Presentation on theme: "Computer Communication and Networks"— Presentation transcript:
1 Computer Communication and Networks Lecture # 08Computer Communication and Networks
2 Signals Why We Need For Signals One of the major concerns of data communication is moving information in the form of electromagnetic signals across mediumInformation can be voice, image, numeric data, characters or any message that is readable and has meaning to the destination user
3 Signals Why We Need For Signals For Example, you cannot roll up a photograph, insert it into the wire and transmit it across the city.You can transmit however an encoded description of the photograph.The binary digits must be converted into a form that transmission medium can accept.This is done by conducting energy along a physical path, so the data stream of 1s and 0s must be turned into energy in the form of EM signals
4 Analog and DigitalBoth data and signals that represent them, can take either analog or digital formWhat is Analog ?What is Digital ?AnalogAnalog: Refers to something that is continuous in time"Continuous" A set of specific points of data and all possible points b/w themDigitalDigital: Refers to something that is discrete"Discrete" A set of specific points of data with no points in between
5 Analog and Digital Data Data can be Analog or DigitalAnalog DataExample of analog data is human voice, when somebody speaks, a continuous wave is created in the air.This can be captured by a microphone and converted to an analog signalDigital DataExample of digital data is data stored in the memory of a computer in the form of 1s and 0s.It is usually converted to a digital signal when it is transferred from one position to the other inside or outside the computer
6 Analog and Digital Signals Signals can be Analog or DigitalSignalDetectable transmitted energy that can be used to carry informationAnalog SignalIt is a continuous waveform that changes smoothly over time.As the wave moves from value ‘ A’ to value ‘B’, it passes through and includes an infinite number of values along its pathDigital SignalA digital signal is discrete. It can have only a limited number of defined values, often as simple as 1s and 0s.The transition of a digital signal from value to value is instantaneous like a light being switched ON and OFF
7 Periodic and Aperiodic Signals Signals (Analog or Digital) can be periodic or aperiodicPeriodic SignalsA signal is called periodic if it completes a pattern within a measurable time frame called a period and then repeats that pattern over identical subsequent periods
8 Aperiodic SignalsAn aperiodic or non-periodic signal is the one that changes constantly without exhibiting a pattern or cycle that repeats over time
9 Characteristics Of Signals Signals can be described by three characteristics:AmplitudePeriod/FrequencyPhaseAmplitude of a signal is the value of the signal at any point on the wave.It is equal to the vertical distance from a given point on the wave form to the horizontal axis.The maximum amplitude of wave is equal to the highest value it reaches on the vertical axis.Amplitude measured in Volts, Amperes or Watts
10 Characteristics Of Signals Period & FrequencyPeriod: Amount of time (in seconds) a signal need to complete one cycle.Frequency: Number of cycles completed in one secondFrequency is measured in hertzPhasePhase describes the position of the waveform relative to time zero.If we think of the wave as something that can be shifted backward or forward along the time axis, phase describes the amount of that shift.Phase is measured in Degrees or Radians.
11 Analog/Digital Conversions Analog data are a function of time and occupy a limited frequency spectrum; such data can be represented by an electromagnetic signal occupying the same spectrum.Digital data can be represented by digital signals, with a different voltage level for each of the two binary digitsBut these are not the only possibilitiesDigital data can also be represented by analog signals by use of a modem (modulator/demodulator).Similarly, analog data can be represented by digital signals. The device that performs this function for voice data is a codec (coder-decoder)
12 Analog/Digital Conversions Data stored in the computer is in the form of 0’s and 1’s. To be carried from one place to the other, data is usually converted to digital signals.This is called “Digital-to-Digital Conversion” or “Encoding digital data into digital signals”Sometimes we need to convert analog data to the digital signal, for example, conversion of telephone conversation to digital signal for a no. of different reasons.This is called “Analog-to-Digital Conversion” or “Digitizing an Analog Signal”
13 Analog/Digital Conversions We might want to send a digital signal coming out of computer through a medium designed for analog signals, for example, to send data from one place to the other using a telephone line.This is called “Digital-to-Analog Conversion” or “Modulating a digital Signal”Often an analog signal is sent over long distances using analog media, for example, voice or music from a radio station which is an analog signal is transmitted through the air, however the frequency of voice or music is not, suitable for this kind of transmission.The signal should be carried by a higher frequency signal. This is called “Analog-to-Analog Conversion” or “Modulating an analog Signal”
15 Topics discussed in this section: Signal:To be transmitted, data must be transformed to electromagnetic signals.Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values.In data communications, we commonly use periodic analog signals and nonperiodic digital signals.Topics discussed in this section:Analog and Digital Data Analog and Digital Signals Periodic and Nonperiodic Signals
17 Topics discussed in this section: PERIODIC ANALOG SIGNALSPeriodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A compositeperiodic analog signal is composed of multiple sine waves.Topics discussed in this section:Sine Wave Wavelength Time and Frequency Domain Composite SignalsBandwidth
19 FigureTwo signals with the same phase and frequency, but different amplitudes
20 Figure Two signals with the same amplitude and phase, but different frequencies
21 Example -1The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows:Ms: 10 3 milliseconds
22 Frequency and period are the inverse of each other. Frequency is the rate of change with respect to time. Change in a short span of timemeans high frequency. Change over a long span of time means low frequency.
23 If a signal does not change at all, its frequency is zero. If a signal changes instantaneously, its frequency is infinite.Phase describes the position of the waveform relative to time 0.
24 Figure Three sine waves with the same amplitude and frequency, but different phases
26 Figure The time-domain and frequency-domain plots of a sine wave
27 A complete sine wave in the time domain can be represented by one single spike in the frequency domain.
28 Example-2 Figure -9 A composite periodic signal Figure 9 shows a periodic composite signal with frequency f. This type of signal is not typical of those found in data communications. We can consider it to be three alarm systems, each with a different frequency. The analysis of this signal can give us a good understanding of how to decompose signals.Figure -9 A composite periodic signal
29 Figure Decomposition of a composite periodic signal in the time and frequency domains
30 Figure The time and frequency domains of a nonperiodic signal
31 The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal.
32 Figure The bandwidth of periodic and nonperiodic composite signals
33 Example-3If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V.SolutionLet fh be the highest frequency, fl the lowest frequency, and B the bandwidth. ThenThe spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz.
35 Topics discussed in this section: DIGITAL SIGNALSIn addition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.Topics discussed in this section:Bit Rate Bit Length Digital Signal as a Composite Analog SignalApplication Layer
36 Figure Two digital signals: one with two signal levels and the other with four signal levels
37 Figure The time and frequency domains of periodic and nonperiodic digital signals
38 Figure Baseband transmission A digital signal is a composite analog signal with an infinite bandwidth.