Communication: “Communication” can be defined as the exchange of information of any kind by any means from one location to another location. Communication System: “Communication system” is the system that is involved to achieve the goal of information exchange.
Example: Exchange of voice signals between two telephones over the same network. Communication between a workstation and a server over a public telephone network. Telecommunication: “Tele” is a Greek word means “Far” and “Communication” means exchange of information of any kind by any means from one location to another location. So, combining both- “Telecommunication” is exchang e of information between distant points. In other word and to be specific, the study of telephones and the systems that transmits and receives telephone signal is called telecommunication. Telecommunication includes telephony, telegraphy, satellite communication, optical fiber communication, radar communication etc. Data Communication: The word “Data” refers to information presented in whatever form is agreed upon by the parties creating and using the data. So, Data communication is the transfer of digital or analog data using digital or analog signals over some form of network. Voice communication:
Components of communication of any kind:
There are three basic components of o f any communication system. They areTransmitter Receiver and Channel.
Principle of basic communication system:
: In the transmission section, first of all, the source generated information is fed to the input transducer, which converts energy of one form to another form, usually in the electrical form. This electrical signal or base band signal is sent to the transmitter. Transmitter: Transmitter modifies the information signal for efficient transmission. It modulates the information signal with a high frequency carrier. After processing the signal transmitter transmits the signal through channel to the receiver. Channel: Channel, media or path implies the medium through which the message travels from the transmitter to the receiver. A channel acts partly as a filter to attenuate the signal and distorts its waveform. The signal attenuation increases with the length of the channel. there are different types of channel exist for different communication system, such as, wire, coaxial cable, wave-guide, optical fiber or radio link through which transmitter output is sent. Receiver: Receiver reprocesses the signal received from the channel by undoing the signal modifications made at the transmitter and the channel. The receiver output is fed to the output transducer, which converts the electrical signal to its original form. By this way, the signal reached to its destination, to which the message is communicated. Digital communication: Digital communication system exchange (both transmit and receive) information to / from digital sources.
A digital (information) source produces a finite set of possible messages. Typewriter is a good example of a digital source. There is a finite no. of characters that can be emitted by this source. Analog communication: Analog communication system exchange (both transmit and receive) information to / from analog sources. A microphone is a good example of a analog source. An analog information source produces messages that are defined on a continuum. Why we use digital not analog? Digital communication has a number of advantages: Relatively inexpensive digital circuits may be used. • Digital systems are relatively easy to design and can be fabricated on IC chips. • Information storage is easy. • Operation can be programmable to update with newly upcoming technologies. • Privacy is preserved by using data encryption. • Greater dynamic range is possible. • Data from voice, video and data sources may be merged and transmitted over a • common digital transmission system. i.e. it is easy to multiplex several digital signals. In long distance communication system, noise does n ot accumulate from repeater • to repeater. Error detection and correction schemes can be employed by using coding. • Does Digital communication system have any limitations? Yes, indeed. Generally, more bandwidth is required than that for analog system. • Synchronization is required. That call for more sophisticated device and • would cost more. How we can get digital signal from analog signal?
Basic steps for A/D conversion: A/D conversion has three steps: a. Sampling b. Quantization c. Coding. Sampling: In this process, Continuous-time signal is converted to Discrete-time signal obtained by taking samples of the continuous-time signal at discrete-time instants. If Xa (t) is the input to the sampler, the output o utput is Xa (nT), where T is called the sampling interval. Quantization : In this process, a Discrete-time Continuous- valued signal is converted into a Discrete-time Discrete-valued (digital) signal. The sampled signal is rounding off to the4 nearest value which is permitted for transmission by the system. The process of rounding
off is called Quantization, while the possible levels permitted for transmission are called Quantizing levels. Coding: In the coding process, each discrete value Xq (n) is represented by a b-bit binary sequence.
How to get analog signal from digital signal? How Sampling theorem or Nyquest theorem determines the sampling needed?
Sampling Theorem / Nyquest theorem:
"If the maximum frequency of any signal spectrum is B (Hz), then the signal can be reconstructed from its samples taken at the rate of not less than 2B samples per second." i.e. the sampling rate must be at least two times the highest frequency contained in the signal. Sampling theorem is also called Nyquist theorem. Explanation:
In
order to transmit
the
information in a continuous-time signal, we need only transmit its samples. But, the sample values are not digital because they lie in a continuous range and can take on any one of the infinite values in the range. The problem is resolved by Quantization, where each sample is approximated or rounded off to the nearest quantized level. Amplitude of the signal m(t) lie in the range (-m p, m p), which is partitioned into L intervals, each of magnitude ∆ V = 2m p / L . Each amplitude of the sample is approximated to the midpoint of the interval in which the sample value falls. Each sample is now approximated to one of the L numbers, the information is thus digitized.
What is system? System: A “System” is an entity that processes a set of signals (inputs) to yield another set of signals (outputs). A system may be made up of physical components, as in electrical, mechanical or hydraulic systems or it may be an algorithm that computes an output from an input signal. What is signal?
Signal: a “Signal” is a set of information or data.
Example includes a telephone or a television signal, monthly sales of a corporation, o r the daily closing prices of a stock market. In all these cases, the signals are functions of independent variable time. How we can classify Signal? Classification of signals: Signals are classified asa. Contin Continuous uous time time and disc discret retee time time signal signalss b. b. Anal Analog og and and digit digital al sign signal als. s. c. Peri Period odic ic and and Aperi Aperiodi odicc signal signalss d. Ener Energy gy and and power power signa signals ls e. Determ Determini inisti sticc and probabil probabilist istic ic signal signals. s. a) Continuous time and discrete time signals: A signal that is specified for every value of t is a continuous – time signal. Example: Telephone and video camera outputs are continuous time signal. A signal that is specified for only at discrete values of t is a discrete time signal. Example: Monthly sales of a corporation, Quarterly gross national product are examples of discrete-time signals.
b. Analog and digital signals. A signal whose amplitude can take on any value in a continuous range is an analog signal. i.e. an analog signal can take on an infinite number of values.
Example: Music, temperature variation throughout the day as displayed on mercury thermometer. A signal whose amplitude can take on only a finite number of values is a digital signal.
Example: Signal associated with a digital computer are digital because they take on only two values.
c. Periodic and Aperiodic signal:
Any continuous time signal that satisfies the condition – x( t ) = x ( t + nT ) Where, n = 1, 2, 3 … T = Fundamental period.
Example: All trigonometric functions ate best example of periodic function. Another periodic function is shown in fig. 3(a) Any continuous time signal which does not satisfies the condition (1) , i.e. not periodic is called aperiodic signal.
d. Energy and Power signal:
a signal with finite energy is an energy signal. A signal g(t) is an energy signal if-
A signal with finite and non-zero power is called power signal.
A signal g(t) is power signal if-
e. Deterministic and Random signal:
A signal whose physical description is known completely, in either a mathematical form or a graphic form is a deterministic signal. A signal which is known by only in terms of probabilistic description, such as mean value, mean squared value, rather than its complete mathematical or graphical description is a random signal.
Does transmitted signal reaches to receiver as it was send?
How signal changes in between generation and reception?
What is the transmission impairment? Signal travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are 1. ATTENUATION: Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it losses some of its energy in ov ercoming the resistance of the medium. That is why a wire carrying electric signals gets warm. i.e. some of the electrical energy is converted to heat energy. 2. DISTORTION: Distortion means that the signal changes its form or shape. Distortion can occur in a composite c omposite signal made of different frequencies. Each signal component has its own propagation speed through a medium and therefore
