Computer Network

1.Definition:

Inter-connection between two or more computers via a transmission media for faster and easier communication and resource sharing

2.Objectives, Uses & Benefits:

Faster and easier communication –

• No need for removable devices (floppies & pen drives)
• No need for multiple printers

Resource sharing –

• A department of 10 users, each requiring 25 GB of hard disk space, can share a single hard disk of 250 GB             centrally located instead of allocating each user with dedicated 25 GB hard disk
• Significant reduction in the per unit cost of storage

Sharing data

• Sharing a document or an image file or a corporate database system
• Multiple users can access the same data at the same time
•  Sharing devices like – printer, scanner, CD-ROM

Sharing Applications (software)

Electronic Mails

• It consists of a mail program that allows the user to send and receive messages
• It provides a messaging system to support store-and-forward functions.
• The system contains a dedicates computer called mail server that acts as the central repository for incoming & outgoing messages
• The central repository stores the messages submitted by a user and waits until the other user logs on and then sends him the messages.
• The users can send and read messages by using client-messaging software provided by the messaging software.
•  Some of the commonly used network messaging software is Microsoft Exchange and Mail, Lotus CC Mail and    Notes and Novel Group Wise.

Fault tolerance (resilience) –

1. Effective system of backing up data (centralized data backup system)
2. RAID Technology (HD Mirroring)
3. Replicating data using backup servers
4. Cluster Technology (multiple backup servers)

Administration – managing security

1. User authentication method (username & password)
2. IP address validation method (access-list technique)
3. Virus security policy (downloading & updating anti-virus software)

3.NETWORKING COMPONENTS:

1. Resource – Anything related to data and communication is a Resource

      Computer Resources:

• Printers
• Scanners
• Storage devices (e.g. HDD, FDD, CD-ROM, etc)
• Fax devices
• Electronic mails
• Modems
• Files, folders & documents

     Types of Resources:

• Local Resources
• Remote Resources

     Resource Accessibility:

• Locally accessible resource
• Remotely accessible resource

2. Transmission Media – Any medium to exchange data or information

     Types of Media:

• Wired Media / Bounded Media
• Wireless Media / Unbounded Media

3. Server – A server is a remote computer that serves information on request. It receives the queries, processes them and then replies to the queries. It has the capability to handle multiple connections from different sources. When a request is sent to the server, it either answers to the request and updates the information, or it sends the request to the right destination.

        Some popular types of servers are:

• Print server : A print server allows the users on a network to share printers
• File server :A file server allows the users on a network to share remote files, folders and storage devices
• Database server :A database server contains a single common database that the users can query and edit from remote locations
• Mail server : A mail server processes and delivers e-mails in an organization having intranet
• FTP server :  A FTP (File Transfer Protocol) server manages file transfers
• Web server :  A web server hosts web pages to be accessed from anywhere in the world

     Classification of server:

• Dedicated server: A server in which local resources are not accessible once the server program is invoked on it

• Non-dedicated server: A server in which local resources are accessible while the server application keeps running in the background. Such servers may hang up due to faulty program handling or execution by the user, causing the whole network to crash.

Note: A Network computer can be termed as a Server only when a server application is running on it that listens for requests and delivers services for those requests

4. Client – A client is a computer on the network on which a client application is running that requests for and receives some services from the server

5. Node – Any computer or any network resource is called a NODE

6. Workstation – A network computer that is fine-tuned to run one particular application. A workstation may be a node, but all nodes may not be workstations
7. Host –  It is a TCP/IP terminology to define any network device that allows a client to access its resources

8. Peer – A computer on the network acting both as a client as well as a server

9. Segment – When a large network is broken down into a number of smaller networks and linked by connecting devices like hubs and switches, we call it a network segment

10. Backbone – The link between a numbers of smaller network segments to create a large network is the network backbone

4.Networking Models: 

• Centralized
• Distributed

Centralized: Data is stored, organized and processed by a centralized server that is fed from remote and dumb terminals (having no hard disk and processing capability) acting only as I/O devices and share no resource.

