Digital communication

ADVANTAGES OF DIGITAL COMMUNICATION SYSTEM OVER ANALOG COMMUNICATION SYSTEM

ABTRACT

Analog communication is a continuous signal for which the time varying feature of the signal is a representation of some other signal. An analog signal has a infinite resolution. Analog systems are very tolerant to noise, make good use of bandwidth, and are easy to manipulate mathematically. Digital communicationsis the physical transfer ofdataover a point-to-point or point-to-multipointtransmission medium. Digital communication enables the data to be transmitted in an efficient manner through the use of digitally encoded information sent through data signals.

COMMUNICATION

Communication means transmitting messages. . Communication can also be defined as the transferring or transmitting the content of data (speech, signals, pulses etc.) from one place to another. A communication system is a combination of processes and the hardware used to accomplish the transfer of the Information.(communication). Acommunications subsystemis afunctional unitoroperational assemblythat is smaller than the larger assembly under consideration

ANALOG COMMUNICATION

Analog communication is a continuous signal for which the time varying feature of the signal is a representation of some other signal. It is helpful in transferring voice, data, video and other information.It could be the transfer of an analog source signal using an analogmodulationmethod such asFMorAM, or no modulation at all. An analog signal uses some property of the medium to convey the signal's information.

At some places anlog communication is still very popular because of its significantly lower costs and complex multiplexing and timing equipment is unnecessary or it simply does not need multiplex digital transmission. An analog signal uses some property of the medium to convey the signal's information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. Electrically, the property most commonly used is voltage followed closely by frequency, current, and charge.Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is achieved using a transducer.

An analog signal has a infinite resolution. In practice an analog signal is subject to noise and a finiteslew rate. Therefore, both analog and digital systems are subject to limitations in resolution and bandwidth. As analog systems become more complex, effects such as non-linearity and noise ultimately degrade analog resolution to such an extent that the performance of digital systems may surpass it. Similarly, as digital systems become more complex, errors can occur in the digital data stream. A comparable performing digital system is more complex and requires more bandwidth than its analog counterpart.In analog systems, it is difficult to detect when such degradation occurs. However, in digital systems, degradation can not only be detected but corrected as well.

Analog systems are very tolerant to noise, make good use of bandwidth, and are easy to manipulate mathematically. However, analog signals require hardware receivers and transmitters that are designed to perfectly fit the particular transmission Analog signals are signals with continuous values. Analog signals are used in many systems, although the use of analog signals has declined with the advent of cheap digital signals.

ADVANTAGES OF ANALOG COMMUNICATION

(1) Uses less bandwidth

(2) More accurate

DISADVANTAGES OF ANALOG COMMUNICATION

(1) The effects of random noise can make signal loss and distortion impossible to recover.

DIGITAL COMMUNICATION

Digital communicationsis the physical transfer ofdata(adigitalbit stream) over a point-to-point or point-to-multipointtransmission medium. Examples of such media arecopper wires,optical fibers,wirelesscommunication media, andstorage media. The data is often represented as anelectro-magneticsignal, such as an electrical voltage signal. The worddigitalcomes from the same source as the worddigitanddigitus(theLatinword forfinger), as fingers are used for discrete counting.

Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard. It may also be ananalogsignal such as a phone call or a video signal,digitizedinto a bit-stream for example usingpulse-code modulation(PCM) or more advancedsource coding(data compression) schemes. This source coding and decoding is carried out bycodecequipment.

Digital communication enables the data to be transmitted in an efficient manner through the use of digitally encoded information sent through data signals. These data signals are easily compressed and, as such, can be transmitted with accuracy and speed.

Unlike in an analog communications where the continuity of a varying signal can not be broken, in a digital communication a digital transmission can be broken down into packets as discrete messages. Transmitting data in discrete messages not only facilitates the error detection and correction but also enables a greater signal processing capability. Digital communication has, in large part, replaced analog communication as the ideal form of transmitting information through computer and mobile technologies.

The information source generates particular symbols at a particular rate. The source encoder translates these symbols in sequences of 0's and 1's. The channel encoder is oriented towards translating sequences of 0's and 1's to other sequences of 0's and 1's, to realize high transmission reliability and efficiency. The modulator accepts streams of 0's and 1's, and converts them to electrical waveforms suitable for transmission.

The communication channel provides the electrical connection between the source and destination. It has a finite bandwidth, and the waveform transmitted suffers from amplitude distortion and phase distortion. In addition to distortion, power is decreased due to attenuation of the channel. Finally, the waveform is corrupted by unwanted electrical signals, referred to as noise. The primary objective of a communication system is to suppress the bad effects of noise as much as possible.

The inverse process takes place at the destination side. The demodulator converts the electrical waveforms to sequences of 0's and 1's, the channel decoder translates the sequence of 0's and 1's to the original sequence of 0's and 1's. It also performs error correction and clock recovery. The source decoder finally translates the sequence of 0's and 1's into symbols.

