DATA COMMUNICATIONS AND COMPUTER NETWORKS

COMPUTER NETWORKING
INTRODUCTION
CHAPTER OVERVIEW

Computer Networks
Types of computer networks
Configurations/topologies
Communication channels
Data Transmission
Wireless networking
Application of computer networks
Emerging issues in Networks
Others


Computer Networks

Is a communications system connecting two or more computers that work to exchange information and share resources (Hardware, software and data). A network may consist microcomputers, or it may integrate microcomputers or other devices with larger computers. Networks may be simple or complex, self-contained or dispersed over a large geographical region.

COMMUNICATION CHANNELS

The transmission media used in communication are called communication channels. Two ways of connecting microcomputers for communication with other equipment is through:-
• Cable
• Air
There five kinds of communication channels used for cable or air connections.

Telephone line (twisted pair) – Made up of copper wires called twisted pair. It is a standard communication channel for both data and voice. (Speed of up to 100 Mbps)

Coaxial cable – A high frequency transmission cable that replaces the multiple wires of telephone lines with a single solid copper core. It has over 80 times the transmission capacity of twisted pair. It is often used to link parts of a computer system in one building (Speed of up to 600 Mbps)

Fibre-optic cable – Transmits data as pulses of light through tubes of glass. It has over 26,000 times the transmission capacity of twisted pair. A fibre-optic tube can be half the diameter of human hair. Fibre-optic cables are immune to electronic interference and more secure and reliable. Fibre optic cable is rapidly replacing twisted pair (telephone).

Microwave – Transmit data as high-frequency transmission radio waves that travel in straight lines through air.

Satellite – Used to amplify and relay microwave signals from one transmitter on the ground to another. They orbit about 22,000 miles above the earth. They rotate at a precise point and speed and can be used to send large volumes of data. Bad weather can sometimes interrupt the flow of data from a satellite transmission.

TYPES OF COMPUTER NETWORKS

Terms used to describe computer networks:

Node – any device connected to a network such as a computer, printer or data storage device.

Client – a node that requests and uses resources available from other nodes. Typically a micro-computer.

Server – a node that shares resources with other nodes. May be called a file server, printer server, communication server, web server or a database server.

Network Operating System (NOS) - The operating system of the network that controls and coordinates the activities between computers on a network, such as electronic communication and sharing of information and resources.

Distributed processing – Computing power is located and shared at different locations. Common in decentralized organizations (each office has its own computer system but is networked to the main computer)

Host computer – a large centralized computer, usually a minicomputer or mainframe.


Data transmission: Analog versus digital

Information is available in analogue or in digital form. Computer-generated data can easily be stored in a digital format, but analog signals, such as speech and video must first be sampled at regular intervals and then into a digital form. This process is known as digitization and has the following advantages:
• Digital data is less affected by noise
• Extra information can be added to digital signals so that errors can either be detected or corrected.
• Digital data tends to be not to degrade over time
• Processing digital data is relatively easy.

Direction of data transmission

There are three directions or modes of data flow in a data communication system.

1. SIMPLEX COMMUNICATION – Data travels in one-direction only e.g. Point-of-sale terminals
2. HALF-DUPLEX COMMUNICATION – Data flows in both directions, but not simultaneously. E.g. Electronic bulletin board
3. FULL DUPLEX COMMUNICATION – Data is transmitted back and fourth at the same time e.g. mainframe communications

Modes of data transmission

Data may be sent over communication channels in either asynchronous or synchronous mode.
Asynchronous transmission – data is sent and received one byte at a time. Used with microcomputers and terminals with slow speeds.

Synchronous transmission – data is sent and received several bytes (blocks) at a time. It requires a synchronized clock to enable transmission at timed intervals.


TYPES OF COMPUTER NETWORKS

Different communication channels allow different types of networks to be formed. Telephone lines may connect communications equipment within the same building. Coaxial cable or fibre-optic cable can be installed on building walls to form communication networks. Communication networks also differ in geographical size. Three important networks according to geographical size are:
1. LANs
2. MANs
3. WANs

LOCAL AREA NETWORK (LAN)

A LAN is a computer network in which computers and peripheral devices are in close proximity. It is a collection of computer within a single office or building that connects to a common electronic connection – commonly known as a network backbone. This type of network typically uses micro-computers in a bus organization linked with telephone, coaxial, or fibre-optic cable. A LAN allows all users to share hardware, software and data on the network. Minicomputers, mainframes or optical disk storage can be added to the network. A network bridge device may be used to link a LAN to other networks with the same configuration. A network gateway may be used to link a LAN to other networks, even if their configurations are different.

METROPOLITAN AREA NETWORK (MAN)

A MAN is a computer network that may be citywide. This type of network may be used as a link between office buildings in a city. The use of cellular phone system expands the flexibility of a MAN network by linking car phones and portable phones to the network.

