mobile computing and wireless communication
TRANSCRIPT
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MOBILE COMPUTING
& WIRELESS COMMUNICATION
Jay Nagar+91-9601957620www.jaynagarblog.wordpress.com
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Introduction
• Wireless Comes of Age
• The Cellular Revolution
• The Global Cellular Network
• Broadband
• The Trouble With Wireless
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Wireless Comes of Age
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1. Transmission Fundamentals
• Transmission Fundamentals
• Provides a basic overview of transmission topics.
• Some data communications concepts
• Signaling techniques.
• Analog and Digital data transmission.
• Channel capacity,
• Transmission media, and the concept of multiplexing.
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Outline
• Signals for Conveying Information
– Time Domain Concepts
– Frequency Domain Concepts
– Relationship between Data Rate and Bandwidth
• Analog and Digital Data Transmission
– Analog and Digital Data
– Analog and Digital Signaling
– Analog and Digital Transmission
• Channel Capacity
– Nyquist Bandwidth
– Shannon Capacity Formula
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Outline
• Transmission Media
– Terrestrial Microwave
– Satellite Microwave
– Broadcast Radio
– Infrared
• Multiplexing
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Signals for Conveying Information
• We are concerned with electromagnetic signals.
• An electromagnetic signal is a function of time, but it can alsobe expressed as a function of frequency;
• Time Domain Concepts:
• An electromagnetic signal can be either analog or digital.
• Analog signal is one in which the signal intensity varies in asmooth fashion over time. In other words, there are no breaksor discontinuities in the signal.
• Digital signal is one in which the signal intensity maintains aconstant level for some period of time and then changes toanother constant level.
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Signals for Conveying Information
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Signals for Conveying Information
• The effect of varying each of the three parameters.In part (a) of the figure, the frequency is 1 Hz; thusthe period is T = 1 second
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Signals for Conveying Information
• Part (b) has the same frequency and phase but apeak amplitude of 0.5. peak amplitude of 0.5.
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Signals for Conveying Information
• In part (c) we have f = 2, which is equivalent to T = 1/2.
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Signals for Conveying Information
• Part (d) shows the effect of a phase shift.
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Signals for Conveying Information
• There is a simple relationship between the two sinewaves, one in time and one in space.
• The wavelength of a signal is the distanceoccupied by a single cycle, or, put another way,the distance between two points of correspondingphase of two consecutive cycles.
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Signals for Conveying Information
• Frequency Domain Concepts
• Relationship between Data Rate andBandwidth
– There is a direct relationship between theinformation-carrying capacity of a signal and itsbandwidth: The greater the bandwidth, the higherthe information-carrying capacity.
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Analog and Digital Data Transmission
• The terms analog and digital correspond, roughly, tocontinuous and discrete, respectively.
• These two terms are used frequently in datacommunications in at least three contexts: data, signals,and transmission.
• Data as entities that convey meaning, or information.
• Signals are electric or electromagnetic representationsof data.
• Transmission is the communication of data by thepropagation and processing of signals.
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Analog and Digital Data
• Analog data take on continuous values in someinterval For example, voice and video arecontinuously varying patterns of intensity.
• Digital data take on discrete values; examples aretext and integers.
• The most familiar example of analog data is audio,which, in the form of sound waves, can beperceived directly by human beings.
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Analog and Digital Signaling• In communications system, data are propagated from one point to
another by means of electromagnetic signals.
• An analog signal is a continuously varying electromagneticwave that may be propagated over a variety of media, dependingon frequency; examples are copper wire media, such as twistedpair and coaxial cable wireless media…
• A digital signal is a sequence of voltage pulses that may betransmitted over a copper wire medium; for example, a constantpositive voltage level may represent binary 0 and a constantnegative voltage level may represent binary l.
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Analog and Digital Signaling
• Digital data can also be represented by analogsignals by use of a modem (modulator-demodulator). The modem converts a series ofbinary (two-valued) voltage pulses into an analogsignal by modulating a carrier frequency.
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Analog and Digital Signaling
• Analog signals: Represent data withcontinuously varying electromagnetic wave
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Analog and Digital Signaling
• Digital signals: Represent data withsequence of voltage pulses
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Analog and Digital Transmission
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Analog and Digital Transmission
• Analog transmission is a means of transmittinganalog signals without regard to their content; thesignals may represent analog data (e.g., voice) ordigital data (e.g., data that pass through a modem).
