that a signal spans from minimum to maximum Bandwidth – Absolute value of the difference between the lowest and highest frequencies of a signal For example, consider an average voice The average voice has a frequency range of roughly 300 Hz to 3100 Hz The spectrum would be 300 – 3100 Hz The bandwidth would be 2800 Hz
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19	Fundamentals of Signals (continued) Phase 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.
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21	Loss of Signal Strength All signals experience loss (attenuation) Attenuation is denoted as a decibel (dB) loss Decibel losses (and gains) are additive
22	Loss of Signal Strength (continued)
23	Loss of Signal Strength (continued) 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 log₁₀ (P₂ / P₁) -3 dB = 10 log₁₀ (X / 100) -0.3 = log₁₀ (X / 100) 10^-0.3 = X / 100 0.50 = X / 100 X = 50
24	Converting Data into Signals There are four main combinations of data and signals: Analog data transmitted using analog signals Digital data transmitted using digital signals Digital data transmitted using analog signals Analog data transmitted using digital signals
25	Transmitting Analog Data with Analog Signals In order to transmit analog data, you can modulate the data onto a set of analog signals Broadcast radio and television are two very common examples of this
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27	Transmitting Digital Data with Digital Signals: Digital Encoding Schemes There are numerous techniques available to convert digital data into digital signals. Let’s examine five: NRZ-L NRZI Manchester Differential Manchester Bipolar AMI
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29	Nonreturn to Zero Digital Encoding Schemes Nonreturn to zero-level (NRZ-L) transmits 1s as zero voltages and 0s as positive voltages Nonreturn to zero inverted (NRZI) has a voltage change at the beginning of a 1 and no voltage change at the beginning of a 0 Fundamental difference exists between NRZ-L and NRZI With NRZ-L, the receiver has to check the voltage level for each bit to determine whether the bit is a 0 or a 1, With NRZI, the receiver has to check whether there is a change at the beginning of the bit to determine if it is a 0 or a 1
30	Manchester Digital Encoding Schemes Note how with a Differential Manchester code, every bit has at least one significant change. Some bits have two signal changes per bit (baud rate = twice bps)
31	Manchester Digital Encoding Schemes (continued)
32	Bipolar-AMI Encoding Scheme The bipolar-AMI encoding scheme is unique among all the encoding schemes because it uses three voltage levels When a device transmits a binary 0, a zero voltage is transmitted When the device transmits a binary 1, either a positive voltage or a negative voltage is transmitted Which of these is transmitted depends on the binary 1 value that was last transmitted
33	4B/5B Digital Encoding Scheme 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 NRZI encoded signal
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35	Transmitting Digital Data with Analog Signals Three basic techniques: Amplitude shift keying Frequency shift keying Phase shift keying
36	Amplitude Shift Keying One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)
37	Amplitude Shift Keying (continued)
38	Amplitude Shift Keying (continued)
39	Frequency Shift Keying One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)
40	Frequency Shift Keying (continued)
41	Phase Shift Keying One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)
42	Phase Shift Keying (continued)
43	Phase Shift Keying (continued) Quadrature Phase Shift Keying Four different phase angles used 45 degrees 135 degrees 225 degrees 315 degrees
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45	Phase Shift Keying (continued) Quadrature amplitude modulation As an example of QAM, 12 different phases are combined with two different amplitudes Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations With 16 signal combinations, each baud equals 4 bits of information (2 ^ 4 = 16)
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47	Transmitting Analog Data with Digital Signals To convert analog data into a digital signal, there are two techniques: Pulse code modulation (the more common) Delta modulation
48	Pulse Code Modulation The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values
49	Pulse Code Modulation (continued)
50	Pulse Code Modulation (continued) When the binary values are later converted to an analog signal, a waveform similar to the original results
51	Pulse Code Modulation (continued)
52	Pulse Code Modulation (continued) The more snapshots taken in the same amount of time, or the more quantization levels, the better the resolution
53	Pulse Code Modulation (continued)
54	Pulse Code Modulation (continued) Since telephone systems digitize human voice, and since the human voice has a fairly narrow bandwidth, telephone systems can digitize voice into either 128 or 256 levels These 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)
55	Pulse Code Modulation (continued) How fast do you have to sample an input source to get a fairly accurate representation? Nyquist says 2 times the highest frequency Thus, if you want to digitize voice (4000 Hz), you need to sample at 8000 samples per second
56	Delta Modulation An analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop
57	Delta Modulation (continued)
58	The Relationship Between Frequency and Bits Per Second Higher Data Transfer Rates How do you send data faster? Use a higher frequency signal (make sure the medium can handle the higher frequency Use a higher number of signal levels In both cases, noise can be a problem
59	The Relationship Between Frequency and Bits Per Second (continued) Maximum Data Transfer Rates How do you calculate a maximum data rate? Use Shannon’s equation S(f) = f x log₂ (1 + S/N) Where f = signal frequency (bandwidth), S is the signal power in watts, and N is the noise power in watts For example, what is the data rate of a 3400 Hz signal with 0.2 watts of power and 0.0002 watts of noise? S(f) = 3400 x log₂ (1 + 0.2/0.0002) = 3400 x log₂ (1001) = 3400 x 9.97 = 33898 bps
60	Data Codes The set of all textual characters or symbols and their corresponding binary patterns is called a data code There are three common data code sets: EBCDIC ASCII Unicode
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63	Unicode Each character is 16 bits A large number of languages / character sets For example: T equals 0000 0000 0101 0100 r equals 0000 0000 0111 0010 a equals 0000 0000 0110 0001
64	Data and Signal Conversions In Action: Two Examples Let us transmit the message “Sam, what time is the meeting with accounting? Hannah.” This message leaves Hannah’s workstation and travels across a local area network
65	Data and Signal Conversions In Action: Two Examples (continued)
66	Data and Signal Conversions In Action: Two Examples (continued)
67	Data and Signal Conversions In Action: Two Examples (continued)
68	Summary Data and signals are two basic building blocks of computer networks All data transmitted is either digital or analog Data is transmitted with a signal that can be either digital or analog All signals consist of three basic components: amplitude, frequency, and phase Two important factors affecting the transfer of a signal over a medium are noise and attenuation Four basic combinations of data and signals are possible: analog data converted to an analog signal, digital data converted to a digital signal, digital data converted to an analog signal, and analog data converted to a digital signal
69	Summary (continued) To transmit analog data over an analog signal, the analog waveform of the data is combined with another analog waveform in a process known as modulation Digital data carried by digital signals is represented by digital encoding formats For digital data to be transmitted using analog signals, digital data must first undergo a process called shift keying or modulation Three basic techniques of shift keying are amplitude shift keying, frequency shift keying, and phase shift keying
70	Summary (continued) Two common techniques for converting analog data so that it may be carried over digital signals are pulse code modulation and delta modulation Data codes are necessary to transmit the letters, numbers, symbols, and control characters found in text data Three important data codes are ASCII, EBCDIC, and Unicode