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1-14
Chapter 1
Planning a Network Topology
■
Application The application layer provides the interface between the
network-
ing protocol stack and the software running on the computer. For example,
this
layer provides the interface for e-mail, file transfers, Telnet and File
Transfer Pro-
tocol (FTP) applications. Applications use the services provided by
application-
layer protocols, which in turn use the services provided by the other layers
beneath them.
It is important to understand that the protocols that implement the
functions of the OSI
model do not correspond exactly to the individual layers. A computer on a
network
does not necessarily run seven different protocols, with one corresponding
to each
layer. Generally speaking, the designer of a network infrastructure selects
a data-link
layer protocol, such as Ethernet or Token Ring, which actually encompasses
both the
physical and data-link layers in its functions, and a protocol suite, such
as TCP/IP,
which implements the functions of the network and transport layers. The
session, pre-
sentation, and application-layer functions are sometimes provided by a
protocol in the
suite or by a separate application-layer protocol.
Selecting a Data-Link Layer Protocol
The selection of a data-link layer protocol is the most important decision
in the design
of the network’s physical infrastructure. The data-link layer protocol is
not only respon-
sible for strictly data-link layer functions, such as media access control,
but also for the
network’s physical layer implementation. Currently, the most commonly used
data-link
layer protocol on networks is Ethernet, with Token Ring running a distant
second.
However, there are several Ethernet variations that provide various levels
of perfor-
mance, and selecting the correct one is crucial.
You need to consider a number of criteria when selecting the data-link layer
protocol
for use on a network. Because the data-link layer protocol dictates the
nature of the
network’s physical infrastructure, you must consider design elements such as
the dis-
tance between workstations and the transmission speed you require. You must
also
consider the nature of the traffic the network will carry and its amount.
Additionally,
your budget is always an important consideration.
Selecting a Media Type
Although there have been other types of media used in the past, most LANs
con-
structed today use either unshielded twisted pair (UTP) cable or fiber-optic
cable. In
most cases, UTP cable is sufficient and much less expensive, but fiber-optic
cable pro-
vides a viable alternative when you have special performance requirements.
Network
media that consist of cables are called bounded media. Recent developments
in wire-
less LAN technologies have also made unbounded media (networks that don’t
use
cables) a viable and practical solution.
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