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Table of Contents
- Communication System Definition
- Analog Communication
- Digital Communication
- Advantages of Digital Communication
- Disadvantages of Digital Communication
- Comparison of Analog and Digital Communication
Analog Communication is a data transmitting technique in a format that utilizes continuous signals to transmit data including voice, image, video, electrons etc. An analog signal is a variable signal continuous in both time and amplitude which is generally carried by use of modulation. Digital communicationsis the physical transfer ofdata over a point-to-point or point-to-multipointtransmission medium. Examples of such media arecopper wires,optical fibers,wirelesscommunication media, andstorage media.
2. COMMUNICATION SYSTEM
Communications is the field of study concerned with the transmission of information through various means. It can also be defined as technology employed in transmitting messages. It can also be defined as the inter-transmitting the content of data (speech, signals, pulses etc.) from one node 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. Examples of communications subsystems in theDefense Communications System(DCS) are:
(a)A satellite linkwith oneEarth terminalinCONUSand one in Europe
(b)The interconnect facilities at each Earth terminal of the satellite link
(c)An optical fiber cablewith its driver and receiver in either of the interconnect facilities.
Communication subsystem (b) basically consists of a receiver,frequency translatorand a transmitter. It also containstranspondersand other transponders in it andcommunication satellitecommunication system receives signals from theantenna subsystem.
A communication system providesus only with a channel for the mutual informationexchange which is not apriori dedicated to certain categories ofinformationonly.
Communication system consists of Analog and Digital communication.
3. ANALOG COMMUNICATION
Analog communication is a communication method of conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable. It could be the transfer of an analog source signal using an analog modulation method such as FM or AM, or no modulation at all.
Analog transmission is still very popular, in particular for shorter distances, due to significantly lower costs and complex multiplexing and timing equipment is unnecessary, and in small "short-haul" systems that simply do not need multiplexed digital transmission.
However, in situations where a signal often has high signal-to-noise ratio and cannot achieve source linearity, or in long distance, high output systems, analog is unattractive due to attenuation problems. Furthermore, as digital techniques continue to be refined, analog systems are increasingly becoming legacy equipment.
Recently, some nations, such as the Netherlands, have completely ceased analog transmissions on certain media, such as television, for the purposes of the government saving money.
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. If you are working on a new system, and you decide to change your analog signal, you need to completely change your transmitters and receivers.
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.
4. DIGITAL COMMUNICATION
Digital communications is the physical transfer of data(a digital bit stream) over a point-to-point or point-to-multipoint transmission medium. Examples of such media are copper wires,optical fibers,wireless communication media, and storage media. The data is often represented as an electro-magnetic signal, such as an electrical voltage signal or an infra-red signal.
Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard. It may also be ananalog signal such as a phone call or a video signal,digitized into a bit-stream for example using pulse-code modulation(PCM) or more advanced source coding(data compression) schemes. This source coding and decoding is carried out by codec equipment.
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.
5. ADVANTAGES OF DIGITAL COMMUNICATION:
a). It is fast and easier.
b). No paper is wasted.
c). The messages can be stored in the device for longer times, without being damaged, unlike paper files that easily get damages or attacked by insects.
d). Digital communication can be done over large distances through internet and other things.
e). 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.
f). It removes semantic barriers because the written data can be easily chaned to different languages using software.
g). It provides facilities like video conferencing which save a lot of time, money and effort.
6. DISADVANTAGES OF DIGITAL COMMUNICATION:
a). It is unreliable as the messages cannot be recognised by signatures. Though software can be developed for this, yet the software can be easily hacked.
b). Sometimes, the quickness of digital communication is harmful as messages can be sent with the click of a mouse. The person does not think and sends the message at an impulse.
c). Digital Communication has completely ignored the human touch. A personal touch cannot be established because all the computers will have the same font!
d). The establishment of Digital Communication causes degradation of the environment in some cases. "Electronic waste" is an example. The vibes given out by the telephone and cell phone towers are so strong that they can kill small birds. In fact the common sparrow has vanished due to so many towers coming up as the vibrations hit them on the head.
e). Digital Communication has made the whole world to be an "office." The people carry their work to places where they are supposed to relax. The whole world has been made into an office. Even in the office, digital communication causes problems because personal messages can come on your cell phone, internet, etc.
f). Many people misuse the efficiency of Digital Communication.
7. COMPARISON OF ANALOG AND DIGITAL COMMUNICATION
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. 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. If you are working on a new system, and you decide to change your analog signal, you need to completely change your transmitters and receivers.
Digital signals are signals that are represented by binary numbers, "1" or "0". The 1 and 0 values can correspond to different discrete voltage values, and any signal that doesnt quite fitinto the scheme just gets rounded off. Digital signals are intolerant to noise, and digital signals can be completely corrupted in the presence of excess noise. In digital signals, noise could cause a 1 to be interpreted as a 0 and vice versa, which makes the received data different than the original data. Imagine if the army transmitted a position coordinate to a missile digitally, and a single bit was received in error? This single bit error could cause a missile to miss its target by miles. Luckily, there are systems in place to prevent this sort of scenario, such as checksums and CRCs, which tell the receiver when a bit has been corrupted and ask the transmitter to resend the data. The primary benefit of digital signals is that they can be handled by simple, standardized receivers and transmitters, and the signal can be then dealt with in software (which is comparatively cheap to change).
Analog communication systems, amplitude modulation (AM) radio being a typifying example, can inexpensively communicate a bandlimited analog signal from one location to another (point-to-point communication) or from one point to many (broadcast). Although it is not shown here, the coherent receiver provides the largest possible signal-to-noise ratio for the demodulated message.An analysis of this receiver thus indicates that some residual error will always be present in an analog system's output.
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: Because of the Noisy Channel Coding Theorem, 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; see this problem for 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.
Consequently, with the increased speed of digital computers, the development of increasingly efficient algorithms, and the ability to interconnect computers to form a communications infrastructure, digital communication is now the best choice for many situations.
Advantages and disadvantages of analog vs digital communication:
1. The first advantage of digital communication against analog is it's 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 the case of analog communication it's difficult to remove the noise once added to the signal.
2. Security is the another priority of messaging the services in the modern 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.
3. In a digital communication the signal is digitised 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.
4. 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 AMPLIFIERS are used for analog communication and REPEATERS are used in digital communication.amplifiers are needed every 2 to 3 Kms apart where as repeaters are neede every 5 to 6 Kms apart.So definitely digital communication is cheaper.Amplifiers also often add non-linearities that distort the actual signal.
5. Bandwidth is another scarce resource.Various Digital communication techniques are available that use the available bandwidth much efficiently than analog communication techniques.
6. When audio and video signals are transmitted digitally an AD(Analog to Digital) converter is needed at tansmitting side and a DA(Digital to Analog) converter is again needed at receiver side.While transmitted in analog communication these devices are not needed.
7. 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 quantisation process.
Digital communication has several advantages over analog communication system. Some are explained below:
1. Analog circuits require amplifiers, and each amplifier adds distortion and noise to the signal.
2. In contrast, digital amplifiers regenerate an exact signal, eliminating cumulative errors. An incoming (analog) signal is sampled, its value is determined, and the node then generates a new signal from the bit value; the incoming signal is discarded. With analog circuits, intermediate nodes amplify the incoming signal, noise and all.
3. Voice, data, video, etc. can all by carried by digital circuits. What about carrying digital signals over analog circuit? The modem example shows the difficulties in carrying digital over analog.