The complexity of modern computer networks has given rise to several conceptual models for explaining how networks work. One of the most common of these models is the International Standards Organization's Open Systems Interconnection (OSI) Reference Model, also referred to as the OSI seven-layer model. The seven layers of the OSI model are numbered beginning at the lowest (physical) layer.
Levels 1 through 3 are network specific, and will differ depending on what physical network you are using. Levels 4 through 7 comprise network-independent, higher-level functions. Each layer describes a particular function (instead of a specific protocol) that occurs in data communications. The seven layers function as follows:
|Physical||Describes the physical media of the network. For example, the fiber optic cable required for a Fiber Distributed Data Interface (FDDI) network is part of the physical layer.|
|Data Link||Provides reliable delivery of data across the physical layer (which is usually inherently unreliable).|
|Network||Manages the connections to other machines on the network.|
|Transport||Ensures error-free data transmission.|
|Session||Manages the connections between applications.|
|Presentation||Ensures that data is presented to the applications in a consistent fashion.|
|Application||Comprises the applications that use the network.|
Note that while the OSI Reference Model is useful for discussing networking concepts, many networking protocols do not closely follow the OSI model. For example, when discussing Transmission Control Protocol/Internet Protocol (TCP/IP), the Application and Presentation Layer functions can be combined into a single level, as can the Session and Transport Layers, as well as the Data Link and Physical Layers.
Each layer in the OSI model communicates with the corresponding layer on the remote machine as shown in the OSI Reference Model figure. The layers pass data only to the layers immediately above and below. Each layer adds its own header information (and, in the case of the Data Link layer, footer), effectively encapsulating the information received from the higher layers.
Individual users as well as organizations use networks for many reasons. A few possibilities are:
Data entry consists of entering data directly into either local or remote data files, reducing the need for such intermediate steps as posting, recording, or punching. Increased accuracy and efficiency are natural by-products of a one-step data transfer. Data queries entail searching data files for specified information. Data updating involves altering, adding, or deleting data stored in local or remote files. Remote batch entry consists of entering batches of data from a remote location, an activity often performed at night or during periods of low system usage. Because of such diverse capabilities, communications and networks are not only desirable but necessary.
Sharing resources is another function of networks. Users can share data as well as programs, file-storage space, and peripheral devices (for example, printers, modems, terminals, and fixed disks). Such sharing of system resources is cost effective (in the case of peripheral sharing), while eliminating the problems of keeping multiple copies of programs and data consistent (in the case of program and file sharing).
A communications network also provides a means for users to communicate using electronic mail. Electronic mail (e-mail) can be used between users on the same system or between users across the world.