OSI Model Definition

The open systems interconnection reference model, commonly referred to as the OSI reference model, OSI seven layer model or OSI model, is a layered, abstract description for communications and computer network protocol design and is the foundation of modern networking.

Developed in 1977 for the purpose of standardizing and simplifying definitions relating to computer networks, this model divides the networking process into seven logical layers, each of to which has unique responsibilities and to which are assigned specific services and protocols (i.e., agreed-upon formats). They are the application, presentation, session, transport, network, datalink and physical layers.

In this model, information and control are passed from one layer to the next, starting at the application layer in the transmitting host (i.e., computer connected to the network), proceeding down the hierarchy to the physical layer, then passing over the communications channel to the destination host, where they proceed back up the hierarchy to the application layer.

The physical layer, also called layer one, defines all physical and electrical specifications for devices used to interface to the network, including the shape and layout of pins in connectors, voltages, cable specifications and radio broadcast frequencies. It provides the means for transmitting raw bits, but it is not concerned with MAC addresses, IP addresses and packets; rather, these are dealt with by layers higher up in the hierarchy.

The data link layer, layer two, is responsible for encoding bits into packets prior to transmission and then decoding the packets back into bits at the destination. It provides reliable data transfer by transmitting packets with the necessary synchronization, error control and flow control.

The network layer, layer three, is responsible for routing, which is the moving of packets across the network using the most appropriate paths. It also addresses messages and translates logical addresses (i.e., IP addresses) into physical addresses (i.e., MAC addresses). It is the layer at which IP (Internet protocol) operates. Other protocols in the TCP/IP (transmission control protocol/Internet protocol) suite of protocols, which forms the basis of the Internet and most other modern networks, that also operate in this layer are ICMP (Internet control message protocol), IPsec (Internet protocol security), ARP (address resolution protocol), RIP (routing information protocol), OSPF (open shortest path first) and BGP (border gateway protocol).

The transport layer, layer four, is responsible for maintaining reliable end-to-end communications across the network. It provides full-duplex (i.e., simultaneously bidirectional) virtual circuits on which delivery is reliable, error free, correctly sequenced and duplicate-free. The best known example of a transport layer protocol is TCP (transmission control protocol). Also operating at this layer are UDP (user datagram protocol), RTP (real-time transport protocol) and SCTP (stream control transmission protocol).

The function of the session layer, layer five, is to maintain communication between hosts after it has been established and then terminate it when no longer needed. An example of a session layer protocol is SQL (structured query language), which provides a user-friendly text interface to relational database systems.

The presentation layer, layer six and the second layer from the top, translates data from programs and protocols in the application layer above it into formats that can be transmitted over networks and used by other applications on other hosts. It is likewise responsible for the delivery and formatting of information to the application layer for further processing or display.

The application layer, layer seven, is the top layer of both the OSI and TCP/IP models. It provides services to connect application programs and communications protocols as well as services to ensure effective communication between application programs over the network.

The OSI model is roughly, but not strictly, adhered to in the computing and networking industry. Its main feature is the interface between layers, which dictates the specifications on how one layer interacts with another. This allows networking protocols to be developed on a modular basis, that is, so that they can easily interact with the protocols on higher and lower layers, regardless of the company or other organization that developed the protocols. Although protocols are usually developed that reside in a single layer, sometimes they are designed to incorporate elements of multiple, adjacent layers.

The seven-layer OSI model is often compared with the five layer TCP/IP model. Development was begun on the former before the latter; and it is more of a theoretical approach, whereas the latter was developed mainly as a practical solution to a specific set of engineering problems. Because of its theoretical approach and despite its greater number of layers, the OSI model is often considered easier for students of networking to comprehend; it is emphasized for this reason and because understanding it first can make it easier to understand the OSI model.

The fact that the real-world Internet and most other networks are based on the TCP/IP model has been attributed to a number of factors, including the facts that early implementations of the OSI model suffered from poor performance and that the TCP/IP model was associated with UNIX, which allowed it to take advantage of the great popularity of that operating system in academia (where the Internet originated). Another objection to the OSI model was that it was poorly designed because the session eand presentation layers are nearly empty, whereas other layers contain numerous protocols.

Created November 25, 2005.
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