PROPERTIES OF DIGITAL SYSTEM

All digital information possesses common properties that distinguish it from analog communications methods:

(1)Synchronization:In digital information is conveyed by the sequence in which symbols are ordered, all digital schemes have some method for determining the beginning of a sequence. In written or spoken human languages synchronization is typically provided by pauses (spaces), capitalization, and punctuation. Machine communications typically use special synchronization sequences.

(2) Language:All digital communications require alanguage, which in this context consists of all the information that the sender and receiver of the digital communication must both possess, in advance, in order for the communication to be successful. Languages are generally arbitrary and specify the meaning to be assigned to particular symbol sequences, the allowed range of values, methods to be used for synchronization, etc.

(3) Errors :Disturbances (noise) in analog communications invariably introduce some, generally small deviation or error between the intended and actual communication. Disturbances in a digital communication do not result in errors unless the disturbance is so large as to result in a symbol being misinterpreted as another symbol or disturb the sequence of symbols. It is therefore generally possible to have an entirely error-free digital communication. Further, techniques such as check codes may be used to detect errors and guarantee error-free communications through redundancy or retransmission. Errors in digital communications can take the form ofsubstitution errorsin which a symbol is replaced by another symbol, orinsertion/deletionerrors in which an extra incorrect symbol is inserted into or deleted from a digital message. Uncorrected errors in digital communications have unpredictable and generally large impact on the information content of the communication.

(4) Copying:Because of the inevitable presence of noise, making many successive copies of an analog communication is infeasible because each generation increases the noise. Because digital communications are generally error-free, copies of copies can be made indefinitely.

(5) Granularity:When a continuously variable analog value is represented in digital form there is always a decision as to the number of symbols to be assigned to that value. The number of symbols determines the precision or resolution of the resulting datum. The difference between the actual analog value and the digital representation is known asquantization error. Example: the actual temperature is 23.234456544453 degrees but if only two digits (23) are assigned to this parameter in a particular digital representation (e.g. digital thermometer or table in a printed report) the quantizing error is: 0.234456544453. This property of digital communication is known asgranularity.

SYMBOL TO DIGITAL CONVERSION

Since symbols are not continuous, representing symbols digitally is rather simpler than conversion of continuous or analog information to digital. Instead ofsamplingandquantizationas inanalog-to-digital conversion, such techniques aspollingandencodingare used.

A symbol input device usually consists of a number of switches that are polled at regular intervals to see which switches are pressed. Data will be lost if, within a single polling interval, two switches are pressed, or a switch is pressed, released, and pressed again. This polling can be done by a specialized processor in the device to prevent burdening the mainCPU. When a new symbol has been entered, the device typically sends aninterruptto alert the CPU to read it.

For devices with only a few switches (such as the buttons on ajoystick), the status of each can be encoded as bits (usually 0 for released and 1 for pressed) in a single word. This is useful when combinations of key presses are meaningful, and is sometimes used for passing the status of modifier keys on a keyboard (such as shift and control). But it does not scale to support more keys than the number of bits in a single byte or word.

Devices with many switches (such as acomputer keyboard) usually arrange these switches in a scan matrix, with the individual switches on the intersections of x and y lines. When a switch is pressed, it connects the corresponding x and y lines together. Polling (often called scanning in this case) is done by activating each x line in sequence and detecting which y lines then have a signal, thus which keys are pressed. When the keyboard processor detects that a key has changed state, it sends a signal to the CPU indicating the scan code of the key and its new state. The symbol is thenencoded, or converted into a number, based on the status of modifier keys and the desiredcharacter encoding.

A customencodingcan be used for a specific application with no loss of data. However, using a standard encoding such asASCIIis problematic if a symbol such as 'ß' needs to be converted but is not in the standard.

WAVEFORM IN DIGITAL SIGNAL

Generally the wave forms are drawn between two voltage aBooleanvalue (0 and 1) is referred to as adigital signal, even though it is an analog voltage waveform, since it is interpreted in terms of only two levels.

Theclock signalis a special digital signal that is used tosynchronizedigital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.

The given diagram is an example of the practical pulse and therefore we have introduced two new terms that are:

(1) Rising edge: the transition from a low voltage (level 1 in the diagram) to a high voltage (level (2) Falling edge: the transition from a high voltage to a low one.

Although in a highly simplified and idealised model of a digital circuit we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can instantly change voltage levels. This means that during a short, finitetransition timethe output may not properly reflect the input, and indeed may not correspond to either a logically high or low voltage.

ADVANTAGES OF DIGITAL COMMUNICATION

The advantages associate with digital communication systems include

(1) A common format for encoding different kinds of message signals (e.g., speech signal, video signal, computer data, etc.) for the purpose of transmission.

(2) An improved security of message.

(3) Increased immunity to noise and external interference.

(4) Flexibility in configuring digital communication systems.

DISADVANTAGES OF DIGITAL COMMUNICATION

The disadvantages associate with digital communication systems include

(1) Increased transmission

(2) Increased system complexity

(3) It is unreliable as the messages cannot be recognised by signatures. Though software can be developed for this, yet the softwares can be easily hacked.