Distributed: Data is entered and processed at the remote terminals but stored into a server. The remote terminals share resources among themselves and also with the server. 95% of the data processing is done at the terminals. The server acts as the BACKEND and the terminals act as FRONTENDS.

Types of Distributed Networks:

• Peer-to-Peer Networking
• Client/Server Networking

Peer-to-Peer Networking:

• Two or more computers share their individual resources, such as, hard drives, CD-ROM drives, printers, etc.
• Each computer acts both as a client and a server
• Each workstation has equal capabilities and responsibilities

   Advantages:

• Network is faster and cheaper
• Easiest way to build a network
• No network administrator is required
• User-level security

   Disadvantages:

• No central administration
• Less security 

Client/Server Networking:

• A computer is dedicated as a server that provides services to multiple computers acting as clients or workstations over the network
• An administrator is assigned to control the overall network and its security
• Implementation of server application and client application is mandatory
• All PCs depend on the central server that does the majority of work
• A request from the client to the server contains the following information –
• The server address
• The request
• The return address

     Advantages:

• Strong central security
• Central file repository provides easy data backup
• Ability of servers to pool available hardware and software, lowering overall costs
• Ability to share expensive equipments, such as laser printers
• Optimized dedicated servers that provide faster sharing of network resources

     Disadvantages:

• Implementation is costly and complicated
• Dedicated Network Administrator is required, which is again a costly affair
• When the server is down, all the network services are unavailable

5. NETWORK OPERATING SYSTEMS:

 Client Operating Systems:

• DOS
• OS/2
• Windows 3.x
• Windows 9x, Me
• Windows NT Workstations (3.x, 4.x)
• Windows 2000 Professional
• Windows XP Professional
• Novell Clients
• UNIX Workstations
• Linux Workstations

Server Operating Systems:

• Novell Netware 2.x, 3.x, 4.x, 5.x
• Windows NT Server 3.x, 4.x
• Windows Server 2000
• Windows Server 2003
• SCO UNIX Server
• Linux Server

Network Classifications:

Categorized on the basis of distance and structure –

• Local Area Network (LAN)
• Campus Area Network (CAN)
• Metropolitan Area Network (MAN)
• Wide Area Network (WAN)
• Internet

LAN:

A network of computers –

• Within a room
• Between rooms within a floor
• Between floors within a building
• Between buildings within a campus

• Can cover a distance of maximum 10 Kilometers
• Much easier and cheaper for implementation
• Data transmission speed is relatively high
• Practically no error in data transmission
• Mostly uses simple cabling methods for connecting computers
• Uses networking devices like hubs, bridges, switches, Ethernet, token ring, FDDI, etc.
• LAN Types:
  • Ethernet
  • Token Ring
  • Token Bus
  • FDDI (Fiber Distributed Data Interface)

MAN:

• A network of multiple LANs (a LAN of LANs)
• Usually expands throughout a city (intra-city) – connects different offices in the same city
• Can cover a distance of maximum 100 Kilometers
• Relatively expensive for implementation than LAN
• Slower data transmission than LAN
• Connects diverse devices using optical services like SONET, SDH and ATM

• SONET = Synchronous Optical Network Standard
• SDH = Synchronous Digital Hierarchy Standard; ATM = Asynchronous Transfer Mode 

WAN:

• A network of computers that expands throughout the world
• Connections are made over lease lines, satellite links or microwave transceivers
• Most expensive communicating devices are used
• Implementation and configuration are very complicated

6.INTRANET

• Network that is limited to a single organization or company
• It is normally implemented as a LAN or WAN network

INTERNETWORK

• When a number of smaller network segments are interconnected using some networking devices such as ROUTERS to form a large network, we call it Internetwork
• Internet is the perfect example of Internetwork

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