Introduction to the OSI Model

{The Open System Interconnection Reference Model (OSI) is a seven layer model that was developed as part of the effort to standardize networking that was started in the late 1970's as part of the Open Systems Interconnection (OSI) initiative}


The Seven Layers of the OSI Model.,

in summary the four layers of the OSI model are broken as follows:

• The Physical Layer defines the electrical and physical properties and the operating specifications for the devices and media in use. The main job of the Physical Layer is the physical "connection" or attachment of given media and how it is configured (e.g. Token Ring cable, size of cable used, termination in place etc.). In some instances, there may be secondary responsibilities of this layer depending on the device for things such as flow control, modulation/demodulation and so forth. The protocol data unit in use at this level of the OSI model is referred to as a "bit."

• The Data Link Layer provides the practical means to transfer data between network nodes as its main job is to transfer data between network nodes in a wide area network or between nodes on the same local area network segment/subnet. It has the secondary responsibility to detect and correct errors (as permissible) that may take place at the Physical Layer. The protocol data unit in use at this level of the OSI model is referred to as a "frame.

• The Network Layer handles the forwarding and routing of data along logical paths between network connected nodes. In addition to routing and forwarding functions of this layer of the model is also performs addressing, error handling, quality of service control, congestion control and packet sequencing. The protocol data unit in use at this level of the OSI model is referred to as a "packet."

• The Transport Layer is responsible for the reliable, end to end transfer, recovery and flow control of the segments between the nodes. The protocol data unit in use at this level of the OSI model is referred to as a "segment."

• The Session Layer addresses the build up and tear down of the connection sessions between nodes on a network. The protocol data unit in use at this level (and all of the subsequent levels) of the OSI model is referred to simply as "data."

• The Presentation Layer is responsible for taking the data from applications at the application layer and breaking it down for use on the session layer as well as the reverse. It also has the task of formatting the data so that it can be sent to other nodes.

• The Application Layer handles the initial connection of a given application to the network. It is where applications and application type activities such as browsing the web, sending and receiving email and performing file transfers take place. There are applications that wholly reside at the level such as Telnet and FTP.