Page 54

Chapter 1

Internetworking

Internetworking Models

hen networks first came into being, computers could typically

communicate only with computers from the same manufacturer. For example,
companies ran either a complete DECnet solution or an IBM solution--not
both together. In the late 1970s, the

Open Systems Interconnection (OSI)

reference model

was created by the International Organization for Stan-

dardization (ISO) to break this barrier.

The OSI model was meant to help vendors create interoperable network

devices so that different vendor networks could work with each other. Like
world peace, it'll probably never happen completely, but it's still a great goal.

The OSI model is the primary architectural model for networks. It describes

how data and network information are communicated from an application
on one computer, through the network media, to an application on another
computer. The OSI reference model breaks this approach into layers.

The Layered Approach

reference model

is a conceptual blueprint of how communications should

take place. It addresses all the processes required for effective communica-
tion and divides these processes into logical groupings called

layers

. When a

Should I just replace all my hubs with switches?

Well, if you can, sure--why not? Switches really add a lot of functionality to

a network that hubs just don't have. However, most of us don't have an

unlimited budget. Hubs still can create a nice network--that is, if you design

and implement the network correctly, of course.

Let's say that you have 40 users plugged into four hubs, 10 users each. At

this point, the hubs are all connected together so that you have one large

collision domain and one large broadcast domain. If you can afford to buy

just one switch and plug each hub into a switch port, as well as the servers

into the switch, then you now have four collision domains and one broad-

cast domain. Not great, but for the price of one switch, your network is

much better.

www.sybex.com