One of the most important concepts in networking is the open-systems interconnection (OSI) reference model. It serves as a framework within which communication protocol standards are developed.

In 1977, the International Organization for Standards (OSI) began an ambitious project to develop a single international standard set of communications protocols. By 1984, ISO had defined an overall model of computer communications calle the Reference Model for Open Systems Interconnection, or OSI Model. The OSI model, described in international standard ISO 7948, documents a generalized model of system interconnection.

ISO also developed a comprehensive set of standards for the various layers of the OSI model. The standards making up the OSI architecture were not widely implemented in commercial products for computer networking. However, the OSI model is still important. The concepts and terminology associated with the OSI model have become widely accepted as a basis for discussing and describing network architectures. The OSI model is often used in categorizing the various communications protocols that are in common use today and in comparing one network architecture to another.

The Seven Layers

The OSI model defines the seven functional layers listed in the table below. Each layer performs a different set of functions, and the intent was to make each layer as independent as possible from all others.

The following sections briefly describe each of the seven layers of the OSI model, working from the bottom up.

Layer 1 - Physical Layer The physical layer defines the physical characteristics of the interface, such as mechanical components and connectors, electrical aspects such as voltalge levels representing binary values, and functional aspects such as setting up, maintaining, and taking down the physical link. Well-know physical layer interfaces for local area networks (LANs) include Ethernet, Token-Ring, and Fiber Distributed Data Interface (FDDI). Layer 2 - Data-Link Layer The data link layer defines the rules for sending and receiving information across the physical connection between two systems. This layer encodes and frames data for transmission, in addition to providing error detection and control. Because the data link layer can provide error control, higher layers may not need to handle such services. However, when reliable media is used, there is a performance advantage by not handling error control in this layer, but in higher layers. Bridges operate at this layer in the protocol stack. Layer 3 - Network Layer The network layer defines protocols for opening and maintaining a path on the network between systems. It is concerned with data transmission and switching procedures, and hides such procedures from upper layers. Routers operate at the network layer. The network layer can look at packet addresses to determine routing methods. If a packet is addressed to a workstation on the local network, it is sent directly there. If it is addressed to a network on another segment, the packet is sent to a routing device, which forwards it on the network. Layer 4 - Transport Layer The transport layer provides a high level of control for moving information between systems, including more sophisticated error handling, prioritization, and security features. The transport layer provides quality service and accurate delivery by providing connection oriented services between two end systems. It controls the sequence of packets, regulates traffic flow, and recognizes duplicate packets. The transport layer assigns packetized information a traffic number that is checked at the destination. If data is missing from the packet, the transport layer protocol at the receiving end arranges with the transport layer of the sending system to have packets re-transmitted. This layer ensures that all data is received and in the proper order. Layer 5 - Session Layer The session layer coordinates the exchange of information between systems by using conversational techniques, or dialogues. Dialogues are not always required, but some applications may require a way of knowing where to restart the transmission of data if a connection is temporarily lost, or may require a periodic dialog to indicate the end of one data set and the start of a new one. Layer 6 - Presentation Layer Protocols at the presentation layer are part of the operating system and application the user runs in a workstation. Information is formatted for display or printing in this layer. Codes within the data, such as tabs or special graphics sequences, are interpreted. Data encryption and the translation of other character sets are also handled in this layer. Layer 7 - Application Layer Applications access the underlying network services using defined procedures in this layer. The application layer is used to define a range of applications that handle file transfers, terminal sessions, network management, and message exchange.

The following table illustrates how some common protocol stacks map into the OSI model:

The following table illustrates how some common services map into the OSI model: