Networking Systems

Prepared by Dr. Iftekhar Hossain | 1

NETWORKS AND NETWORKING SYSTEM

A group of computers and other devices connected together is called a network and the concept of connected computers sharing resources is called networking. TYPES OF NETWORKING:

a) LAN - Local Area Network b) WAN - Wide Area Network

LAN: A Local Area Network, LAN, is a number of computers connected to each other by cable in a single location, usually a single floor of a building or a campus. WAN: Often businesses have offices throughout a large region. Local area networks are perfect for sharing resources within a building or campus and they cannot be used to connect distant sites. Wide Area Network, WAN, fills these needs. WAN are the set of connecting links between LANs. These links are made over telephone lines leased from various telephone companies. In rare instances, WAN can be created with satellite links and radio or microwave transceivers. NETWORK SHARING HARWARE RESOURCES: Computers that are not networked cannot effectively share resources. For instance, a small office with 10 standalone computers and 1 printer, allows only the user with the printer attached to his/her computer to print. Others must put their data on a floppy disk, transfer it to the computer with the printer and print it from there. This of course interrupts the user who would normally be using the computer with the printer attached. A network allows anyone connected to the network to use the printer, not just the individual sitting at the computer to which the printer is attached. Networked computers can also share:

1. Fax modem 2. Scanner 3. Hard disk 4. Floppy disk drive 5. CD ROM drive 6. Tape backup unit 7. Plotters

NETWORK SHARING SOFTWARE RESOURCES: Software resources can also be used more effectively over a network. With standalone computers, the software used on the computers must be present in the hard disk drives of each of the computers whether or not that computer is used at that moment for that task. Software costs can become prohibitive for a large number of computers. It is also difficult and time consuming to install and configure the software individually on every one of the computers. With a network, you can centrally install and configure the software, vastly reducing the work required to make computer programs available to an organization. You can also restrict access to the programs. For instance, to make sure the number of people using the word processor does not exceed the number of copies of the program you have installed.


NETWORK PROTECTING INFORMATION A network provides a more secure environment for important information. With standalone computers, the access to the computer means access to the information on the computer. Networks provide an additional layer of security by way of passwords. You can give each network user a different account name and password allowing the network server to distinguish among those who need access and protecting the information from unauthorized use. CLIENTS, SERVERS, PEERS There are three roles of a computer in a Local Area Network.

a) Clients: which use but do not provide network resources b) Servers: which provide network resources c) Peer: which both use and provide network resources

SERVER BASED NETWORK Server based networks are defined by the presence of servers on a network that provide security and administration of the network. Server based networks divide processing times between clients and servers. Clients often called front-end, request services such as file storage and printing and servers, often called back-end, deliver them. Server computers are typically more powerful than client computers. ADVANTAGES OF SERVER BASED NETWORK

1) Strong central security 2) Central file storage that allows all users to work from the same set of data and provides easy

backup of critical data. 3) Ability to share expensive equipments such as laser printers, scanners, plotters. 4) Central organization that keeps data from getting lost among computers.

DISADVANTAGES OF SERVER BASED NETWORK

1) Expensive dedicated hardware is required 2) Expensive network operating system software and client license is required. 3) A dedicated network administrator is required.

PEER TO PEER NETWORK Peer networks are defined by a lack of central control over the network. There are no servers in peer networks. Users simply share disk space and resources such as the printers and scanners. There is no central login process, if you have logged in to one peer on the network, you will be able to use any resource on the network that are not controlled by a specific password. ADVANTAGES OF PEER NETWORK

1) No extra investment in server hardware or software is required. 2) Easy setup 3) No network administration is required. 4) Ability of users to control resource sharing.


DISADVANTAGES OF PEER NETWORK

1) Additional loads on computers because of resource sharing. 2) Lack of central organization that can make data hard to find. 3) Weak security. 4) Inability of peers to handle as many network connections as servers.

TYPES OF SERVERS Not all servers are alike in a server-based network. A server in a network is dedicated in performing a specific task in support of other computers on the network. One server may perform all these tasks or a separate server may be dedicated to each task. 1) File server: File servers offer services that allow network users to share files. File services are the network applications that store, retrieve and move data. With network file services users can exchange, read, write and manage shared files and the data contained in them. There are several popular types of file servers such as WINDOWS NT, Netware and Apple Share. The following sections considers these types:

a) File transfer b) File storage and data migration c) File update and synchronization d) File archiving

2) Printer server: A printer server accepts and queues jobs from workstations, the user may be informed when printing is complete. The printer server may also provide certain print management functions, for example, to attach priorities to different printing jobs so that certain jobs are printed before others.

a) Allows users to share printers b) Allows you to place printers where convenient c) Achieves better workstation performance by using high-speed network data transfer,

print queuing and spooling.


NETWORK TOPOLOGY

TOPOLOGY The manner in which computers in a network are geometrically arranged and connected is known as the topology of the network. RING TOPOLOGY:

In a Ring Topology, consecutive computers are connected by point-to-point links, which are arranged to form a single closed path. Data are transmitted from node to node around the ring. The interface at each computer has the ability to recognize packets (data) destined to it. An obvious disadvantage is that if there is a fault in any part of the circle, all of the nodes will be affected. BUS (LINE) TOPOLOGY:

In a Bus Topology all the computers are connected to a common transmission pair of computers on the network can communicate at the same time. Each computer has a unique address, which is used when information is transmitted. When a data packet is sent out, it propagates throughout the medium, and is received by all the computers. To receive messages, each computer continuously controls (monitors) the medium and copies those messages that are addressed to itself as the data packets go by. Since the transmission medium in a bus is generally time shared, there must be some type of control mechanism to prevent several stations from transmitting simultaneously. A bus network…

Is cheap to install (just one long cable) Can be quite slow since all computers share the same cable when communicating Will stop working if there is a break in the central bus cable.


STAR TOPOLOGY:

In this type of network a central controller forms the principal node (server), while the subsidiary nodes form the points of the star. As the central machine controls the whole system, the whole system will be affected if it breaks down. Star Topologies use more cabling than other topologies and this makes them more expensive. However, communication is fast because there is a direct path from the central controller to each terminal. HEIRARCHICAL TOPOLOGY: In a hierarchical network one or more computers are more powerful than the rest. The relationship between the nodes is called a client-server relationship. The more powerful server looks after printing, file maintenance and other peripherals. Less powerful computers called clients are connected to the network. The clients may have neither disk drives nor processing power of their own. They make use of the functions provided by the server.


