|
The Open Systems Interconnection
(OSI)
reference model is an industry standard framework that is used to
divide the functions of networking into seven distinct layers. It is
one of the most commonly used teaching and reference tools today in
networking. The International Organization for Standardization (ISO,
not to be confused with OSI) developed the OSI model in the 1980s.
There are seven layers in the OSI reference model.
How does the OSI model work? Communication across the layers of the reference model is
achieved because of special networking software programs called
protocols. Protocols are discussed in the sections that follow.
The OSI was intended to be a model for developing networking
protocols. However, most of the protocols now used on LANs do not
necessarily respond exactly to these layers. Some protocols fall
neatly within the boundaries between these layers, while others
overlap or provide services that overlap or span several layers.
This explains the meaning of "reference" as it is used
in conjunction with the OSI reference model. Table
The following sections discuss examples of the networking protocols mentioned in this section.
|
Each layer provides specific services to the layers above and
below it in order for the network to work effectively. At the top
of the model is the application interface (layer), which
enables the smooth usage of such applications as word processors
and web browsers. At the bottom is the physical side of the
network. The physical side includes the cabling (discussed earlier
in this chapter), NIC, hubs, and other networking hardware.
and 
demonstrate the process of data being encapsulated on the sending end
and data being de-encapsulated on the receiving end.
summarizes the functions of each layer, the type of data it
handles and the type of hardware that can work at that layer.