2.1 Concept of Communication System (Unit 2)

        The term communication refers to the sending, receiving, and processing of information by electronic media. It started with wire telegraphy in the eighteen forties (1840s), developing with telephony some decades later and radio at the beginning of this century. Radio communication was made possible by the invention of the triode tube (one kind of vacuum tube). It was greatly stimulated by the work done during World War II. Consequently, it became even more widely used and refined through the invention and use of the transistor, integrated circuits, and other semiconductor devices. More recently, the use of satellites and fiber optics has made communications even more widespread, with an increasing emphasis on computer and other data communications. 

        A modern communication system is first concerned with the processing and storing of information before its transmission. The forms of communication include radio telephony and telegraphy, broadcasting, point-to-point, and mobile communications, computer communications, and radio aids to navigation.

        When the computer was first introduced in scientific work and business - a medium-sized company or university might have one or few computers, while large institutions had at most a few dozens. 

            The merging of computers and communications has had a profound influence on the way the computer system is organized. The old model of a single computer serving all the organization's computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. This system is called-"Computer Network.”

A computer network is defined as two or more computing devices linked together to communicate, sharing information (files and software programs) and other hardware resources (printers, hard disks, plotters). The concept of connected computers sharing resources is called networking. Most networks are constructed around a cable connection that links the computers. This connection permits the computers to share (talk and listen) through a wire. More recently, several wireless solutions have become available: Infrared ports, Bluetooth, radio links, Wi-Fi, satellite, and other protocols allow a variety of new devices to link with PCs.

Advantages of using Computer Network
Some of the advantages of using computer network are:
1. Resource sharing: Sharing a resource is an important area in which a network exceeds the stand-alone computer. Most institutions cannot afford enough laser printers, fax machines, MODEMs, scanners, and CD-ROM players for each computer. However, if these peripherals are added to a network, they can be shared by all users in a node (computers are in a network is known as a node.)
2. Centralized control and management: The networks provide centralized control and management. The entire computers in the network are normally connected to a server, which defines the policy and security measures for the resources used by the network members.
3. Speedy and cost-effective communication: Since the entire computer is connected, the message can send from one computer to another within few seconds.
4. Backup and recovery: Data are securely handled by the server and provide the mechanism for the backup in other devices with the use of the network. Similarly, the network is also used to recover data and system in case of system crash using backup or through the server.
5. Flexible access: The network allows people to easily access their files from any computer throughout the network.
6. Workgroup computing: Workgroup software (such as Microsoft Office) allows many users to work on a document or project concurrently. For example, experts located at various schools within a county could simultaneously contribute their ideas about new curriculum standards to the same document.
Disadvantages of using Computer Network
Some of the disadvantages of using computer network are:
1. Expensive: For connecting computer, some extra devices and resources like NIC, Hub, cables are required, which increases the cost of operation.
2. Security problem: Data are shared in a public medium so extra precaution is needed for secure transmission and storage. In addition, due to the connected computers, there is an increase in computer crimes like sending viruses, software piracy, hacking, plagiarism, pornography, spoofing.3. Needs special technical knowledge: Skilled manpower is required to establish, maintain, and operate a networking system.

4. Network Failure: Failure of the network may hamper the day-to-day operation of the organization. For example, network failure in a bank may affect the transaction.
5. Complex: The network itself is difficult to establish, manage and operate

2.2 Basic Elements of Communication System
The communication system is a system which describes the information exchange between two points. The process of transmission and reception of information is called communication. The basic elements of communication system:
1. Transmitter of information: The transmitter's function is to process the message signal into a form suitable for transmission over the communication channel. It is the sender which sends data to the receiver.
2. Channel or medium of communication: Its function is to provide a pathway between the transmitter's output and the receiver's input. It transmits data from the sender to the receiver.
3. Receiver of information: The job of the receiver is to process the received signal to recover the appropriate message signal. It receives data sent by the sender.
2.3 Block Diagram of Communication System/Model

àStudy from book

2.4 Elements of Data Communication/Transmission
The elements of data communication/transmission are:
1. Information or input signal: The communication systems have been developed for communicating useful information from one place to other. The information can be in the form of sound signals like speech or music or it can be in the form of pictures.

2. Input transducer: The information in the form of sound, picture or data signals cannot the transmitted as it is. First it must be converted into a suitable electrical signal. The input transducers commonly used in communication systems are microphones, TV, etc.
3. Transmitter: The function of the transmitter block is to convert the electrical equivalent of the information to a suitable form. It increases the power level of the signal. The power level should be increased to cover a large range. The transmitter consists of the electronics circuits such as amplifier, mixer, oscillator, and power amplifier.
4. Communication channel or medium: The communication channel is the medium used for the transmission of electronic signals from one place to another. The communication medium can be conducting wires, cables, optical fibres or free space.
5. Noise: Noise is an unwanted electrical signal which gets added to the transmitted signal when it is travelling towards receiver. Due to noise, the quality of the transmitted information will degrade. One added the noise cannot be separated from the information. Hence, noise is a big problem in communication systems.
6. Receiver: The reception is exactly the opposite process of transmission. The received signal is amplified and demodulated and converted in a suitable form. The receiver consists of electronic circuits like a mixer, oscillator, detector, and amplifier.
7. Output transducer: It consists of the electrical signal at the output of the receiver back to the original form i.e. sound or TV pictures. The typical example of output transducers are loud speakers, picture tubes, etc.
2.5 Communication Mode
Communication mode defines the direction of data flow in the communication. There are three possible modes:

1. Simplex: In this mode transmission is possible in only one direction similar to a one-way street. A simplex line can either only receive or send data. The full capacity of the channel is used for one way communication. For example: Communication between keyboard and CPU, remote and television, loudspeaker, Fire alarm system.