FORMATION OF NETWORK - COMPONENTS A network is not just a number of computers connected by cables. Parts found in a typical network are:

Network Software: This may be part of the operating system or it can be software designed specifically to manage a network.

Cables: Connecting cables are usually used to connect devices on a network, although some networks make use of radio or microwaves to provide the link. Cables vary in both performance and cost.

Connectors: Connectors are used to connect network cables to terminals or other devices.

Network Cards: If a personal computer is to be used as a terminal in a network, a device called a network card must be built into it. The network card looks like a small circuit board and slots into one of the connectors on the main board (mother board) inside the computer.

Types of Network Local Area Network (LAN) A Local Area Network is a network confined to one building or site. Often a LAN is a private network belonging to an organisation or business. Because LANs are geographically small, they usually use cables or low-power radio (wireless) for the connections.


Wireless Local Area Network (WLAN) A wireless LAN (WLAN) is a LAN that uses radio signals (WiFi) to connect computers instead of cables. At the centre of the WLAN is a wireless switch or router - a small box with one or two antennas sticking out the back - used for sending and receiving data to the computers. (Most laptops have a wireless antenna built into the case.) It is much more convenient to use wireless connections instead of running long wires all over a building.

However, WLANs are more difficult to make secure since other people can also try to connect to the wireless network. So, it is very important to have a good, hard-to-guess password for the WLAN connections.

Typically, the range of a wireless connection is about 50m, but it depends how many walls, etc. are in the

way.

Wide Area Network (WAN) A Wide Area Network is a network that extends over a large area. A WAN is often created by joining several LANs together, such as when a business that has offices in different countries links the office LANs together. Because WANs are often geographically spread over large areas and links between computers are over long distances, they often use quite exotic connections technologies: optical fibre (glass) cables, satellite radio links, microwave radio links, etc.


The Internet is an example of a global WAN .In fact it is the world’s largest WAN. Computers on the

International Space Station are linked to the Internet, so the you could say the the Internet is now the first

off-planet WAN!


TRANSMISSION OF DIGITAL DATA INTERFACES & MODEMS

Information generated by a source need to be encoded into a suitable format for transmission. To transmit the encoded signals generated by the Information-processing equipment over a communication link, assistance is needed. For example, a PC generates a digital signal but needs an additional device to modulate a carrier frequency before it is sent over a telephone line. Encoded data is sent from the generating device to the next device by a bundle of wires, a sort of mini communication link, called an interface.

Because an interface links two devices not necessarily made by the same manufacturer, its characteristics must be defined and standards must be established. Characteristics of an interface include its mechanical specifications (how many wires are used to transport the signal), its electrical specifications (the frequency, amplitude, and phase of the expected signal), and its functional specifications (if multiple wires are used, what does each one do?). These characteristics are all described by several popular standards and are incorporated in the physical layer of the OSI model.

Data transmission The transmission of binary data across a link can be accomplished either in parallel mode or serial mode. In parallel mode, multiple bits are sent with each clock pulse. In serial mode, one bit is sent with each clock pulse. While there is only one way to send parallel data, there are two subclasses of serial transmission; synchronous and asynchronous.


Parallel Transmission Binary data, consisting of 1s and 0s, may be organised into groups of n bits each. By grouping, we can send data n bits at a time instead of one. This is called parallel transmission. We use n wires to send n bits at one time. That way each bit has its own wire, and all n bits of one group can be transmitted with each clock pulse from one device to another. The Figure bellow shows how parallel transmission works for n = 8. Typically, the eight wires are bundled in a cable with a connector at each end.

The advantage of parallel transmission is speed. All else being equal, parallel transmission can increase the transfer speed by a factor of n over serial transmission. A significant disadvantage of parallel transmission is cost. Parallel transmission requires n communication lines (wires in the example) just to transmit the data stream. Because this is expensive, parallel transmission is usually limited to short distances.

Serial Transmission In serial transmission one bit follows another, so we need only one communication channel rather than n to transmit data between two communicating devices. The advantage of serial over parallel transmission is that with only one communication channel, serial transmission reduces the cost of transmission over parallel by roughly a factor of n. Since communication within devices is parallel, conversion devices are required at the interface between the sender and the line (parallel-to-serial) and between the line and the receiver (serial-to-parallel).


Asynchronous Transmission Asynchronous transmission is so named because the timing of a signal is unimportant. Instead, information is received and translated by agreed-upon patterns. Patterns are based on grouping the bit stream into bytes. Each group, usually eight bits, is sent along the link as a unit. To alert the receiver to the arrival of a new group, an extra bit is added to the beginning of each byte. This bit, usually a 0, is called the start bit. To let the receiver know that the byte is finished, one or more additional bits are appended to the end of the byte. These bits, usually 1s, are called stop bits. By this method, each byte is increased in size to at least 10 bits, of which 8 are information and 2 or more are signals to the receiver. In addition, the transmission of each byte may then be followed by a gap of varying duration. This gap can be represented either by an idle channel or by a stream of additional stop bits. In asynchronous transmission, we send one start bit (0) at the beginning and one or more stop bits (1s) at the end of each byte. There may be a gap between each byte. The start and stop bits and the gap alert the receiver to the beginning and end of each byte and allow it to synchronise with the data stream. This mechanism is called asynchronous because, at the byte level, sender and receiver do not have to be synchronised. But within each byte, the receiver must still be synchronised with the incoming bit stream. That is, some synchronisation is required, but only for the duration of a single byte. The receiving device resynchronises at the onset of each new byte. When the receiver detects a start bit, it sets a timer and begins counting bits as they come in. After n bits, the receiver looks for a stop bit. As soon as it detects the stop bit it ignores any received pulses until it detects the next start bit. Asynchronous here means ‘‘asynchronous at the byte level,’’ but the bits are still synchronised; their durations are the same.

The following Figure is a schematic illustration of asynchronous transmission. In this example, the start bits are 0s, the stop bits are 1s, and the gap is represented by an idle line rather than by additional stop bits.

