Computer network is a group of two or more computers that are interconnected to share resources (data, hardware, software...) Two computers are said to be interconnected if they are able to exchange information.
Experts in the field of networking debate whether two computers that are connected together using some form of communications medium constitute a network but we will take above definition as relevant.
Network may involve other devices such as routers, switches, network printers, IP telephones etc. so, in general, we speak about nodes of a network.
Main purpose of networks is to transfer data from source to destination. Data is transferred by different types of signal and represented with sequence of bits (ones and zeros). Various physical media can be used for transmission of signal: coaxial cable, twisted pair, fiber optics and air.
Suppose that we want to transfer data from host A to host B over the network. Data may be e-mail, .mp3 file, web page etc. Furthermore that data have to be segmented into smaller pieces of information, transferred over the network and delivered to host B without errors.
Objective of this book is to try to explain this basic procesess in computer networks and why they are important.
Network is organized hierarchically. Main concepts of hierarchical systems are:
Communication protocol is a set of uniquely defined syntactic and semantic rules for exchanging information between two entities that includes:
Entity may be computer, software, mobile phone, sensor etc.
Communication architecture is a set of protocols that enables communication between two entities.
We will talk about these concepts and architectures later. For now let's explain concept of layers. The concept of layers is used to describe communication from one node of network to another.
A conversation between two people provides a good opportunity to use a layered approach to analyze information flow. In a conversation, each person wishing to communicate begins by creating an idea. Then a decision is made on how to properly communicate the idea.
For example, a person could decide to speak or sing, and what language to use. Finally the idea is delivered. The person creates the sound which carries the message.
This process can be broken into separate layers that may be applied to all conversations. The top layer is the idea that will be communicated. The middle layer is the decision on how the idea is to be communicated. The bottom layer is the creation of sound to carry the communication.
The same method of layering explains how a computer network distributes information from a source to a destination.
We use models to describe communication in networks. Two models that will be explained in this book are OSI model and TCP/IP model.
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