THE INTERNET

Contents:

1. Introduction
2. Objectives
3. Classification of Networks
4. Networking Models

Introduction

The Internet is worldwide computer network that interconnects,
millions of computing devices throughout the world.
Most of these devices are PC's, and servers that store and transmit information such as web pages and e-mail messages.
Internet is revolutionizing and enhancing the way we as humans communicate,
both locally and around the globe.
Everyone wants to be a part of it because the Internet literally puts
a world of information and a potential worldwide audience at your fingertips.

The Internet evolved from the ARPANET (Advanced Research Projects Agency)
to which other networks were added to form an inter network.
The present Internet is a collection of several hundred thousand of networks rather than a single network.
From there, evolved a high-speed backbone of Internet access for sharing these of networks.
The end of the decade saw the emergence of the World Wide Web,
which heralded a platform-independent means of communication enhanced with a pleasant and relatively easy-to-use graphical interface.

World Wide Web is an example of an information protocol/service
that can be used to send and receive information over the Internet.

It supports:

1. Multimedia Information (text, movies, pictures, sound, programs . . . ).
2. HyperText Information (information that contains links to other information resourses).
3. Graphic User Interface (so users can point and click to request information instead of typing in text commands).

The World Wide Web model follows Cient/Server software design.
A service that uses client/server design requires two pieces of software to work:
Client Software, which you use to request information, and
Server Software, which is an Information Provider.

The server software for the World Wide Web is called an HTTP server (or informally a Web server).
Examples are Mac HTTP, CERN HTTP, and NCSA HTTP. The client software for World Wide Web is called a Web browser.

Examples are:

Netscape, and Internet Explorer.

Objectives

After going through this unit students should be able to:

• Make classification of networks
• understand two types of networking models
• understand the concept of packet switching
• understand how to access to the internet
• list the services available on Internet and
• understand how does the Internet works

Classification of Networks

There are different approaches to the classification of compute Networks.
One such classification is based on the distance approach.
In this section we will discuss such networks.
The networks can be classified into LAN, MAN and WAN networks.
Here, we describe them into brief to understand the difference between the types of network.

Local Area Network (LAN)

LAN is a privately-owned computer networks confined to small geographical area,
such as an office or a factory widely used to connect office PC's to share information and resources.
In a Local area network two or more computers are connected by same physical medium, such as a transmission cable.
An important characteristic of Local Area networks is speed.
i.e. they deliver the data very fast compared to other
types of networks with typical data transmission speed are 10-100 Mbps.

A wide variety of LANs have been built and installed, but a few types have more recently become dominant.
The most widely used LAN system is the Ethernet system.
Intermediate nodes (i.e. repeaters, bridges and switches)
allow LANs to be connected together to form larger LANs.
A LAN may also be connected to another LAN or to WANs and MANS using a "router".

In summary, a LAN is a communications network, which is:

• local (i.e. one building or group of buildings)
• controlled by one administrative authority
• usually high speed and is always shared

LAN allows users to share resources on computers within an organization.

Metropolitan Area Network (MAN)

A MAN, basically a bigger versions of a LAN is designed to extend over an entire city.
It may be single network such as a cable television network,
or it may be a means of connecting a number of LANs into a large network so that resources may be shared for example, a company can use a MAN to connect the LANs in all of its offices throughout a city.

A MAN typically covers an area of between 5 and 50 km diameter.
Many MANS cover an area the size of a city, although in some cases MANS may be as small as a group of buildings
The MAN, its communications links and equipment are generally owned by either a consortium of users or by a single network provider
who sells the service to the users. This level of service provided to each user must therefore be negotiated with the MAN operator,
and some performance guarantees are normally specified.

A MAN often acts as a high-speed network to allow sharing of regional resources (similar to a large LAN).
It is also frequently used to provide a shared connection to other networks using a link to a WAN.

Wide Area Network (WAN)

The term Wide Area Network (WAN) usually refers to a network,
which covers a large geographical area, and use communications subnets (circuits) to connect the intermediate nodes.
A major factor impacting WAN design and performance is a requirement that
they lease communication subsets from telephone companies or other communications carriers.
Transmission rates are typically 2 Mbps, 34 Mbps, 155 Mbps, 625 Mbps (or sometimes considerably more).
The basic purpose of the subnet is to transmit message from one end to another end through intermediate nodes.