Asynchronous transmission is slower than other forms of transmission because of the addition of control information. But it is cheap and effective, two advantages that make it an attractive choice for situations like low-speed communication. For example, the connection of a terminal to a computer is a natural application for asynchronous transmission. A user types only one character at a time, types extremely slowly in data processing terms, and leaves unpredictable gaps of time between each character.


Synchronous Transmission In synchronous transmission, the bit stream is combined into longer "frames," which may contain multiple bytes. Each byte, however, is introduced onto the transmission link without a gap between it and the next one. It is left to the receiver to separate the bit stream into bytes for decoding purposes. In other words, data are transmitted as an unbroken string of 1s and 0s, and the receiver separates that string into the bytes, or characters, it needs to reconstruct the information. In synchronous transmission, we send bits one after another without start/stop bits or gaps. It is the responsibility of the receiver to group the bits. The following Figure gives a schematic illustration of synchronous transmission. We have drawn in the divisions between bytes. In reality, those divisions do not exist; the sender puts its data onto the line as one long string. The receiver counts the bits as they arrive and groups them in eight-bit units. The advantage of synchronous transmission is speed. With no extra bits or gaps to introduce at the sending end and remove at the receiving end and, by extension, with fewer bits to move across the link, synchronous transmission is faster than asynchronous transmission. For this reason, it is more useful for high-speed applications like the transmission of data from one computer to another.


Modem Transmission Modes Three transmission modes are used by modems: simplex (SDX), half-duplex (HDX), and fullduplex (FDX). Simplex (SDX) Data are sent or received in one direction only. Simplex modems are used in applications such as weather and news services wires, which send data from a central location to the newsrooms. Of newspapers and radio and TV stations over based telephone circuits. Simplex modems use full bandwidth of the telephone circuit.

Half–duplex (HDX) The communication channel is shared btw. Sending and receiving stations. In this mode communications can take place in either direction, but in only one direction at a time. The time that it takes for transmission directions to change between two stations is referred to as modem turnaround time. The disadvantage of HDX modem is that each time the direction of communication is reversed, the telephone circuit must be "turned around". Another disadvantage of HDX communication is that the receiving terminal can not provide an immediate feedback in case of errors. Full–duplex (FDX) Modems capable of operating in FDX can transmit and receive data simultaneously. In two–wire telephone circuits, this is performed by using frequency-division multiplexing (FDM), where it separates the band of channel by two. Low band and high band are located within the pass band of the telephone lines. One modem transmits on the low band and receives on the high band; the other modem transmits on the high band and receives on the low band, thus allowing the FDX operation. Another technique is called echo cancelling, in which both modems transmit simultaneously on the same frequency. The FDX modems have been designated to operate in one of the two modes: - Originate mode (Terminal originate the cell) - Answer mode (Terminal answer the cell) In the originate mode, transmission occurs in the low band frequencies and reception occurs in the high band frequencies. In the answer modern, transmission occurs in the high band frequencies and band frequencies and reception occurs in low band frequencies.


Error Detection and Correction Introduction

There are many reasons such as noise, cross-talk etc., which may help data to get corrupted during transmission. The upper layers works on some generalized view of network architecture and are not aware of actual hardware data processing. So, upper layers expect error-free transmission between systems. Most of the applications would not function expectedly if they receives erroneous data. Applications such as voice and video may not be that affected and with some errors they may still function well.

Data-link layer uses some error control mechanism to ensure that frames (data bit streams) are transmitted with certain level of accuracy. But to understand how errors is controlled, it is essential to know what types of errors may occur.

There may be three types of errors:

Single bit error:

[Image: Single bit error]

In a frame, there is only one bit, anywhere though, which is corrupt.

Multiple bits error:

[Image: Multiple bits error]

Frame is received with more than one bits in corrupted state.

Burst error:

[Image: Burst error]

Frame contains more than1 consecutive bits corrupted.

Error control mechanism may involve two possible ways:

Error detection Error correction


Error Detection

Errors in the received frames are detected by means of Parity Check and CRC (Cyclic Redundancy Check). In both scenario, few extra bits are sent along with actual data to confirm that bits received at other end are same as they were sent. If the checks at receiver’s end fails, the bits are corrupted.

Parity Check

One extra bit is sent along with the original bits to make number of 1s either even, in case of even parity or odd, in case of odd parity.

The sender while creating a frame counts the number of 1s in it, for example, if even parity is used and number of 1s is even then one bit with value 0 is added. This way number of 1s remain even. Or if the number of 1s is odd, to make it even a bit with value 1 is added.

[Image: Even Parity]

The receiver simply counts the number of 1s in a frame. If the count of 1s is even and even parity is used, the frame is considered to be not-corrupted and is accepted. If the count of 1s is odd and odd parity is used, the frame is still not corrupted.

If a single bit flips in transit, the receiver can detect it by counting the number of 1s. But when more than one bits are in error it is very hard for the receiver to detect the error

Error Correction

In digital world, error correction can be done in two ways:

Backward Error Correction: When the receiver detects an error in the data received, it requests back the sender to retransmit the data unit.

Forward Error Correction: When the receiver detects some error in the data received, it uses an error-correcting code, which helps it to auto-recover and correct some kinds of errors.

The first one, Backward Error Correction, is simple and can only be efficiently used where retransmitting is not expensive, for example fiber optics. But in case of wireless transmission retransmitting may cost too much. In the latter case, Forward Error Correction is used.

To correct the error in data frame, the receiver must know which bit (location of the bit in the frame) is corrupted. To locate the bit in error, redundant bits are used as parity bits for error detection. If for example, we take ASCII words (7 bits data), then there could be 8 kind of information we need. Up to seven information to tell us which bit is in error and one more to tell that there is no error.

For m data bits, r redundant bits are used. r bits can provide 2r combinations of information. In m+r bit codeword, there is possibility that the r bits themselves may get corrupted. So the number of r bits used must inform about m+r bit locations plus no-error information, i.e. m+r+1.


Communication and Information System Anatomy of Internet: Anatomy refers to the study of structure. While studying the structure of the internet, it can be broadly said that the Internet is a networks of computer networks. Internet brings together these computers through the communication media and protocols. It also enable the computers to communicate with one another. A study of the internet outlines the following major components of the structure:

Internet services.

Elements of the Internet.

