PRESENTATION LEVEL/LAYER
The presentation level consists of the presentation
layer. The presentation layer formats data to be
presented to the application layer. It can be viewed as
the translator for the network. This layer provides a
common representation for data which can be used
between the application processes. The presentation
layer relieves the applications from being concerned
with data representation, providing syntax
independence as follows:
Encodes data in a standard way (integers,
floating point, ASCII, and so on)
Provides data compression to reduce the
number of bits that have to be transmitted
Provides data encryption for privacy and
authentication
APPLICATION LEVEL/LAYER
The final level is the application level, which
consists of the application layer. The application layer
serves as the window for the application process to
access the OSI environment. This layer represents the
services that directly support users and application
tasks. It contains a variety of commonly needed
protocols for the following items:
Network virtual terminals
File transfers
Remote file access
Electronic mail
Network management
USING THE OSI MODEL
A communications system that does not use a
layered architecture can be designed. A specifically
designed communications system is faster, more
efficient, requires less software code, and eliminates
redundant functions and activities. Why, then, is the
OSI reference model considered the standard in
designing networks and writing software? It is
considered the standard primarily because the use of
a layered architecture, such as the OSI reference
model, provides the network with flexibility and
migration.
The greatest advantage of your using layer
architecture in a network is hardware independence.
As advances in technology continue, it is not
necessary to scrap a network completely because one
component has been superseded. For example, if you
have a network and need to upgrade the cable to a
type that can handle increased data at a faster rate, the
layered architecture of the OSI model will allow you
to make this replacement to the physical layer without
changing the other layers.
LAN TOPOLOGIES
The physical arrangement of the components of a
LAN is called its configuration or topology. The three
major types of configurations, or topologies, of a LAN
are the linear bus, the star, and the ring. You can
also create hybrid topologies by combining features of
these configurations. For example, several bus
networks can be joined together to form a ring of
buses.
Each topology requires the components of a LAN
to be connected in a different arrangement. These
components are also referred to as nodes. A node is
any point on a network where data can be sent
(transmitted) or received—a workstation, a server,
and so on.
LINEAR BUS NETWORK
The linear bus topology is like a data highway.
That is, all components or nodes are connected to the
same cable, and the far ends of this cable never meet,
as shown in figure 8-2. Linear bus LANs are best
suited to applications involving relatively low usage
of the bus coupled with the need to pass relatively
short messages from one node to another. In many
such networks, the workstations check whether a
message is coming down the cable before sending
their messages. Since all nodes share the bus, all
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