Open System Interconnection (OSI) is a protocol and set of rules for communication.In order to get data over the network - such as an email from your computer to some computer at the other end of the world - lots of different hardware and software needs to work together.All these pieces of hardware and the different software programs speak different languages. Consider your email program: it's able to talk to the operating system, through a specific protocol, but it's not able to talk to the computer hardware.We need a special program in the operating system that performs this function. In turn, the computer needs to be able to communicate with the telephone line or other Internet hookup method. Behind the scenes, the network connection hardware needs to be able to communicate in order to pass your email from one appliance to the other, all the way to the destination computer. OSI Layers and ServicesEach layer of the OSI Model provides specific services that contribute to overall network functioning.
Application LayerThe Application Layer supports user and application tasks and overall system management, including resource sharing, file transfers, remote file servers and database and network management.Presentation LayerThe Presentation Layer translates and converts transmitted encoded data into formats that can be understood and manipulated by users.Session LayerThe Session Layer establishes and controls system-dependent aspects of communication sessions provided by the Transport Layer and the logical functions running under the operating system in a participating node.Transport LayerThe Transport Layer provides end-to-end control of a communication session once the path has been established, which enables the reliable and sequential exchange of data independent of both the systems that are communicating and their locations in the network.Network-Control LayerThe Network-Control Layer addresses messages, sets up the path between communicating nodes, routes messages across intervening nodes to their destinations and controls the flow of messages between nodes.Data-Link LayerThe Data-Link Layer establishes an error-free communication path between network nodes over the physical channel, frames messages for transmission, checks the integrity of received messages, manages access to and use of the channel and ensures the sequence of transmitted data.Physical LayerThe Physical Layer defines the electrical and mechanical aspects of interfacing to a physical medium for transmitting data, as well as setup, maintenance and disconnection of physical links. This layer includes the software driver for each communications device, plus the hardware itself—interface devices, modems and communications lines.