Uniform Resource Locators.

Internet Protocol. Internet Services: The Internet is a combination of many networks, and a large number of databases and other services. The major services offered on the internet are given below:

E-mail: E-mail is the most common service of the Internet. It can be used on old PC’s also. Mailing Lists: Mailing Lists are a group-based messaging service. Once subscribed, you would

receive mailing list message via a standard e-mail account. There are currently over 90,000 Internet mailing lists.

FTP (File Transfer Protocol): An Internet service designed for transferring files among computers. Files are available for downloading on the Internet using FTP.

Newsgroups (Usenet): Usenet is a public messaging and “bulletin board” system. It comprises of more than 34,000 individual forums, and each one pertains to a specific topic.

World Wide Web(WWW): This service features user-friendly publishing and multimedia documents and files. Web pages are created using HTML, JavaScript, and Java.

INTERNET: The Internet is a network of networks, linking computers to computer within the same protocol, mainly TCP/IP (Transmission Control Protocol and Internet Protocol). The network allows all of the computers to communicate with one another. A home computer is usually linked to the Internet using a normal phone line of a MODEM (Modulator-Demodulator) that talks to the ISP (Internet Service Provider). A computer in a business or home has a Network Interface Card (NIC) that directly connects it to a LAN inside the business. The business then connects its LAN to an ISP using a high-speed phone line like a T1 line. A T1 line can handle approximately 1.5 million bits per second while a normal phone line using a modem can handle 30,000 to 56,000 bits per second. ISPs then connect to larger ISPs and the larger ISPs maintain fibre optics “backbones” for an entire nation or region. Backbones all around the world are connected through fibre optics lines by under sea cables, or satellite links. In this way every computer in the Internet is connected to each other. In general, all of the machines in the Internet can be catalogued into two types: Servers and clients. Those machines that provide services, like web servers, FTP (File Transfer Protocol), to other machines are known as servers. The machines that are used to connect those servers are called clients. When you connect to Yahoo, at www.yahoo.com, to read a page, yahoo is providing a machine for use on the Internet to serve your request. Yahoo is providing a server; your machine on the other hand is probably providing no service to anyone else on the Internet. Therefore, it is a user machine, also known as client. It is possible and common for a machine to be both a server and a client.


Server machines may provide one or more services in the Internet for example; the server machines might have software running on it that allows it to act as a web server, an email server and a FTP server. Clients that come to a server machine do so with specific intent, so clients direct their request into a specific software server running on the overall server machine. For example, you are running a web browser on your machine, it would likely want to talk to the web server on the serve machine. To keep all these machines straight, each machine on the Internet is assigned to a unique address called an IP address. A typical IP address looks like 192.168.1.109. Because most people have trouble remembering the strings of numbers, that make up the IP address and because IP addresses sometimes need to change, servers on the Internet also have human readable names called domain names, for example, www.sunbeamsbd.com.

DOMAIN NAME SERVER (DNS):

1) Host name (www) 2) Domain name (sunbeamsbd) 3) Top level domain name (com)

BASIC INTERNET TERMINOLOGIES

A simplified hierarchical model of the internet includes following basic terminologies: INTERNET PROTOCOL: Internet Protocol (IP) is responsible for the addressing and sending data from one computer to another computer. IP is a method by which data is sent from one computer to another over the network. Each computer which is connected to the Internet has at least one IP address which uniquely identifies this computer from other computers. INTERNET INFORMATION SERVER(IIS): IIS is a group of Internet servers(HTTP server and FTO server) including the additional capabilities of Windows NT and Windows 2000. TRANSMISSION CONTROL PROTOCOL(TCP): It uses a set of rules to exchange messages with other Internet points at the information packet level. INTERNET SERVICE PROVIDER(ISP): An Internet Service Provider, or ISP is one gateway to the Internet. In most cases, you connect to an ISP by using a PC modem to dial into your ISP’s modems over a standard telephone line. Your modem connects to a single modem among a bank of modems at your ISP. This is called a dial-up connection. Users within corporations and large organizations typically connect an ISP via a high-speed link(typically over fiber optic cabling but not phone lines) called a direct connection.


INTERNET APPLICATIONS: One can find many applications of the Internet such as the following:

To exchange electronic mail with friends all over the globe for a very small price.

To participate in group discussions on topics of interest, through public news groups.

To find educational tools, universities around the world, book stores, and libraries are sharing online information.

Commercial electronic store fronts are growing in numbers in the USA and Europe. One can order different products over the Internet.

In the case of business one can get technical support for products one is using. One can publish information such as technical or marketing literature.

Because the Internet is electronic, one can make changes to reflect late breaking news that would be impossible for printed publications. Change becomes easier, updates are simpler and information is instant.

The Internet has become the first global venue for publishing information. This network has enough users now and it is benefiting from positive feedback loop: the more users it gets, the more content it gets, and more content it gets, the more users it gets.

a) What is a World Wide Web: The World Wide Web(WWW) is simply the multimedia part of the internet. Using the WWW you can view graphics and video as well as listen to sound. There are also hyperlinks to other pages or media. The World Wide Web has its own protocol called HTTP (hypertext transfer protocol). Specialized software called a web browser is needed to make use of the features of the World Wide Web (WWW). The different packages on offer work in similar ways. The WWW is divided into millions of sites called websites. These are files made of groups of pages, designed and set up by companies or individuals who wish to communicate with internet users throughout the world. The aim may be simply to spread information, talk to other people, or to sell, or find, services or products. Web pages are written in a programming language called Hyper Text Markup Language (HTML) and the web browser automatically recognizes these pages and tries to display them.

b) Handshaking: Handshaking is the name given to the method of controlling the flow of serial communication between two devices so that transmission only happens when the device at the receiving end is ready to receive it. There are two methods in use, the hardware method and the software method. With the hardware method, a separate wire is used to send a signal to tell the sending device that the receiving device is ready. This method is only really suitable when the devices are near to each other so that a special cable can be used. The hardware method is used when a computer communicates with a serial printer. Communication systems which make use of the telephone cable only have a single wire, so hardware handshaking is not possible. Instead software handshaking is used, where special control characters are sent by the receiver to let the sending device know when to send data. One software method used is called XON/XOFF and this uses Ctrl + S to pause sending data and Ctrl + Q to resume transmission.


EMAIL AND THE WORLD WIDE WEB: Electronic messages can be sent from one computer to another by the use of email. Email uses two servers called SMTP (Simple Mail Transfer Protocol) and POP3 server, (Post Office Protocol). The SMTP server is used for the transfer of mails that is the outgoing mails. The POP3 server is used for the transfer of incoming mails. Advantages:

1) Email provides a quick way of sending messages all around the world. 2) The email can be printed and kept as a traditional letter and in a commercial context, could

provide proof that a person has agreed to something. 3) The same message can be sent to many recipients at the same time. 4) Documents prepared in any type of software, can be attached to emails and sent with them.

This involves pictures and video clips. 5) The World Wide Web provides an almost limitless source of information and if it is used

carefully, can provide valuable information for research. 6) The results of research can be published immediately on the web so that information available

is also up to date. Disadvantages:

1) In working situations, the use of emails and the surfing on the World Wide Web can lead to a lot of time wasting.

2) Messages deliberately designed to cause trouble or upset people, can easily be sent to many targets.

3) Young people may gain access to sites that are not designed for them. 4) Some people will say that certain user often young people become addicted to surfing the web

and are no longer interested in other activities. 5) Virus can easily be sent to millions of recipients can cause massive systems breakdown. 6) A lot of junk mail can be sent in addition of being unwanted, the recipient has to pay a lot of

charges while this mail is being downloaded.


What is attenuation? Attenuation is the data loss experienced in a transmission media as a function of length. Attenuation increases as the distance a signal increases. Attenuation will eventually prevent a signal reaching its intended destination.


SERVERS In some small networks, every machine is equally likely to have a resource that another machine needs to use.

For example a small home network may be set up like this:

There are files stored on each computer. One machine is linked to the scanner, whilst another is linked to a printer. The game machine is linked to the internet, which all three machines can use.

This is fine for small networks as the number of requests to use a resource is not going to be too high.

Printer Server.

But now imagine there are a dozen computers on the network and each one needs to print out a document every few minutes. The machine that is connected to the printer is going to be tied up most of the time.

In this case it makes sense to allocate a machine exclusively to service printer requests. This machine is called a 'printer server'

File server.

Consider a network of a dozen computers. There could be thousands of files scattered across each machine. Two problems arise:

a) how do you find the file you need?

b) how do you back up your data to keep it safe?

The solution is to keep all your files on one computer. This machine is called the 'file server'.

Advantages:

Files are simple to find

Files are easily shared

User computers can be switched off without any problem of a file becoming unavailable.

Data is easily backed up.


Database server.

Many companies rely on a central store of information to run their operations. Information such as sales and stock data is usually kept in a database.

Very often a machine will be dedicated to run a specific database. This machine is called a 'database server'

Advantages:

All data is available from anywhere in the network

A specially designed computer can be selected to run the database e.g lots of memory.

Database is easily backed up.

Other kinds of server.

A server is any machine that provides a service for other users on the network.

Common services include:

Email server

Internet Proxy server

Intranet server.

The email server will provide all the usual facilities such as address books, spam filtering and so on.

Quite often, staff want to use the same website over and over again. The Internet Proxy server will store a local copy of often-used web pages to speed up access and to reduce bandwidth consumption (which costs money).

Many companies run their own private internal web services. This is called an Intranet and is run from the Intranet server.


PROXY SERVER A 'Proxy' is another word for 'Substitute'.

Imagine you have become a multi-millionaire overnight. All the world's press want to know your story. Instead of handling all this yourself, you hire a 'public relations' professional

You tell her what you want to say and she will present your story to the press in the best light. On the other hand, she will also reject unwanted interviews, thus saving you the hassle.

She is your 'Proxy'.

The Proxy Server.

This machine has the two main tasks:

Quite often staff tend to use the same web sites over and over again. So to speed up access and reduce bandwidth costs, the Proxy server will keep a local copy of the web pages and serve these to the user instead. Of course rules running on the proxy server will determine how often these local pages need to be updated. The proxy server also acts as a filter: Prevents unauthorized users from accessing external networks such as the internet. Prevents unauthorized web sites to be accessed (stops time wasting!) Provides a web service to external clients, but does not allow them through to the internal network itself. For example a bank web site would be running a proxy service.

In a way, the proxy server is the outward facing aspect of the company linked to the internet.

Supply authorised internal users with web pages Supply external users with authorised information and services.


THE NETWORK INFRASTRUCTURE REQUIRED TO SUPPORT THE WORLD WIDE WEB

The Internet started life as the ARPANET in 1969 and it consisted of four computers. It now consists of several million computers linked together by cables, wires and satellites. Data travelling from one computer to another is transmitted from one link in the network to another using the best possible route it can find. If some links are out of service, the data will be routed through different links. The major communication links that for the Internet are called the "backbone". A handful of network service providers (NSPs) e.g. BT, maintain a series of nationwide links. More links are being added as Internet use increases.

Unlike online services, which are centrally controlled, the Internet is decentralized by design. Each Internet computer, called a host, is independent. Its operators can choose which Internet services to use and which local services to make available to the global Internet community. Remarkably, this anarchy by design works exceedingly well. The Internet is not synonymous with World Wide Web. There are a variety of ways to access the Internet. Most online services, such as America Online, offer access to some Internet services. It is also possible to gain access through a commercial Internet Service Provider (ISP). Internet Service Provider, a company that provides access to the Internet. For a monthly fee, the service provider gives you a software package, username, password and access phone number. Equipped with a modem, you can then log on to the Internet and browse the World Wide Web and USENET, and send and receive e-mail.

Internet Backbone

the main network connections composing the Internet.

Network Service Provider

A company that provides Internet access to ISPs. Sometimes called backbone providers, NSPs offer direct access to the Internet backbone and the Network Access Points (NAPs). What does a browser do?

Web Browser

A software application used to locate and display Web pages. The two most popular browsers are Netscape Navigator and Microsoft Internet Explorer. Both of these are graphical browsers, which means that they can display graphics as well as text. In addition, most modern browsers can present multimedia information, including sound and video, though they require plug-ins for some formats.


ROUTERS

A Router is a device that transfers data from one network to another in an intelligent way. It has the task of forwarding data packets to their destination by the most efficient route.

