connecting computers across small and large distances. Networks are used to provide

easy access to information, thus increasing productivity for users. This section covers

some of the components involved with networking, as well as the basic types of

topologies used to connect networking devices, including computers.

One of the main components of networking is applications, which enable users to

you to access and use resources that are not located on your local computer. Some of

the more common networking applications include e-mail (sending mail electronically),

FTP (transferring files), andWWW(providing a graphical representation to information).

The number of networking applications ranges in the thousands, but those listed are the

most commonly used.

To build a network, you need three types of devices or components: computers,

networking devices, and cabling. Computers—devices such as PCs and file servers

running Microsoft Windows, Macintosh OS, Unix (including Linux), or other

operating systems—are responsible for providing applications to the users. Networking

devices—such as hubs, bridges, switches, routers, firewalls, modems, NT1s (an ISDN

network termination device), and channel service units / data service units (CSU/

DSUs)—are responsible for moving information between computers.. Cabling, such

as copper or fiber cabling, is needed to connect the computers and networking devices

so that information can be shared between components.Wireless communication also

falls in this category.

When you are cabling up your computers and networking devices, various types of

topologies can be used. A topology defines how the devices are connected. Figure 1-1

shows examples of topologies that different media types use.

topology, two devices can directly communicate without interference from other

devices. These types of connections are not common when many devices need to be

connected together. An example of a point-to-point topology is when you connect

two routers across a dedicated WAN circuit.

devices. Star topologies are used in environments where many devices need to be

connected. An example of a media type that uses a star topology is 10BaseT Ethernet.

When connecting devices together, you connect your computers to a hub or switch

(the center of the star). An extended star topology is basically multiple star topologies

interconnected.

media types, like 10Base5 and 10Base2 Ethernet, use a bus topology. Typically, special

In a ring topology, device one connects to device two, device two connects

The International Organization for Standardization (ISO) developed the Open Systems

Interconnection (OSI) Reference Model to describe how information is transferred

from one machine to another, from the point when a user enters information using a

keyboard and mouse to when that information is converted to electrical or light signals

transferred along a piece of wire or radio waves transferred through the air. It is

important to understand that the OSI Reference Model describes concepts and terms

in a general manner, and that many network protocols, such as IP and IPX, fail to fit

nicely into the scheme explained in ISO’s model. Therefore, the OSI Reference Model

is most often used as a teaching and troubleshooting tool. By understanding the basics

of the OSI Reference Model, you can apply these to real protocols to gain a better

understanding of them as well as to more easily troubleshoot problems.

There are seven layers in the OSI Reference Model, shown in Figure 2-1: application,

presentation, session, transport, network, data link, and physical. The functions of the

application, presentation, and session layers are typically part of the user’s application.

The transport, network, data link, and physical layers are responsible for moving

information back and forth between these higher layers.

Each layer is responsible for a specific process or role. Remember that the seven

layers are there to help you understand the transformation process that data will