Communication Protocols - Open Systems Interconnect Model

OPEN SYSTEMS INTERCONNECT MODEL

The Open Systems Interconnect Model (OSI), created by the International Standards Organization in 1974, provides a solid framework for understanding how communication protocols work. As its name suggests, OSI is a model, not a type of computer program or an actual device. The model was designed so that many different kinds of computer systems could exchange data in a seamless, universal way. Many companies design their products to follow the OSI model when communicating.

The OSI Model divides computer networks into seven different layers (the physical, data link, network, transport, session, presentation, and application layers). Each layer plays a different role in the transmission of information. Many different kinds of protocols can be involved at each layer, and individual protocols can be involved in more than one layer of the model. For example, TCP/IP plays a role on levels seven, four, and three. The first four layers of the OSI Model ensure that data sent from one machine arrives intact on the receiving end.

The physical layer resides at the bottom of the OSI Model. It deals exclusively with the transmission and reception of electronic or mechanical signals (which contain bits of information) between two mediums. This level includes the actual cables and hardware involved in the communication process.

When data is sent over a local area network (LAN)—a network of computers and servers in a specific area, such as within a company—it is often done so in packets or chunks known as frames, which conform to the type of network involved. At the data link layer, bits of information are changed into frames, such as Ethernet frames. Ethernets are one very common type of way of accessing LANs with PCs and Macintosh computers.

At the network layer, the most efficient pathway is determined for the transmission of information between a sender and receiver. This involves different points on the networks involved, which are called nodes. At this level, data from one network is relayed to other networks via devices called routers.

Unlike the first three levels of the OSI Model, which are concerned with the movement of information from one node to another, the transport layer is responsible for determining the overall integrity of a message. In other words, this level determines that data sent arrives intact on the receiving end. This level also ensures data is received in a timely way, and in the correct order or sequence. End-to-end communication between the sending and receiving sources is possible at this level. From the transport layer on, the focus is on how information moves between processes or programs, instead of nodes or points on a network.

The session layer is where communication links between two network stations originate, are managed, and terminated. The timing, direction (one-way or two-way), and flow of a connection are managed at this level. According to Tech Encyclopedia, this layer is sometimes unused, or the steps at this level often occur at the transport layer.

The translation of data—encryption, decryption, and presentation—is determined at the presentation layer. This layer is responsible for taking data from many different systems and putting it into a format that can be read by computers on a network. Like the session layer, the presentation layer is not always used by all protocols.

Finally, the application layer involves the management of interactions between users and programs, or between two programs. At this level, files are opened, closed, transferred, and written; and e-mail messages are sent.