In order to do this, the router has a micro computer inside it. This holds a table in memory that contains a list of all the networks it is connected to, along with the latest information on how busy each path in the network is, at that moment. This is called the 'routing table'.

When a data packet arrives, the router does the following:-

Reads the data packet's destination address

Looks up all the paths it has available to get to that address.

Checks on how busy each path is at the moment

Sends the packet along the least congested (fastest) path.

Other tasks the Router can perform:

Exchange Protocol information across networks.

Filter traffic - useful for preventing hacker attacks for example Routers operate at the network level of the OSI model.


REPEATERS All signals fade as they travel from one place to another.

Each type of network cable has a maximum useable length. If you go beyond that length, the signal will be too weak to be useful.

Of course, computers on a real network can easily be more than 200 metres apart. Therefore the network cable is split up into segments. Each segment is less than the maximum length allowed. Joining the segments together is a device known as a 'Repeater'.

A Repeater boosts the signal back to its correct level.

Here are some typical maximum cable lengths:

Copper cable - 100 m

Thick Ethernet -500m

Thin Ethernet - 185m


BRIDGES A Bridge does just what you would expect it to do - it joins two networks together so as far as data packets are concerned it looks like one large network

A bridge is not as capable as a Router - but it is less expensive.

Both networks have to be use the same protocol


HUB

There are many network topologies available:

Star - uses a hub

Tree - uses a hub

Bus - does not use a hub

Ring - does not use a hub

To allow the Star and Tree network topologies to work properly, each computer must be able to send data packets to any other computer on the network.

The network 'Hub' allows computers to share data packets within a network.

Each computer will be connected to a single 'port' on the hub. So if you purchase an '8 port hub', you will be able to connect up to eight computers together.

You can also 'daisy chain' hubs to allow even more computers to join the network.


Typical network making use of a hub:

SWITCHES A network cable can only have one data packet in it at any instant.

So if two or more computers want to place a data packet on to the network at exactly the same time, then a 'data collision' will take place.

The network protocol is set up to deal with this. Basically it declares the collided data as unusable and forces the two computers to re-send their data packets at a slightly different time.

This is fine for a lightly loaded network with only a few computers on-line. You will not notice the small delay caused by data collisions.

But imagine what happens when a hundred PCs are sharing the same network and they are all wanting to send their data packets. This will most likely result in thousands of data collisions per second - each one costing a small amount of time. You will certainly notice the network 'slowing down'.

A switch has a number of ports and it stores the addresses of all devices that are directly or indirectly connected to it on each port.

As a data packet comes into the switch, its destination address is examined and a direct connection is made between the two machines.


PROTOCOLS


BANDWITH:


GATEWAYS

There are many different network protocols in use today. For example the large internet company called AOL has its own special email protocol.

If your computer does not use the AOL protocol (and the chances are it won't). Then how do you get email to your friend on the AOL network?

Answer: A gateway.

A gateway converts the data passing between dissimilar networks so that each side can communicate with each other. i.e converts data into the correct network protocol.

The gateway is a mixture of hardware components and software. This is unlike a standard 'Bridge' which simply joins two networks together that share the same protocol.


FILTERS Not all data packets are equal. Some are more equal than others ( a bit of Animal Farm quotations creeping in!)

If your network is to be kept secure it is often essential that some filtering takes place.

For example, some staff wish to work from home with their laptops and they need to access files from within the company network.

In this case a filter would be set up that accepts data packets coming from that particular laptop. Other filtering rules would block unwanted packets trying to come in.

Just like Gateways, a Filter can be a mix of hardware and software components.

Note that a filter can also prevent data packets from leaving the company network. For example, a rule could be set up that only allows an authorised server within the network to send data outside the local network.

A filter is an essential component of a 'Firewall'.


MODEMS Before the days of broadband Internet connections, most computers connected to the Internet via telephone lines (dial-up connections). The problem with using telephone lines is that they are designed to carry voices, which are analogue signals. They are not designed for digital data.

The word modem is an abbreviation of MOdulator DEModulator. A modulator acts as a DAC, and a demodulator acts as an ADC. The solution was to use a special device to join the digital computer to the analogue telephone line. This device is known as a modem. A modem contains a DAC and an ADC. The DAC in the modem is required so that the digital computer can send data down the analogue telephone line (it converts digital data into noises which is exactly what the telephone line is designed to carry.) The ADC in the modem is required so that the analogue signals (noises) that arrive via the telephone line can be converted back into digital data.

So, simply put, a modem is required because computers are digital devices and the telephone system is analogue. The modem converts from digital to analogue and from analogue to digital. The ADC in the modem is required so that the analogue signals (noises) that arrive via the telephone line can be converted back into digital data. The reason telephone lines were used is that almost every building in the world is already joined to

every other via the telephone system.

Using the telephone system for connecting computers meant that people didn’t have to install new

wires to their houses and offices just for computer use.

In the last few years however, this is exactly what people have done. Special cables have been

installed just for Internet access. These special cables are designed to carry digital data, so no modem

is required.

http://www.igcseict.info/theory/5/anadig/index.html

http://www.igcseict.info/theory/5/anadig/index.html


Wi-Fi modems

In addition to telephone modems, radio has now become very popular as a means of connecting to the internet. The device that allows you to do this is called the Wi-Fi modem

NETWORK CARDS

Network cards are required in every machine connected to the network. They allow the signal from the network to be transmitted to the machine – this could be via a fixed cable, infra red or radio waves.

A network card that uses a standard cable network socket.

A Wi-Fi network card with an external aerial for the signal. CABLES How do you transfer data from one point to another on a network?

This has to be done by some physical effect.

There are three main methods of transferring data:

Electrical

Radio or Microwave

Infrared


Firewall A firewall is a device, or a piece of software that is placed between your computer and the rest of the network (where the hackers are!) If you wish to protect your whole LAN from hackers out on the Internet, you would place a firewall between the LAN and the Internet connection.

A firewall blocks unauthorised connections being made to your computer or LAN. Normal data is allowed through the firewall (e.g. e-mails or web pages) but all other data is blocked.

In addition to physical devices, firewalls can also be software. In fact most computer operating

systems have a software firewall built in (e.g. Windows, Linux and Mac OS).


