The OSI Model for Open Systems Interconnection
In 1978, the International Standards Organisation (ISO) created a universal standard for exchanging information between and within networks and across geographical boundaries. This standard for network architecture is the seven-layer model for Open Systems Interconnection (OSI), which has encouraged conformity in designing communications networks and controlling distributed processing.
Communication among these distributed systems and networks requires a standard approach to network design, one that defines the relationships and intersections between network services and functions via common interfaces and protocols.
In the OSI model the network architecture specifies a hierarchy of independent layers that contain modules for performing defined functions. This translates into a set of rules that defines the way participating network nodes must interact to communicate and exchange information. The OSI Model defines standard relationships between the hardware and software in today’s complex computer systems.
Services per layer
Each layer of the OSI Model provides specific services that contribute to overall network functioning.
The Application Layer supports user and application tasks and overall system management, including resource sharing, file transfers, remote file servers and database and network management.
The Presentation Layer translates and converts transmitted encoded data into formats that can be understood and manipulated by users.
The 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.
The 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.
The 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.
The 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.
The 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.
What layer does it operate on?
Sometimes network devices are described as being, for instance, Layer 2 devices or Layer 3 devices. This describes which OSI layer the device operates on. Ethernet switches are usually Layer 2 (Data Link Layer) devices and sort traffic using physical (MAC) addresses. Routers are Layer 3 (Network Control Layer) devices and manage network traffic using network addresses. Media converters, which provide only simple interface and electrical conversion, are usually Layer 1 (Physical Layer) devices.
Generally, the higher the network layer a device operates on, the more sophisticated (and expensive!) it is. For instance, if an Ethernet switch is described as a "Layer 3" switch, that means it has some routing functions and more functionality than an ordinary Layer 2 Ethernet switch. A Layer 2 media converter provides more than a simple Layer 1 physical conversion and has some of the functionality of a Layer 2 switch.