WIDE AREA NETWORK (WAN)

A WAN is a computer network that may be countrywide or worldwide. It normally connect networks over a large physical area, such as in different buildings, towns or even countries. A modem connects a LAN to a WAN when the WAN connection is an analogue line. For digital connection a gateway connects one type of LAN to another LAN, or WANS, and a bridge connects a LAN to similar types of LAN. This type of network typically uses microwave relays and satellites to reach users over long distances. The widest of all WANs is the internet, which spans the entire globe.


CONFIGURATIONS/TOPOLOGIES

A computer network configuration is also called network topology. The topology is the method of arranging and connecting the nodes of a network. There are four principal network topologies:
1. Star network
2. Bus network
3. Ring network
4. Hierarchical (hybrid) network

STAR NETWORK

In a star network there are a number of small computers or peripheral devices linked to a central unit called hub. The central unit may be a host computer or a file server. All communications pass through the central unit and control is maintained by polling. This type of network can be used to provide a time- sharing system and is common for linking micro-computers to a mainframe.
Star network

Advantages of star network

It is easy to add new and remove nodes
A node failure does not bring down the entire network
It is easier to diagnose network failures through a central hub
Disadvantages of star network
If the central hub fails the whole network ceases to function
It costs more to cable a star configuration than other topologies

BUS NETWORK

In a bus network each device handles its communication control. There is no host computer, however there may be a file server. All communications travel along a common connecting cable called a bus. It is a common arrangement for sharing data stores on different microcomputers. It is not as efficient as star network for sharing common resources, but is less expensive. The distinguishing feature is that all devices (nodes) are linked along one communication line – with endpoints – called the bus or backbone.

Advantages of bus network

• Reliable in very small networks as well as easy to use and understand.
• Requires the least amount of cable to connect the computers together and therefore is less expensive than other cabling arrangements.
• Is easy to extend. Two cables can be easily joined with a connector, making a longer cable for more computers to join the network. A repeater can also be used to extend a bus configuration.

Disadvantages of bus network

• Heavy network traffic can slow a bus considerably. Because any computer can transmit at any time, bus networks do not coordinate when information is sent. Computers interrupting each other can use a lot of bandwidth.
• Each connection between two cables weaken the electrical signals.
• The bus configuration can be difficult to troubleshoot. A cable break or malfunctioning computer can be difficult to find and can cause the whole network to stop functioning.


RING NETWORK

In ring network each device is connected to two other devices, forming a ring. There is no central file server or computer. Messages are passed around the ring until they reach their destination. Often used to link mainframe, especially over wide geographical areas. It is used in decentralized organization called a distributed data processing.

Advantages of Ring network

• Offer high performance for a small number of workstations or for larger networks where each station has a similar work load.
• Can span longer distances than other types of networks.
• Ring networks are easily extendable.

Disadvantages of Ring network

• Relatively expensive and difficult to install
• Failure of one component of the network can lead to failure of the entire network
• It is difficult to troubleshoot the entire network.
• Adding or removing computers can disrupt the network.

HIERARCHICAL (HYBRID) NETWORK

A hierarchical network consists of several computers linked to a central host computer. It is similar to a star network. Other computers are also hosts to other, smaller computers or to peripheral devices in this type of network. It allows various computers to share databases, processing power, and different output devices. It is useful in centralized organizations.

Advantages of Hybrid Network

• Improves sharing of data and programs across the network
• Offers reliable communication between nodes

Disadvantages of Hybrid Network

• Difficult and costly to install and maintain
• Difficult to troubleshoot network problems

COMPONENTS AND DEVICES OF A NETWORK

All types of computer networks whether wireless or wired requires hardware and software components when it is established. There are many types of hardware devices which are either installed or connected to the computer terminals in order to construct a network.
1. BRIDGE – Links two networks of the same type.

2. GATEWAY – Connects two networks of the dissimilar type.

3. ROUTER – A router is required to connect multiple networks, like your LAN to the Internet. Routers work by storing large tables of networks and addresses, then using algorithms to determine the shortest routes to individual addresses within those networks. In this way efficient routers not only facilitate intra-network communications, but also play a role in overall network performance. delivering the information faster.

4. SWITCH - A network switch is a small hardware device that joins multiple computers together within one local area network (LAN). Technically, network switches operate at layer two (Data Link Layer) of the OSI model.

Client/Server environment

A client/Server network environment is one in which one computer acts as a server and provides data distribution and security functions to other computers that are independently running various applications. An example of the simplest client/server model is a LAN whereby a set of computers is linked to allow individuals to share data.