Electrical:

A multi-wired cable with a socket at each end is used to connect the various devices together e.g. computer to hub, hub to switch or switch to router etc.

The Ethernet network cable shown above transfers data by means of electrical signals. A typical network cable called 'Cat 5' is used which is especially designed to carry the signal as efficiently as possible.

Radio or Microwave

Microwaves are just a small part of the radio spectrum, but because they are so widely used, they tend to be called by their own name.

Data is sent out through aerials mounted on tall towers. The 'cable' is effectively the microwave link between towers. Some large companies use microwave towers spread along hilltops to allow one office to communicate with others in the same country.

They do this because it is cheaper than renting telephone lines for carrying the same amount of data.

On a much smaller scale, laptops can communicate with the local area network with radio links:

Infra-Red

This is a very familiar method of transferring data if you are at all aware of your remote control ! The television remote control makes use of an infra-red link.

PDA and personal organisers often make use of an infra-red link to synchronise calendars and 'to-do' lists.


Ways of securing computer networks

Access authorization restricts access to a computer to group of users through the use of authentication systems. These systems can protect either the whole computer - such as through an interactive logon screen - or individual services, such as an FTP server. There are many methods for identifying and authenticating users, such as passwords, identification cards, and, more recently, smart cards and biometric systems.

Anti-virus software consists of computer programs that attempt to identify, thwart and eliminate computer viruses and other malicious software (malware).

Application with known security flaws should not be run. Either leave it turned off until it can be patched or otherwise fixed, or delete it and replace it with some other application. Publicly known flaws are the main entry used by worms to automatically break into a system and then spread to other systems connected to it.

Backups are a way of securing information; they are another copy of all the important computer files kept in another location. These files are kept on hard disks, CD-Rs, CD-RWs, and tapes. There is also a fourth option, which involves using one of the file hosting services that backs up files over the Internet for both business and individuals.

Encryption is used to protect the message from the eyes of others. It can be done in several ways by switching the characters around, replacing characters with others, and even removing characters from the message. These have to be used in combination to make the encryption secure enough, that is to say, sufficiently difficult to crack.

Firewalls are systems which help protect computers and computer networks from attack and subsequent intrusion by restricting the network traffic which can pass through them, based on a set of system administrator defined rules.

Honey pots are computers that are either intentionally or unintentionally left vulnerable to attack by crackers. They can be used to catch crackers or fix vulnerabilities.

Intrusion-detection systems can scan a network for people that are on the network but who should not be there or are doing things that they should not be doing, for example trying a lot of passwords to gain access to the network.

Pinging: The ping application can be used by potential crackers to find if an IP address is reachable. If a cracker finds a computer they can try a port scan to detect and attack services on that computer.

Social engineering awareness keeps employees aware of the dangers of social engineering and/or having a policy in place to prevent social engineering can reduce successful breaches of the network and servers.


The advantages of networking:

Computers can communicate and share data and files. Storage facilities, Hardware peripherals such as printers can be shared. Data from all the computers can easily be backed up centrally.

The disadvantages of networking:

One of the major reasons behind the spread of computer viruses. As data is shared there is a greater need for security. Users of the network have to have user

ids and passwords. If the server fails, all the workstations are affected. Work stored on shared hard disk drives

will not be accessible and it will not be possible to use network printers either. Installing a network is expensive as it involves purchasing of networking hardware such as

Hubs, cables, network cards, bridges, routers etc. Large networks require specialist to maintain them, which again involves time and money.

Use of Bluetooth and Wifi in Network Use of Bluetooth and Wifi in Network Bluetooth (Personal Area Network) Bluetooth is a wireless networking technology designed for very short-range connections (typically just a few metres). The idea of Bluetooth is to get rid of the need for all of those cables (e.g. USB cables) that connect our computer to peripheral devices such as printers, mice, keyboards, etc. Bluetooth devices contain small, low-power radio transmitters and receivers. When devices are in range of other Bluetooth devices, they detect each other and can be 'paired' (connected) Because Bluetooth networking only works over very short distances, and with devices belonging to

one user, this type of network is sometimes called a 'Personal Area Network'

http://dineshbakshi.com/igcse-gcse-ict/computer-networks/62-revision-notes/1519-use-of-bluetooth-and-wifi-in-network


Typical uses of Bluetooth:

Connecting a wireless keyboard to a computer Connecting a wireless mouse to a computer Using a wireless headset with a mobile phone Printing wirelessly from a computer or PDA Transferring data / music from a computer to an MP3 player Transferring photos from a phone / camera to another device

Advantages of Bluetooth

Bluetooth can connect devices from point-to-point and probably better on security than Wifi, as it can cover shorter distances.

Bluetooth offers an optional two levels of password protection. Up to seven devices can be connected at any one time which makes it easier to find and

connect the device being searched for, as Bluetooth highlights itself to the other devices. Bluetooth technology is also more ideal for consumer electronics devices as it has a smaller

power requirement. Another advantage is concerning voice communication. For an office or home environment,

Bluetooth can be used in a cordless phone within a 10m range, without the need to be passed around.

What is Wifi?

Wifi is short for 'wireless fidelity' and is a limited-range wireless networking code which is used in many airports, hotels or other services, who offer public access to Wifi networks, to allow people to log on to the Internet and receive emails whilst on the move. As Wifi is a reasonably fast method of transmitting information in wave form, it is often used in computers and also notebooks. In future, it will become possible to access the Internet from just about anywhere, without the use of any wires. The advantages of using Wifi are that the networks are fairly cheap and straight-forward to set up. Wifi is also quite inconspicuous and can hardly be noticed unless it is being looked for specifically, whilst in a Wifi 'hotspot.' For a wireless network to be created, communication is transferred like a two way radio, using radio waves.

Advantages of Wifi

The most prominent advantage which Wifi has over Bluetooth is that Wifi operates at a much faster rate - of about 11mbps, whereas Bluetooth only operates at a much slower rate of around 720kbps.

Wifi is also designed to link up entire networks, rather than computer to computer. Wifi can achieve this too but it is not its real purpose.

It is now often being used for increasingly more applications, such as Internet access, gaming and basic connectivity for home electronic appliances such as televisions or DVD players. Wi-Fi may be used by cars in highways, as standards rise in development, in support of an Intelligent Transportation System to increase safety, gather statistics, and enable

Disadvantages of Wifi

Security problem

Limited connections

Limited range


E-Commerce Shopping over the Internet is increasingly popular.

