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Chapter 1

Planning a Network Topology

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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.