NETWORK PROTOCOLS

Protocols are sets of communication rules for exchange of information. Protocols define speeds and modes for connecting one computer with another.
Common network protocols include:

• 3 layer logical model
• TCP/IP (Transmission Control Protocol / Internet Protocol.
• ISO/OSI model (International Organizations for Standards/ Open System Interconnection)

DATA COMMUNICATION

An Internet Protocol address (IP address) is a numerical label assigned to each device (e.g., computer, printer) participating in a computer network that uses the Internet Protocol for communication.[1] An IP address serves two principal functions: host or network interface identification and location addressing. Its role has been characterized as follows: "A name indicates what we seek. An address indicates where it is. A route indicates how to get there."[2]
The designers of the Internet Protocol defined an IP address as a 32-bit number[1] and this system, known as Internet Protocol Version 4 (IPv4), is still in use today. However, due to the enormous growth of the Internet and the predicted depletion of available addresses a new addressing system (IPv6), using 128 bits for the address, was developed in 1995,[3] standardized as RFC 2460 in 1998,[4] and is being deployed worldwide since the mid-2000s.
IP addresses are binary numbers, but they are usually stored in text files and displayed in human-readable notations, such as 172.16.254.1 (for IPv4), and 2001:db8:0:1234:0:567:8:1 (for IPv6).


Application of computer networks


Connectivity is the ability and means to connect a microcomputer by telephone or other telecommunication links to other computers and information sources around the world. The connectivity options that make communication available to end-users include:
• Fax machines (Facsimile transmission machines)
• E-Mail (Electronic Mail)
• Voice messaging systems
• Video conferencing systems
• Shared resources
• Online services

FAX MACHINE

Fax machines convert images to signals that can be sent over a telephone line to a receiving machine. They are extremely popular in offices. They can scan the image of a document and print the image on a paper. Microcomputers use fax/ modem circuit boards to send and receive fax messages.

E-MAIL (Electronic Mail)

Email is a method of sending an electronic message between individuals or computers. One can receive e-mail messages even when one is not on the computer. E-mail messages can contain text, graphics

VOICE MESSAGING SYSTEMS

Voice messaging systems are computer systems linked to telephones that convert human voice into digital bits. They resemble conventional answering machines and electronic mail systems. They can receive large numbers of incoming calls and route them to appropriate ‘voice mailboxes’ which are recorded voice messages. They can forward calls and deliver the same message to many people.

VIDEO CONFERENCING SYSTEMS

Video conferencing systems are computer systems that allow people located at various geographic locations to have in-person meetings. They can use specially equipped videoconferencing rooms to hold meetings. Desktop video conferencing systems use microcomputers equipped with inexpensive video cameras and microphones that sit atop a computer monitor.

SHARED RESOURCES

Shared resources are communication networks that permit microcomputers to share expensive hardware such as laser printers, chain printers, disk packs and magnetic tape storage. Several microcomputers linked in a network make shared resources possible. The connectivity capabilities of shared resources provide the ability to share data located on a computer.

ONLINE SERVICES

Online services are business services offered specifically for microcomputer users. Well-known service providers are America Online (AOL),WorldNet, CompuServe, Africa Online, Kenyaweb, UUNET, Wananchi Online and Microsoft Network.

Typical online services offered by these providers are:

Teleshopping – a database which lists prices and description of services. You place an order, charge the purchase to a credit card and merchandise is delivered by a delivery service.

Home banking – banks offer this service so you can use microcomputers to pay bills, make loan payments, or transfer money between accounts.

Investing – investment firms offer this service so you can access current prices of stocks and bonds. You can also buy and sell orders.

Travel reservations – travel organizations offer this service so you can get information on airline schedules and fare, order tickets, and charge to a credit card.

Internet – You can get access to the World Wide Web.

INTERNET

The internet is a giant worldwide network. The internet was started in 1969 when the United States government funded a major research project on Computer networking called ARPANET (Advanced Research Project Agency NETwork). When on the internet you move through Cyberspace. Cyberspace is the space of electronic movement of ideas and information. The web provides a multimedia interface to resources available on the internet. It is also known as WWW or World Wide Web. The web was first introduced in 1992 at CERN (Centre for European Nuclear Research) in Switzerland. Prior to the web, the internet was all text with no graphics, animation, sound or video.

Common internet applications

Communicating

Communicating on the Internet includes e-mail, discussion groups (newsgroups), and chat groups
You can use email to send or receive messages to people around the world.

Shopping

Shopping on the internet is called e-commerce
You can window shop at cyber malls called storefronts
You can purchase goods using cheques, credit cards or electronic cash called electronic payment.

Researching

You can do research on the internet by visiting virtual libraries and browse through stacks of books.
You can read selected items at the virtual libraries and even check out books.