• To achieve longer distances, the analogtransmission system includes amplifiers that boostthe energy in the signal.
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Analog and Digital Transmission
• Digital transmission, in contrast, is concernedwith the content of the signal. We havementioned that a digital signal can be propagatedonly a limited distance.
• To achieve greater distances, repeaters are used. Arepeater receives the digital signal, recovers thepattern of ones and zeros, and retransmits a newsignal. Thus, the attenuation is overcome.
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Channel Capacity
• A variety of impairments can distort or corrupt asignal. A common impairment is noise.
• Noise is something that degrades signal quality.
• The maximum rate at which data can be transmittedover a given communication path, or channel, undergiven conditions is referred to as the channelcapacity.
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Channel Capacity
• There are four concepts here that we are trying to relateto one another:
1. Data rate: This is the rate, in bits per second (bps), at whichdata can be communicated.
2. Bandwidth: This is the bandwidth of the transmitted signal asconstrained by the transmitter and the nature of the transmissionmedium, expressed in cycles per second, or Hertz.
3. Noise: For this discussion, we are concerned with the averagelevel of noise over the communications path.
4. Error rate: This is the rate at which errors occur, where an erroris the reception of a 1 when a 0 was transmitted or the receptionof a 0 when a 1 was transmitted.
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Channel Capacity - NyquistBandwidth
• let us consider the case of a channel that is noisefree.
• Nyquist, states that if the rate of signal transmission is2B, then a signal with frequencies no greater than Bis sufficient to carry the signal rate.
• C = 2B log2 M
– M is the number of discrete signal elements or voltagelevels.
– B is bandwidth
– For Example : M = 8, a value used with some modems, abandwidth of B = 3100 Hz yields
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Shannon Capacity
• Nyquist's formula indicates that, all other thingsbeing equal, doubling the bandwidth doubles thedata rate.
• Consider the relationship among data rate, noise,and error rate.
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Shannon Capacity
• The key parameter involved in this reasoning is thesignal-to-noise ratio (SNR, or S/N).
• This ratio is measured at a receiver.
• A high SNR will mean a high-quality signal.
• Shannon's result is that, the maximum channel capacity,in bits per second, obeys the equation.
C = B log2(1 + SNR)
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Transmission Media
• Physical path between transmitter and receiver.
• Can be classified as guided or unguided. In both cases,communication is in the form of electromagnetic waves.
• In Guided media, the waves are guided along a solidmedium, such as copper twisted pair, copper coaxialcable, or optical fiber.
• Unguided media, which provide a means oftransmitting electromagnetic signals but do notguide them; The atmosphere and outer space areexamples
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Transmission Media
• There are basically two types of configurations forwireless transmission: directional andomnidirectional.
• For the directional configuration, the transmittingantenna puts out a focused electromagnetic beam;the transmitting and receiving antennas musttherefore be carefully aligned.
• In the omnidirectional case, the transmitted signalspreads out in all directions and can be received bymany antennas.
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Transmission Media
• Three general ranges of frequencies are categories.
• Frequencies in the range of about 1 GHz (gigahertz =109 Hz) to 100 GHz are referred to as microwavefrequencies. microwave is quite suitable for point-to-point transmission.
• Frequencies in the range 30 MHz to 1 GHz are suitablefor omnidirectional applications. We refer to this range asthe radio range.
• Another important frequency range, for localapplications, is the infrared portion
• of the spectrum. This covers, roughly, from 3 X 1011 to 2X 1014 Hz.
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Multiplexing
• To make efficient use of the transmission system, itis desirable to carry multiple signals on a singlemedium. This is referred to as multiplexing.
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Multiplexing
• Two techniques for multiplexing intelecommunications networks are in common use:
• Frequency division multiplexing (FDM)
• Time division multiplexing (TDM).
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Multiplexing - FDM
• A number of signals can be carried simultaneously ifeach signal is modulated onto a different carrierfrequency and the carrier frequencies aresufficiently separated so that the bandwidths ofthe signals do not overlap.
• the channels are separated by guard bands, which are unused portions of the spectrum.
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Multiplexing - TDM
• Multiple digital signals can be carried on a singletransmission path by interleaving portions of eachsignal in time.
• The interleaving can be at the bit level or in blocksof bytes or larger quantities.
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Synchronous TDM System -Transmitter
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Synchronous TDM System -Receiver