Cbse Additional Material for Physics

Supplementary Material on Additional Sub-Topics included in Class XII Physics Theory Syllabus in the Chapter on Communication Systems for March – 2015 Examination

A. INTERNET Introduction
Invention of computers changed the working style of people in twentieth century. Its capability to tirelessly and sequentially do arithmetical and logical operations made the human life simpler and faster. Offices, universities, banks, schools etc. nothing remained unaffected by use of computers. This was not enough and before the end of twentieth century we succeeded in creating a global network of computers that provides ways to exchange information and to communicate among all computers connected to the network. This global network of computers is what we now call Internet (or simply net). Internet, in fact, is the short form of INTER-NETwork which is the interconnected network of all worldwide servers.

Networking of computers: The way Internet works
Two or more than two computers are said to be networked when they are able to exchange information between them. This sharing of information can be through wires connecting these computers or some wireless means of communications like Wi-Fi. Networking of computers at small scale (e.g. within an office, a building or a school) is called Local Area networking (LAN). One can also connect devices like printer, scanner, etc. to a LAN as shown in the figure below.

A Local Area Network (LAN)

One can built such a local area network of computers within an institution by connecting all or some of their computers. Every LAN has some main computers called server computers. These servers are used to connect LAN to other networks through telephone lines or satellites. In this way, by connecting various LAN, a Wide Area Network (WAN) is created as shown in the figure. Formation of a WAN

Various interlinked WAN together constitute what we call an Internet as shown in the figure. All information related to a local network is stored in server...