# Computer Communication and Networks

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Computer Communication and Networks
Lecture # 08 Computer Communication and Networks

Signals Why We Need For Signals
One of the major concerns of data communication is moving information in the form of electromagnetic signals across medium Information can be voice, image, numeric data, characters or any message that is readable and has meaning to the destination user

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

Analog and Digital Both data and signals that represent them, can take either analog or digital form What is Analog ? What is Digital ? Analog Analog: Refers to something that is continuous in time "Continuous" A set of specific points of data and all possible points b/w them Digital Digital: Refers to something that is discrete "Discrete" A set of specific points of data with no points in between

Analog and Digital Data
Data can be Analog or Digital Analog Data Example 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 signal Digital Data Example 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

Analog and Digital Signals
Signals can be Analog or Digital Signal Detectable transmitted energy that can be used to carry information Analog Signal It 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 path Digital Signal A 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

Periodic and Aperiodic Signals
Signals (Analog or Digital) can be periodic or aperiodic Periodic Signals A 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

Aperiodic Signals An aperiodic or non-periodic signal is the one that changes constantly without exhibiting a pattern or cycle that repeats over time

Characteristics Of Signals
Signals can be described by three characteristics: Amplitude Period/Frequency Phase Amplitude 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

Characteristics Of Signals
Period & Frequency Period: Amount of time (in seconds) a signal need to complete one cycle. Frequency: Number of cycles completed in one second Frequency is measured in hertz Phase Phase 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.

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 digits But these are not the only possibilities Digital 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)

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”

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”

Analog/Digital Conversions

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

Topics discussed in this section:
PERIODIC ANALOG SIGNALS Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves. Topics discussed in this section: Sine Wave Wavelength Time and Frequency Domain Composite Signals Bandwidth

Figure A sine wave

Figure Two signals with the same phase and frequency, but different amplitudes

Figure Two signals with the same amplitude and phase, but different frequencies

Example -1 The 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

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 time means high frequency. Change over a long span of time means low frequency.

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.

Figure Three sine waves with the same amplitude and frequency, but different phases

Figure Wavelength and period

Figure The time-domain and frequency-domain plots of a sine wave

A complete sine wave in the time domain can be represented by one single spike in the frequency domain.

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

Figure Decomposition of a composite periodic signal in the time and frequency domains

Figure The time and frequency domains of a nonperiodic signal

The bandwidth of a composite signal is the difference between the
highest and the lowest frequencies contained in that signal.

Figure The bandwidth of periodic and nonperiodic composite signals

Example-3 If 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. Solution Let fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then The spectrum has only five spikes, at 100, 300, 500, 700, and 900 Hz.

Figure The bandwidth for Example-4

Topics discussed in this section:
DIGITAL SIGNALS In 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 Signal Application Layer

Figure Two digital signals: one with two signal levels and the other with four signal levels

Figure The time and frequency domains of periodic and nonperiodic digital signals

Figure Baseband transmission
A digital signal is a composite analog signal with an infinite bandwidth.

Figure Bandwidths of two low-pass channels

Figure Baseband transmission using a dedicated medium