In most WAN a subnet consists of two types of elements:

1. Transmission lines
2. Switching element

Transmission lines also called channels move about from one machine to another machine.
The basic purpose of the switching element is to select the outgoing path for forwarding the message.
Numerous WANs have been constructed, including public switched networks,
large corporate networks, military networks, banking networks, stock brokerage networks,
and airline reservation networks. A WAN is wholly owned and used by a single company is often referred to as an enterprises network.

Networking Models

There are two types of networking models available:
OSI reference Model and the TCP/IP Network Model for the design of computer network system.
In this section we shall look at these models.

OSI (Open System Interconnection) Networking Model

An open system is a model that allows any two different systems to communicate regardless of their underlying architecture.
The purpose of the OSI model is to open communication between different devices without
requiring changes to the logic of the underlying hardware and software.
The OSI model is not a protocol, it is a model for understanding and designing
a network architecture that is inter- operable, flexible and robust.
The OSI model has a seven-layered architecture.

These are:

• Application layer
• Presentation layer
• Session layer
• Transport layer
• Networl layer
• Datalink layer
• Physical layer

Physical layer: the physical layer is concerned with sending raw bits between the source and destination nodes over a physical medium.
The source and destination nodes have to agree on a number of factors.
Signal encoding: how are the bits 0 and 1 to be represented?
Medium: what is the medium used and its properties?
Bit synchronization: is the transmission synchronous or asynchronous?
Transmission type: whether the transmission is serial or parallel?
Transmission mode: is the transmission simplex, half-duplex or full duplex?
Topology: what is the network topology i.e. star, mesh, ring or bus?

Data link layer: the data link layer is responsible for transmitting a group of bits between the adjacent nodes. The group of bits is known as frame.
The network layer passes a data unit to the data link layer and data link layer adds the header information to this data unit.
The data link layer performs the following functions:
Addressing: Headers and trailers are added containing the physical addresses of the adjacent nodes and removed on a successful delivery.
Framing: Grouping of bits received from the network layer into manageable units called frame.
Flow control: to regulate the amount of data that can be sent to the receiver.
Media access control (MAC): who decide who can send data, when and how much.
Synchronization: this layer also contains bits to synchronize the timing to know the bit interval to recognize the bit correctly.
Error control: it incorpoprates the CRC to ensure the correctness of the frame.
Node to node delivery:it's also responsible for error-free delivery of the entire framelpacket to the next adjacent node.

Network layer: The network layer is responsible for routing a packet within the subnet that is,
from source to destination nodes across multiple nodes in the same network or across multiple networks.
This layer also ensures the successful delivery of a packet to the destination node.
The network layer performs the following functions:
Routing:To find the optimal route
Congestion control: which is based on two approaches (i) Increase on the resources
(ii) Decrease the word

Transport layer: this layer is the first end-to-end layer. Header of the transport layer contains information that helps send the message to the corresponding layer at the destination node.
The message is broken into packets and may travel through a number of intermediate nodes.
This layer takes care of error control and flow control both at the source and destination for the entire message.
The responsibilities of the transport layer are:
Host-to-host message delivery
Flow Control
Segmentation and re-assembly

Session layer: the main functions of this layer are to establish, maintain and synchronize the interaction between two communication hosts.
It makes sure that once a session is established it must be closed gracefully.
It also checks and establishes connections between the hosts of two different users.
The session layer also decides whether both users can send as well as receive data at the same time or whether only one host can send and the other can receive.
The responsibilities of session layer are:
Sessions and sub sessions: this layer divides a session into sub session for avoiding retransmission of entire message by adding the checkpoint feature.
Synchronization: this layer decides the order in which data needs to be passed to the transport layer.
Dialog control: this layer also decides which user application sends data and at what point of time and whether the communication is simplex, half duplex or full duplex.
Session closure: this layer ensures that the session between the hosts is closed gracefully.

Presentation layer: when two hosts are communicating with each other they might use different coding standards and character sets for representing data internally.
This layer is responsible for taking care of such differences.
This layer is responsible for:
Data encryption and decryption for security
Compression
Translation

Application layer: it's the top most layer in the OSI model, which enables the user to access the network.
This layer provides user interface for network applications such as remote login, World Wide Web and FTP.
The responsibilities of the application layer are:
File access and transfer
Mail services
Remote login
World Wide Web