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(a) What is OSI?
(b) What are protocols? Describe briefly.
(c) What is the function of network layer?
(d) Describe circuit switched network.

(a) OSI stands Open System Interconnection. An open system is a model that allows any two systems to communicate even if their architectures are different. OSI is complete model that cover all aspects of network communications. It was developed by International Standards Organization(ISO) in 1983. OSI consists of seven layers. Each layer performs a specific function in network communication.

(b) A set of rules for exchanging data between computers connected to a network is called protocol. It represents an agreement between the communication devices. A protocol defines what is communicated, how it is communicated. A device cannot communicate without a protocol. Some examples of protocols are TCP/IP, SLIP, PPP, HTTP and FTP.

(c) The network layer is responsible for establishing, maintaining and terminating network connections. It manages the delivery of data from source to destination. Network layer determines logical path between sender and receiver. There may be many networks between two computers. This layer manages to send data from the source to destination computer. The common protocols that operate at this level are IP, IPX and X.25.

(d) In a circuit switched network, a dedicated connection is established between two network nodes before the nodes can communicate. A temporary circuit is setup between the source and destination. The circuit remains connected for the duration of the communication session. The circuit functions as if the nodes were physically connected with an electric circuit. An example of circuit switched network is telephone network.