(4) Sometimes, the quickness of digital communication is harmful as messages can be sent with the click of a mouse. The person oes not think and sends the message at an impulse.

(5) Digital Communication has completely ignored the human touch. A personal touch cannot be established because all the computers will have the same font

Although digital communication is becoming increasingly attractive for practical use, it is a fact that analog communication systems are still in existence. Most of the broadcasting systems and a large part of telephone networks in use today are analog in nature.

There are numerous ways of classifying signals depending on the requirement of study. A signal is usually described by a function of one or more independent variable. The value of the signal may be real or complex. We would consider the signal to be a function of a signal independent variable i.e. time t. Analog signals or continuous-time signals are defined for every value of time and they take on continuous values in a given time interval.

ADVANTAGES OF DIGITAL COMMUNICATION OVER ANALOG COMMUNICATION

Analog communication systems, amplitude modulation (AM) radio being a typifying example, can inexpensively communicate a band limited analog signal from one location to another (point-to-point communication) or from one point to many. Although analog systems are less expensive in many cases than digital ones for the same application, digital systems offer much more efficiency, better performance, and much greater flexibility.

1. Efficiency: The Source Coding Theorem allows quantification of just how complex a given message source is and allows us to exploit that complexity by source coding (compression). In analog communication, the only parameters of interest are message bandwidth and amplitude. We cannot exploit signal structure to achieve a more efficient communication system.

2. Performance: As there is noise in different channels,we have a specific criterion by which to formulate error-correcting codes that can bring us as close to error-free transmission as we might want. Even though we may send information by way of a noisy channel, digital schemes are capable of error-free transmission while analog ones cannot overcome channel disturbances; seethis problemfor a comparison.

3. Flexibility: Digital communication systems can transmit real-valued discrete-time signals, which could be analog ones obtained by analog-to-digital conversion,andsymbolic-valued ones (computer data, for example). Any signal that can be transmitted by analog means can be sent by digital means, with the only issue being the number of bits used in A/D conversion (how accurately do we need to represent signal amplitude). Images can be sent by analog means (commercial television), but better communication performance occurs when we use digital systems (HDTV). In addition to digital communication's ability to transmit a wider variety of signals than analog systems, point-to-point digital systems can be organized into global (and beyond as well) systems that provide efficient and flexible information transmission.Computer networks, explored in the next section, are what we call such systems today. Even analog-based networks, such as the telephone system, employ modern computer networking ideas rather than the purely analog systems of the past.

4. One of the main advantage of digital communication over analog is the noise immunity. In any transmission path some unwanted voltage or noise is always present which cannot be eliminated fully. when signal is transmitted this noise gets added to the original signal causing the distortion of the signal. However in a digital communication at the receiving end this additive noise can be eliminated to great extent easily resulting in better recovery of actual signal. In case of analog communication it's difficult to remove the noise once added to the signal.

5. Security is very important these days. The Digital communication provides better security to messages than the analog communication. It can be achieved through various coding techniques available in digital communication.

6. In a digital communication the signal is digitized to a stream of 0 s and 1 s. So at the receiver side a simple decision has to me made whether the received signal is a 0 or a 1.Accordingly the receiver circuit becomes simpler as compared to the analog receiver circuit.

7. When signal is travelling through it's transmission path gets faded gradually. So on it's path it needs to be reconstructed to it's actual form and re-transmitted many times. For that reason amplifier are used for analog communication and reapeters are used in digital communication. Amplifiers are needed every 2 to 3 Kms apart where as repeaters are needed every 5 to 6 Kms apart. So definitely digital communication is cheaper. Amplifiers also often add non-linearity's that distort the actual signal.

8. Bandwidth variation is another scarce resource. Various Digital communication techniques are available that use the available bandwidth much efficiently than analog communication techniques.

9. Digitally an AD(Analog to Digital) converter is needed when audio and video signals are transmitted at transmitting side and a DA(Digital to Analog) converter is again needed at receiver side. While transmitted in analog communication these devices are not needed.

10. Digital signals are often an approximation of the analog data(like voice or video) that is obtained through a process called quantisation.The digital representation is never the exact signal but it's most closely approximated digital form. So it's accuracy depends on the degree of approximation taken in quantization process.

CONCLUSION

The use of digital communication system is increasing in day to day life. Digital communication can be done over large distances through internet and other things. It is comparatively cheaper and the work which requires a lot of people can be done simply by one person as folders and other such facilities can be maintained. It removes semantic barriers because the written data can be easily changed to different languages using software. It provides facilities like video conferencing which save a lot of time, money and effort.

REFRENCES

[1].http://www.tutorvista.com/content/physics/physics-iv/communication-systems/communication-systems-advantages.php

[2].http://www.answerbag.com/q_view/31169

[3].http://en.wikipedia.org/wiki/

[4].http://nptel.iitm.ac.in/courses/Webcourse-contents/IISc-BANG/Data%20Communication/Learning%20Material%20-%20DataCommunication.pdf

[5]. http://cnx.org/content/m0074/latest/

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