Q. Explain function of application layer?

Layers of OSI model are as follows:

(1) The Physical Layer: This defines an interface in terms of connections, voltage levels and data rate in order for signals to be transmitted bit by bit.

(2) The link Layer: This provides error detection as well as correction for a link to make sure that exchange of data is reliable. It may require data stream to be divided into blocks, termed as packets, for inserting error-checking bits or for synchronization. Though transparency is preserved for the data bits in these blocks.

(3) The network layer: This is concerned with operation of the network between terminals. It is responsible for establishing the corrections between appropriate network nodes.

(4) The transport layer: This is responsible for establishing a network independent communication path appropriate for particular terminal equipments (for example providing the appropriate data rate and end-to-end error control). It thus relieves the user from being concerned with such details.

(5) The session layer: This is concerned with setting up and maintaining an operational session between terminals. For illustrations 'signing on' at the commencement of a task and 'signing off' at its end.

(6) The presentation layer: This is concerned with format of the data presented, in order to overcome difference in representation of information as supplied to one terminal and required at the other. Its function is to make communication over the network machine-independent.

(7) The application layer: This defines nature of the task to be performed. It supplies user with the applications programs needed. Illustrations include word processing, electronic mail, banking transactions, etc.