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 layer (Physical).
Levels 1 through 3 are network specific, and 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 from lowest level (machine
level) to highest level (the level at which most human interaction takes
place), 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||Assures 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 are combined, as are the Session and Transport Layers and 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.
Figure 1-1. OSI Reference Model. This illustration shows the various communication levels of the OSI Model as described in the above text.
The layers pass data only to the layers immediately above and below. Each layer adds its own header information (and footer information, in the case of the Data Link), effectively encapsulating the information received from the higher layers.
Individual users as well as organizations use networks for many reasons, including:
Data entry consists of entering data directly into either local or remote data files. 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 like printers, modems, terminals, and fixed disks. Sharing of system resources is cost effective because it eliminates the problems of keeping multiple copies of programs and it keeps data consistent (in the case of program and file sharing).