e numbers and sizes ofnetworks. Many of the networks, however, were built using different implementations of hardware and
software. As a result, many of the networks were incompatible and it became difficult for networks using
different specifications to communicate with each other. To address this problem, the
International Organization for Standardization (ISO) researched many network schemes. The ISO
recognized that there was a need to create a network model that would help network builders
implement networks that could communicate and work together (interoperability) and therefore,
released the OSI reference model in 1984.
This chapter explains how standards ensure greater compatibility and interoperability
between various types of network technologies. In this chapter, you will learn how the OSI reference
model networking scheme supports networking standards. In addition, you will see how
information or data makes its way from application programs (such as spreadsheets) through a network medium (such as wires) to other application programs located on other computers on a
network. As you work through this chapter, you will learn about the basic functions that occur at each
layer of the OSI model, which will serve as a foundation as you begin to design, build and troubleshoot
The concept of layers will help you understand the action that occurs during communication from one computer to another. Shown in the Figure are questions that involve the movement of physical objects
such as highway traffic, or electronic data. This motion of objects, whether it is physical or
logical, is referred to as flow. There are many layers that help describe the details of the flow process.
Other examples of systems that flow, are the public water system, the highway system, the postal
system, and the telephone system. Now examine the Figure Comparing Networks
chart. What network are you examining? What is flowing? What are the different forms of the object
that is flowing? What are the rules for flow? Where does the flow occur? The networks listed in this chart give you more analogies to help you understand computer networks.
Another example of how you might use the concept of layers to analyze an everyday subject is to
examine human conversation. When you create an idea that you wish to communicate to another
person, the first thing you do is choose how you want to express that idea, then you decide how to
properly communicate it, and finally, you actually deliver the idea.
Imagine a young boy seated at one end of a very long dinner table. On the other end of the table, quite a distance away, sits the young boy’s grandmother. The youngster speaks English. The grandmother
prefers to speak Spanish. The table has been set with a wonderful meal that the grandmother has prepared.
Suddenly the young boy shouts at the top of his lungs, Hey, you! Give me the rice! and reaches
across the table to grab it. In most places, this action is considered quite rude. What should the young
boy have done to communicate his wishes in an acceptable manner?
To help you find the solution to this question, analyze the communication process by using layers. First there is the idea ? the young boy wants rice; then there is the representation of the idea? spoken English (instead of Spanish); next is the method of delivery ? Hey, you; and finally, the medium ? shouting (sound) and grabbing (physical action) across the table for the rice.
From this group of four layers, you can see that three of them prevent the young boy from
communicating his idea in an appropriate/acceptable manner. The first layer (the idea) is
acceptable. The second layer (representation), using spoken English instead of Spanish, and
the third layer (delivery), demanding instead of a politely requesting, most definitely do not
follow acceptable social protocol. The fourth layer (medium), shouting and grabbing from the
table rather than politely requesting assistance from another person seated nearby, is
unacceptable behavior in most any social situation. By analyzing this interaction in terms of layers you can understand more clearly some of the problems of communication in both humans or computers, and how you might solve them.
As you learned in chapter 1, the most basic level of computer information consists of binary
digits, or bits (0s and 1s). Computers that send one or two bits of information, however, would
not be very useful, so other groupings – bytes, kilobytes, megabytes, and gigabytes – are
necessary. In order for computers to send information through a network, all communications on a
network originate at a source, then travel to a destination.
Cisco Newtwork Chapter2