Retailers create websites which display the items they are selling.

Customers load the web-site and add the items they wish to buy to a 'shopping-basket', and then pay by credit or debit card. (Credit card details are encrypted on secure sites)

The goods are delivered to the buyer's house.

Advantages of e-commerce:

greater range of goods can be done from home goods are delivered to the door-step

Disadvantages :

there is a couple of days delay before receiving goods. possibility of credit card information getting into the wrong hands you cannot see or touch the goods before buying


What is the difference between workgroups and domains in a network? A workgroup is often referred to as a peer-to-peer network, and there is no centralized administration. A domain is a server-based network, where the server (often called a domain controller) manages user accounts and security for the network. Workgroups are best suited for networks with 10 or fewer computers and low security requirements.

What is the difference between a LAN, MAN and WAN? A LAN is a local area network, which typically means a network in one centralized location. A WAN is a wide area network, which means several LANs in remote locations connected to each other. MAN is a metropolitan area network connecting different places in a city.

What hubs, switches, and routers are? These are all network connectivity devices. Hubs and switches are used to connect several computers or groups of computers to each other. Routers are more complex devices that are often used to connect network segments or networks to each other. Routers route data packets/frames received from hosts and other routers.

What is a VPN? A virtual private network is a network where computers are connected via a WAN connection, but the connection is secured and appears to be a LAN connection

A virtual private Network (VPN) extends a private network across a public network, such as the Internet. It enables a computer to send and receive data across shared or public networks as if it were directly connected to the private network, while benefiting from the functionality, security and management policies of the private network.[1] This is done by establishing a virtual point-to-point connection through the use of dedicated connections, encryption, or a combination of the two.

A virtual private network connection across the Internet is similar to a wide area network (WAN) link between the sites. From a user perspective, the extended network resources are accessed in the same way as resources available from the private network.[2]

VPNs allow employees to securely access their company's intranet while traveling outside the office. Similarly, VPNs securely and cost-effectively connect geographically disparate offices of an organization, creating one cohesive virtual network. VPN technology is also used by ordinary Internet users to connect to proxy servers for the purpose of protecting one's identity.

http://en.wikipedia.org/wiki/Private_network

http://en.wikipedia.org/wiki/Public

http://en.wikipedia.org/wiki/Internet

http://en.wikipedia.org/wiki/Virtual_private_network#cite_note-1

http://en.wikipedia.org/wiki/Point-to-point_%28network_topology%29

http://en.wikipedia.org/wiki/Wide_area_network

http://en.wikipedia.org/wiki/Virtual_private_network#cite_note-2

http://en.wikipedia.org/wiki/Proxy_server


What are WEP, WPA, and WPA2?

Wired Equivalent Privacy (WEP)

Wired Equivalent Privacy (WEP) is the most widely used Wi-Fi security algorithm in the world. This is a function of age, backwards compatibility, and the fact that it appears first in the encryption type selection menus in many router control panels.

WEP was ratified as a Wi-Fi security standard in September of 1999. The first versions of WEP weren’t particularly strong, even for the time they were released, because U.S. restrictions on the export of various cryptographic technology led to manufacturers restricting their devices to only 64-bit encryption. When the restrictions were lifted, it was increased to 128-bit. Despite the introduction of 256-bit WEP encryption, 128-bit remains one of the most common implementations.

Despite revisions to the algorithm and an increased key size, over time numerous security flaws were discovered in the WEP standard and, as computing power increased, it became easier and easier to exploit them. As early as 2001 proof-of-concept exploits were floating around and by 2005 the FBI gave a public demonstration (in an effort to increase awareness of WEP’s weaknesses) where they cracked WEP passwords in minutes using freely available software.

Despite various improvements, work-arounds, and other attempts to shore up the WEP system, it remains highly vulnerable and systems that rely on WEP should be upgraded or, if security upgrades are not an option, replaced. The Wi-Fi Alliance officially retired WEP in 2004.

Wi-Fi Protected Access (WPA)

Wi-Fi Protected Access was the Wi-Fi Alliance’s direct response and replacement to the increasingly apparent vulnerabilities of the WEP standard. It was formally adopted in 2003, a year before WEP was officially retired. The most common WPA configuration is WPA-PSK (Pre-Shared Key). The keys used by WPA are 256-bit, a significant increase over the 64-bit and 128-bit keys used in the WEP system.

Some of the significant changes implemented with WPA included message integrity checks (to determine if an attacker had captured or altered packets passed between the access point and client) and the Temporal Key Integrity Protocol (TKIP). TKIP employs a per-packet key system that was radically more secure than fixed key used in the WEP system. TKIP was later superseded by Advanced Encryption Standard (AES).

Despite what a significant improvement WPA was over WEP, the ghost of WEP haunted WPA. TKIP, a core component of WPA, was designed to be easily rolled out via firmware upgrades onto existing WEP-enabled devices. As such it had to recycle certain elements used in the WEP system which, ultimately, were also exploited.

WPA, like its predecessor WEP, has been shown via both proof-of-concept and applied public demonstrations to be vulnerable to intrusion. Interestingly the process by which WPA is usually breached is not a direct attack on the WPA algorithm (although such attacks have been successfully demonstrated) but by attacks on a supplementary system that was rolled out with WPA, Wi-Fi Protected Setup (WPS), designed to make it easy to link devices to modern access points.


Wi-Fi Protected Access II (WPA2)

WPA has, as of 2006, been officially superseded by WPA2. One of the most significant changes between WPA and WPA2 was the mandatory use of AES algorithms and the introduction of CCMP (Counter Cipher Mode with Block Chaining Message Authentication Code Protocol) as a replacement for TKIP (still preserved in WPA2 as a fallback system and for interoperability with WPA).

Currently, the primary security vulnerability to the actual WPA2 system is an obscure one (and requires the attacker to already have access to the secured Wi-Fi network in order to gain access to certain keys and then perpetuate an attack against other devices on the network). As such, the security implications of the known WPA2 vulnerabilities are limited almost entirely to enterprise level networks and deserve little to no practical consideration in regard to home network security.

Networking Systems

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