1.1 INTRODUCTION: Ø Computer Networks: A collection of autonomous computers interconnected by a single technology to facilitate data communication. · Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be of used. · The computers are autonomous, which are not forcibly started, stopped or controlled by other one. · A system with one control unit and more than one slave is not a computer network. · Computer network consists of end systems or nodes which are capable of transmitting information and which communicate through a transit system interconnected them. The transit system also called as interconnection subsystem or sub network. · The nodes in the computer network comprise the computer, terminals, software and peripherals forming an autonomous system capable of performing information processing. · End system has an interface or interaction through which it is physically connected with subnet. · The interaction point has an address by which end system is identified. · Each end system hosts one or more application entities by which the communication takes place between end systems. · The subnet performs all transmission and switching activities. · Transmission media connect end system and subnet and carry information.