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OSI (Open Systems Interconnection) Model
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
The ISO (International Organization for Standardization) is a worldwide federation that promotes international standards. In the early 1980s, it began work on a set of protocols that would promote open networking environments that would let multivendor computer systems communicate with one another using internationally accepted communication protocols. It eventually developed the OSI reference model.
Before reading further, keep in mind that OSI was once considered to be the ultimate protocol for worldwide interoperability. However, it failed at gaining widespread acceptance and is now referred to mostly as a model against which other protocols are compared. One of the reasons OSI did not take off is because it was a complete set of specifications that was never subjected to any real implementations worth mentioning.
In contrast, the Internet was built from the ground up by building things that worked, and then writing specifications so that other people could follow the same "recipe" and build interoperable networks. While the OSI protocols were a good idea, the Internet protocols were easy to implement, and the Internet grew at the grass-roots level. At one time, it was thought OSI protocols would replace the Internet protocols on the government-funded Internet. There are quite a few Internet RFCs, now mostly historical, that discuss how the Internet would make the transition to OSI protocols.
Keep in mind that the Internet Protocol suite predates the OSI model. RFC 871 (A Perspective on the Arpanet Reference Model, September 1982) states that "it is an historical fact that many now widely-accepted, fundamental concepts of intercomputer networking were original to the ARPANET Network Working Group" and that these concepts existed before they were originally standardized in the OSI model. The author further states that "the designers of the ARPANET protocol suite have had a reference model of their own all the long" but that "workers in the ARPA-sponsored research community were busy with their work or were perhaps somehow unsuited temperamentally to do learned papers on abstract topics."
The remainder of this section describes OSI as a "reference" model and oultines the levels of networking protocols and the relationship they have with one another. The layered OSI model is pictured in Figure O-5. The stacks on the left and right represent two systems that are engaged in an end-to-end communication session. The middle devices are routers that provide internetwork connections between the devices.
Each layer in the protocol stack provides a particular function. These functions provide services (as discussed later) to the layer just above. In addition, each layer communicates to its peer layer in the system to which it is connected. For example, the transport layer on a server communicates with its peer transport layer on a client. This takes place through the underlying layers and across the network.
Peer-layer communication is handled via message exchange between peers. For example, assume the receiver is getting data faster than it can process it. To "slow down" the sender, it needs to send it a message. The transport layer handles this sort of "throttling." So the transport layer creates a message for its peer transport layer in the sending system. The message is passed down the protocol stack where it is packaged and sent across the network. The message is then passed up to the protocol stack where it is read by the transport protocol, which then initiates a procedure to throttle back.
The main point is that lower layers provide services to upper layers. Applications are the usual source of messages and data that are passed down through the protocol stack, but each protocol layer may also generate its own messages in order to manage the communication session.
One other thing to note is that the lower-layer physical and data link protocols operate across physical point-to-point links while the transport layer protocols operate on end-to-end virtual circuits that are created across the underlying network. See "Network Architecture."
Each layer of the OSI model is described here for what it defines. The lowest layer is discussed first because it represents the physical network components.
The Physical Layer
The physical layer defines the physical characteristics of the interface, such as mechanical components and connectors, electrical aspects such as voltage levels representing binary values, and functional aspects such as setting up, maintaining, and taking down the physical link. Well-known physical layer interfaces for data communication include serial interfaces, parallel interfaces, and the physical specifications for LAN systems such as Ethernet and token ring. Wireless systems have "air" interfaces that define how data is transmitted using radio, microwave, or infrared signals.
The Data Link Layer
The data link layer defines the rules for sending and receiving information across a physical connection between two systems. Data links are typically network segments (not internetworks) and point-to-point links. Data is packaged into frames for transport across the underlying physical network. Some reliability functions may be used, such as acknowledgment of received data. In broadcast networks such as Ethernet, a MAC (Medium Access Control) sublayer was added to allow multiple devices to share the same medium. See "Data Link Protocols."
The Network Layer
This layer provides internetworking services that deliver data across multiple networks. An internetwork addressing scheme assigns each network and each node a unique address. The network layer supports multiple data link connections. In the Internet Protocol suite, IP is the network layer internetworking protocol. In the IPX/SPX suite, IPX is the network layer protocol. See "Network Layer Protocols," "Internetworking," and "IP (Internet Protocol)."
The Transport Layer
The transport layer provides end-to-end communication services and ensures that data is reliably delivered between those end systems. Both end systems establish a connection and engage in a dialog to track the delivery of packets across the internetwork. The protocol also regulates the flow of packets to accommodate slow receivers and ensures that the transmission is not completely halted if a disruption in the link occurs. TCP and SPX are transport layer protocols. See "TCP (Transmission Control Protocol)" and "Transport Protocols and Services" for more information.
The Session Layer
The session layer coordinates the exchange of information between systems by using conversational techniques, or dialogs. Dialogs 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.
The Presenatation Layer
Protocols at this layer are part of the operating system and application the user runs on 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.
The 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, and message exchange (for example, electronic mail).
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.