EVOLUTION OF MODERN COMMUNICATION

The history of modern electronic communications begun with Alexander Graham Bells telephone experiments, where speech was able to be converted into electrical energy, transmitted along physical wires and reconstructed at the receiver. Speech, which is actually vibration of the air, vibrated a paper cone to which a small coil was attached. This induced an electrical signal into the coil, which was proportional to the vibration of the paper cone. Alexander Graham Bell, born in Edinburgh, Scotland, 1847, emigrated to the United States, settling in Boston. Bell, interested in the education of deaf people, invented the microphone, and later, in 1876, an electrical speech machine, called the telephone.

By 1878, Bell set up the first telephone exchange in New Haven, Connecticut, and in 1884, long distance connections were made between Boston, Massachusetts and New York City.

In these early experiments at transmitting speech. copper wire was used as the connection media over which the signals traveled. This is due to copper wire being a very good conductor, which lets electrical signals flow down the wire easily.
As the telephone became popular, more and more people wanted to communicate with each other, so a switching center (telephone exchange) evolved. Each customer was connected to the telephone exchange via a pair of wires, which carried the signal from their telephone.

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 As the need to inter-connect telephone customers grew, they were connected via overhead wires to a central switching center, where the physical wires from each customer was connected to the physical wires of another customer via a manual operator.

As more and more customers were connected, the need for more and more operators to connect calls were required. This quickly became unworkable, so development began on automating the connection process between customers, hence, automatic telephone switching exchanges became a reality and replaced local operators, who were still used to connect toll (long distance) calls. The customers wires from their home to the telephone switching center were overhead, installed in the same way as a lot of existing electrical (power) wires are today, on top of long poles inserted into the ground, about 15 or so feet high. This quickly became very cumbersome and difficult to manage. Transmission quality was affected by the weather, as the rain created leakage paths for the signals to go to ground instead of along the wire. At the same time, demand grew to connect customers who lived further and further away, in rural areas. Electrical signals can only travel so far, and to provide service to rural customers meant using better cable that allowed the signals to travel longer distances. Unfortunately, this was costly, so something had to be done to provide them with service. In addition, customers also wanted to be able to talk to other people in different cities, so there developed a need to interconnect telephone exchanges together.

The cost of cable was very high, and before long special equipment was used to carry more than one speech conversation on one pair of wires. This was done by a technique known as multiplexing (specifically Frequency Division Multiplexing, which separates each conversion by frequency).

This bought down the cost of providing speech circuits (one circuit=one speech conversation) to customers. Whereas previously one cable equated to one speech circuit, now a cable could be equated to hundreds of speech circuits. This allowed an increase in revenue to be collected per cable by the telephone companies.

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Radio Paging Systems

Radio Paging Systems:

As a matter of fact, two persons are required for any form of human communication however, for good communication everyone needs to be contactable. Mobile radio is for communication with people on the move. Paging is a simple land mobile radio system to solve the all-times, all-places, all-people communication problems with very economic use of the radio spectrum, paging system can keep thousands of people in touch, with a small, light-weight, on obstructive, body-worm equipment. Present day paging developments are available with several innovative features and powerful message capabilities and are supported by nation-wide and are international transmitting and control facilities. Even in India, a sophisticated modern paging system with many useful features is being planned Mumbai and other metropolises too.

This article reviews the basic paging concepts, on site paging history, wide area paging developments operations of a public paging system constituents of a paging control and transmitting system anatomy of miniaturised  pager unit, innovative features/facilities of an ultra modern system, user’s multiple benefits, numerous beneficiaries, global growth rate and future trends.

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Analog and Digital Communication

Analog and Digital communication:

Analog communication is that type of communication in which message or information signal to be transmitted is analog in nature. This means that in analog communication the modulation signal is an analog signal. This analog message signal may be obtained from sources as speech, video shooting etc… In analog communication, the analog message signal modulates some high carrier frequency inside the transmitter to produce modulated signal.

In digital communication, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form.

Advantages of Digital Communication:

1. The digital communication systems are simpler and cheaper compare to analog communication systems because of advances made in IC technologies.
2. In digital communication, the speech, video and other data may be merged and transmitted over a common channel using multiplexing.
3. Using data encryption, only permitted receivers may be allowed to detect the transmitted data.
4. Since the transmission is digital and the channel encoding is used, therefore the noise does not accumulate from repeater or repeater in long distance communications.

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Classification of Communication Technology

Classification of Communication Technology:

Regarding the mode of propagation, communication may be divided in the following two forms:

1. Lines communication:

In line communication, the medium of transmission is a pair of conductors called transmission line. This is also called as line as line channel. This means that in line also called as line communication, the transmitter and the receiver are connected through a wire or line. However, the installation and maintenance of a transmission line is not only costly and complex but also overcrowds the open space. Apart from this, its message transmission capability is also limited.

2. Wireless communication:

In wireless communication or radio communication, a message is transmitted through open space by electromagnetic waves called as radio waves. Radio waves are radiated from the transmitter in open space through a device called antenna. A receiving antenna intercepts the radio waves at the receiver. All the mobile, radio, TV and satellite broadcasting are wireless communication. The advantages of wireless communication are cost effectiveness, possible long distance communication and simplicity.

    

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Communication Channel

 1.Telephone channels:Telephone channels are based on circuits switching mechanism to establish an end to end communication link on a temporary basic. Telephone channels support only the transmission of electrical signal. Therefore appropriate transducers are used at the transmitting and receiving end of the channel. Further telephone channel are essentially a linear band width limited channel.

2.Optical fiber:

An optical fiber is a dielectric wave guide which transports light signals from one place to another. Just as a metallic wire pair or a coaxial cable. Just as a metallic wire pair or a coaxial cable transport electrical signal. There are numerous advantages of optical fibers.

1. It has innermost potential bandwidth which is around 2*10^14Hz.
2. Low transmission losses.
3. Immunity to electromagnetic interference
4. Small size and weight.
5. Ruggedness and flexibility.

Mobile Radio Channel:

It extends the capability of public telecommunication network by introducing mobility into the network by virtue of its ability to broadcast. Basically there is no ‘line-sight’ path for communication rather radio propagation takes place multi path and mainly by the way scattering from the surface of surrounding building and by defraction over them. Therefore, the energy reaches the receiving antenna via more than one path. Therefore, in a mobile radio environment, we face a problem of multi path phenomenon in the sense that the various incoming radio waves reach their destination from different directions and with different time delays. Because of this, the received signal strength varies with location in a very complicated manner and therefore, a mobile radio channel may be viewed as a linear time- varying channel that is statistical in nature.

Satellite channel:

It provides broad area coverage in a continential as well as intercontinential sense. In almost all satellite communication system satellite are placed in Geo stationary orbit which is at an altitude of 22,300 miles (36000kms) satellite communication provides broad area coverage, reliable transmission links with wide transmission bandwidth. Most popular frequency band for satellite communication is 601Hz-uplink and 401-downlink. These frequencies are used because these used relatively in expensive microwave equipment the signal are less attinuatated due to rainfall. In 6/401 Hz typical satellite is assigned, a 500 MHZ bandwidth i.e. divided amount 12 transponders on board the satellite. Signal transponders can carry at least one color television signal, 12 hundred voice signal or digital data at a rate of 50Mbps.

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SOURCES OF COMMUNICATION TECHNOLOGY

A source of information may be characterized in terms of the signal which carries the information. Further, a signal is defined as a signal- valued function of time that plays the role of the dependent variable. At every instant of time, the function has a unique value.

Speech:

It is the primary method for human communication. Basically, the speech communication process involves the transfer of information from a speaker to a listener, which takes place in three successive stages as under:

1.      Production: An intended message in the speaker’s mind is represented by a speech signal that consists of sounds generated inside the speaker’s mouth and whose arrangement is governed by the rules of language.

2.      Propagation: The sound waves propagated through the air reaching the listener’s ears.

3.      Perception: The incoming sounds are deciphered by the listener into a received message, and thus completing the chain of events that result in the transfer of information from the speaker to the listener.

Television:

The second source of information, television (TV), refers to the transmission of pictures in motion by means of electrical signals. To accomplish this transmission, each complete picture has to be sequentially scanned. The scanning process is carried out in a TV camera. In a back –and-white TV, the camera contains optics designed to focus an image on a photo cathode consisting of a large number of photo sensitive elements. The charge pattern so generated on the photo sensitive surface is scanned by an electron beam, and thus producing an output current which varies temporally in accordance with the way in which the brightness of the original picture varies spatially from one point to another. In television, a picture is divided into 525 lines, which constitute a frame. Now each frame is decomposed into two interlaced fields, each one of which consists of 262.5 lines.

Facsimile:

The purpose of the third source of information, facsimile (fax) machine, is to transmit still pictures over a communication channel. Such a machine provides a highly popular facility for the transmission of hand written or printed text from one point to another. Further, transmitting text by facsimile is treated simply like transmitting a picture. The basic principle employed for signal generation in a facsimile machine is to scan an original document and used an image sensor to convert the light to an electrical signal.

Personal computer:

It is becoming increasingly an important part of our daily lives. We use them for electronics mail, exchange of software, and sharing of resources. It is estimated that over 30% of the personal computers in use today are already networked and the number is increasing rapidly. The text transmitted by a PC is usually encoded using the American standard code for Information Interchange (ASCII), which the first code developed specifically for computer communications. Personal computers are often connected via their RS-232 ports. When ASCII data are transmitted through these ports, a start bit, set too, and one or more stop bits, set to 1, as depicted are added to provide character framing. When the transmission is idle, a long series of 1s is sent so as to keep the circuit connective a live.

Cellular and mobile communication:

Although the concept of cellular communication was developed in 1947 in AT and T Bell laboratories, U S A but the first tests were conducted to explore the possibility in commercial applications. After that it then took another 8yrs when the Federal communication commission in USA, set aside new radio frequency for “Land mobile communication”. In this particular year, AT and T proposed to establish the very first high capacity cellular telephone system. It was then called as the advanced mobile phone service or Amps.

NOISE IN COMMUNICATION TECHNOLOGY:

As the message signal travels from the transmitter to the receiver through a medium called channel. Now noise is present in every communication system. The channel introduces an additive noise in the message signal and thus the message which is received at the receiver is distorted. Since, the receiver detects both message signal and the noise signal. It will reproduce a message signal which contains noise. The noise calculation in a communication system is carried out in form of a parameter which is known as figure of merit.

      Channel noise is always white and Gaussian we assume that the channel noise n(t) is always a white noise. This means that it is uniformly distributed over the entire band of frequencies under consideration. Hence, the power spectrum density of channel noise will be uniform over the frequency range under consideration. Thus the total noise power N may be obtained as:

      N=n/2*bandwidth.

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