Appendix I: The Open Systems Interconnection (OSI) Model

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Overview

The OSI is a standard description or reference model for how information from one point in a network is transmitted from one endpoint, through a network, to another endpoint. The OSI reference model is purely a conceptual model, it does not "communicate" itself. The OSI model is composed of an architecture or framework of seven layers, each specifying particular network functions. Everything from a cable to a web browser fits into this layered framework.

OSI was the first worldwide effort to standardize the entire field of computer communications, or data networking, in the form of a networking framework for implementing hardware and protocols. The OSI model was developed by the International Organization for Standardization (ISO). OSI was originally a detailed specification of computer internetworking interfaces formulated by representatives of major computer and telecommunication companies during committee meetings held beginning in 1983. But the committee established a common reference model for which others could develop interfaces, which in turn could become standards.

Note 

Because "International Organization for Standardization" would have different abbreviations in different languages ("IOS" in English, "OIN" in French for Organisation internationale de normalisation), it was decided at the outset to use a word derived from the Greek isos, meaning "equal." Therefore, whatever the country, whatever the language, the short form of the organization's name is always ISO, not to be confused with the Open Systems Interconnection reference model's acronym, OSI.

The OSI model, completed in 1984, is still considered the chief architectural model for intercomputer communications. OSI continues to be administered by the ISO, so any new standard that seeks validation as an ISO standard for computer communications must be compatible with the OSI reference model. The model also can be used to guide product developers so that their products will consistently interoperate with other communications products. Finally, since the OSI reference model is a common point of reference for categorizing and describing network devices, protocols, and issues, it has value as a recognized, single view of communications that gives everyone a common reference point for education and discussion about communications.

The tasks that move information between networked computers or communicating devices are divided into seven task groups, with each task or group of tasks then assigned to the appropriate OSI layer. The layers are in two categories: The upper layers, sometimes called the Application Layers, and the lower layers, or Data Transport Layers. The upper three or four layers are used whenever a message passes from or to an end-user and are generally implemented only in software. The lower three layers (up to the Network Layer) handle data transport issues and are used when messages travel through the host computer or device. The bottom two layers, the Physical Layer and Data Link Layer, are implemented in hardware and software, though with IP networks only the bottom layer (the Physical Layer) need actually be hardware, since it is closest to the physical network medium and is responsible for putting information on it.

Each layer is basically self-contained and has its own function and, so that the tasks assigned to each layer can be implemented independently. Data going to and from the network is passed layer to layer. Each layer is able to communicate with the layer immediately above it and the layer immediately below it. This way, each layer is written as an efficient, streamlined software component. When two computers or other devices communicate on a network, the software at each layer on one device assumes it is communicating with the same layer on the other device. This can occur because, when a layer receives a packet of information, it checks the destination address, and if its own address is not there, it passes the packet to the next layer. One layer's functionality can thus be updated without affecting adjacent layers.

Although manufacturers and telecom / datacom product developers do not always strictly adhere to OSI in terms of keeping related functions together in a well-defined layer, practically all communications products are described in relation to the OSI model. Different network devices are designed to operate at certain protocol levels, and each network protocol can be mapped to the OSI reference model.

Figure A.1: The OSI Model.

The OSI model assumes that each communicating user is at a computer or device equipped with hardware and software adhering to these seven functional layers. When one person sends a message to another, the data at the sender's end will pass down through each layer in that device to the bottom layer, then over the channel, and at the other end, when the message arrives, data will flow back up through the layer hierarchy in the receiving device, and through the application to the end user.

The seven layers are (from the top, downward):

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