Entertainment

There are many entertainment sites on the internet such as live concerts, movie previews and book clubs.
You can participate in interactive live games on the internet

How to get connected to the internet

You get connected to the internet through a computer. Using a provider is one of the most common ways users can access the internet. A provider is also called a host computer and is already connected to the internet.
ISPs – Internet Service Providers offer access to the internet for a fee.
URLs (Uniform Resource Locators)
URLs are addresses used by browsers to connect to other resources. URLs have at least two basic parts.
Protocol - used to connect to the resource, HTTP (Hypertext Transfer Protocol) is the most common
Domain name – the name of the server where the resource is located. e.g. yahoo.com
Many URLs have additional parts specifying directory paths, file names, and pointers
Connecting to a URL means that you are connecting to another location called a website.
Moving from one website to another is called Surfing.

Web portals

Web portals are sites that offer a variety of services typically including email, sports updates, financial data, news and links to selected websites. They are designed to encourage you to visit them each time you access the web e.g. MSN, Yahoo etc

Web pages

A web page is a document file sent to your computer when the browser has connected to a website. The document file may be located on a local computer or half-way around the world. The document file is formatted and displayed on your screen as a web page through the interpretation of special command codes embedded in the document called HTML (Hypertext markup Language)
Typically the first web page on a website is referred to as the homepage.

SEARCH TOOLS

Search tools developed for the internet help users locate precise information. To access a search tool, you must visit a website that has a search tool available. There are two basic types of search tool available:
Indexes
Search engines
Search engines
• Search engines are also known as web crawlers or web spiders
• They are organized like a database
• Key words and phrases can be used to search through a database
• Databases are maintained by special programs called agents, spiders or bots
• Widely used search engines are Google, HotBot and AltaVista

E-MAIL(Electronic Mail)

Email is the most common internet activity. It allows you to send messages to anyone in the world who has an internet email account. You need internet and email account to use this type of communication. Two widely used email programs are Microsoft outlook Express and Netscape’s communicator.
Email has three basic elements
i)Header – appears first in an email message and contains the following information.
Address – the address of the person(s) that is to receive the email
Subject – a one line description of the message displayed when a person checks their mail.
Attachment – Files that can be sent by the email program
ii) Message – The text of the email communication
iii) Signature – May include sender’s name, address and telephone number (optional)

Advantages of email

• It is normally much cheaper than using the telephone
• Many different types of data can be transmitted, such as images, documents, speech etc
• It is much faster than the postal service
• Users can filter incoming email easier than incoming telephone calls
• It is normally more secure than the traditional methods
• It is relatively easy to send to groups of people

Disadvantages of email

• It stops people using the telephone
• It cannot be used as a legal document
• Not everyone reads his or her email on a regular basis
• Email messages can be sent on the spur of the moment and may be regretted later on (sending by traditional means allows for a rethink) for extreme cases messages can be sent to the wrong person.

Internets: Intranets and Extranets

An organization may experience two disadvantages in having a connection to the WWW and the internet:
The possible use of the internet for non-useful applications (employees)
The possible connection of non-friendly users from the global connection into the organization’s local network.
For these reasons, many organizations have shied away from connection to the global network and have set up intranets and extranets.
Intranet – a private network without within an organization
Extranet – a private network that connects more than one organization

INTRANETS

Intranets are in-house, tailor-made networks for use within the organization and provide limited access (if any) to outside services and also limit the external traffic (if any) into the intranet. An intranet might have access to the internet but there will be no access from the internet to the organization’s intranet. Intranets centralize their information on central servers which are then accessed by a single browser package. The stored data is normally open and can be viewed by any compatible WWW browser.
Other properties defining an intranet are:
 Intranets use browsers, websites, and web pages to resemble the internet within the business
 They typically provide internal email, mailing lists, newsgroups and FTP services
 These services are accessible only to those within the organization

EXTRANETS

Extranets (external intranets) allow two or more companies to share parts of their intranets related to joint projects. For example two companies may be working on a common project, an extranet would allow them to share files related with project.
Extranets allow other organizations, such as suppliers, limited access to the organization’s network. The purpose of extranet is to increase efficiency and to reduce costs.

Firewalls

A firewall (security gateway) is a security system designed to protect organizational networks. It protects a network against intrusion from outside sources. It includes a special computer called a proxy server, which acts as a gate keeper.

Blogs – Are informal yet structured websites where subscribing individuals can publish stories, opinions, and links to other websites of interest.
Rich Site Summary or Really Simple Syndication (RSS) – Is a simple way for people to have content they want pulled from websites and fed automatically to their computer, where it can be stored for later viewing. Its commonly used with blogs.
Wikis – are collaborative websites where visitors can add, delete, or modify content on the site, including work of previous authors.