Computer Networks and Network Secuirty 3 - [PDF Document] (2024)

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COMPUTER NETWORKS AND NETWORK SECURITY

Network

A computer network is a collection of computers and terminaldevices connected together by a communication system. The set ofcomputers may include large-scale computers, medium scalecomputers, mini computers and micro computers. The set of terminalsmay include dumb terminals, intelligent terminals, workstations andmiscellaneous devices such as telephone lines, printers, etc. Thereis no specific definition of a computer network. However, Computernetworks increase the reliability of computer resources, facilitateoverall system development and also satisfy the primary objectiveof resource sharing such as device sharing, file sharing, programsharing and program segmentation.

Need and Scope of Networks:

File sharing

Print sharing

E- Mail

Fax- sharing

Remote access

Shared data base

Fault tolerance

Internet access and security

Communication & collaboration

Organization

Benefits of Network:

Improved Communication

Improved efficiency

Reduction in cost

Reduction in errors

Most commonly found networks:

_ Local Area Networks (LAN)

_ Metropolitan Area Networks (MAN)

_ Wide Area Networks (WAN)

_ Virtual Private Networks (VPN) Types of Network based onarea:

LAN: The local area network (LAN) evolved as a means tointerconnect a variety of computing platforms in a high-speed,reliable, and robust environment. First and foremost, a LAN is anetwork. Furthermore, the network serves as a communicationsinfrastructure for computerized systems. This includes computers,printers, storage systems, and any other device that has aprocessor, and an ability and need to communicate with anothersystem.

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The main attributes of present day LAN:- 1. Inexpensivetransmission media 2. Inexpensive media (modems, repeaters andTrans- receiver) to interface to the media. 3. Easy physicalconnection of devices to the media. 4. High data transmission rates5. A high degree of interconnection between devices 6. There isseldom central controlling processor which polls the attacheddevices on the

network.

A LAN should have the following requisite features: a.Compatibility b. Internetworking c. Growth Path and ModularityD.System Reliability and Maintenance Few reasons for growth of LANare mentioned as under:-

1. Security- Locking of servers by software and hardware means,security for programs and data files can be achieved. Disklessnodes also offer security by not allowing users to downloadimportant data on floppies or upload unwanted software ofviruses.

2. Expanded PC usage through inexpensive workstation- In a LANalready set up; cost to automate additional employees throughdiskless PCs is less.

3. Distributed processing- Many companies operate as if they haddistributed system in place. If numerous PCs are installed aroundthe office, these machines represent the basic platform for a LANwith inter user communication and information exchange.

4. Electronic mail and message broadcasting- Electronic mailallow users to communicate more easily among themselves. This canbe done by providing each user with a mailbox on the server.

5. Organizational benefits- Benefits of LANs are numerous. Theseinclude reduced costs in computer hardware, software andperipherals and a drastic reduction in the time and cost oftraining and re-training manpower to use the benefits.Communication is easier and faster. Information flow betweendepartments also becomes smoother.

6. Data management benefits- Data located on the central serverhence much easier to manage and back it up. No file is transferredto user through floppies.

7. Software cost and up- gradation Since a single server is usedhence the software is to be purchased only once instead of buyingmultiple copies, hence, resulting in reduced cost of software forevery machine in organization. Also up gradation is mucheasier.

Components of LAN are as follows- 1. File servers 2. The NetworkOperating system 3. Workstations 4. Network Interface cards 5.Network cabling Twisted pair, Coaxial cables, Fiber opticcables.

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Wireless LAN (WLANs)

Provides network connectivity over wireless media

An Access Point (AP) is installed to act as Bridge betweenWireless and Wired Network

The AP is connected to wired network and is equipped withantennae to provide wireless connectivity

WAN:

WAN is a digital communication system which interconnectsdifferent sites, computers and user terminals that are spread overa wide area (a state, a country or even world wide). It also helpsLANs to communicate with each other.

It covers a wider geographical area with various communicationfacilities such as long distance

telephone lines, satellite transmission and even under seacables. This type of communication network may be developed tooperate nationwide or worldwide.

It is a commercial data network that provides data communicationservices for business and

government agencies. Using WAN, users can send electronicmessages, data, graphics, programs, documents and

even video information rapidly and economically. Examples ofWANs are inter state banking networks, airline reservation systems,etc.

Characteristics of WAN: The transmitting and receivingequipments are distributed over wider geographical area.

The basic components in a common WAN include a host computerwhich is linked through

various inter connected communication lines to sub-ordinatecomputers. Communication channels between machines are usuallyfurnished (= provided by) by a third

party (for e.g.Telephone Company, public data network, satellitecarrier, etc.). WAN typically operate at lower link speeds (about 1MBPS)

Channels are of relatively low capacity.

Channels are relatively error prone (a bit error rate of 1 in105 bits transmitted)

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S.No Basic Local Area Network Wide Area Network

1 Coverage LAN covers limited geographical area

WAN covers wide geographical area

2 Control Mostly privately owned or owned by anyorganization.

No single owner of the WAN. Public authority involved for usingthe communication channel.

3 Communication mode

Direct cabling is the commonly used mode of communication inLAN

Communication mode of WAN is telecommunication cables,satellites or microwaves.

4 Cost Transmitting data is cheaper Transmitting data iscostlier

5 Application LAN can be set within the department or divisionthat belongs to the organization.

WAN can be set within the departments or division which aredispersed outside the range of locations.

6 Transmission Transmission of data generally error free

Transmission of data generally error prone.

MAN a. A Metropolitan Area Network (MAN) is somewhere between aLAN and a WAN. b. Sometimes MAN refers to a network that connectssystems or local area networks within a metropolitan area (roughly40 kms in length from one point to another). c. MANs are based onfiber optic transmission technology and provide high speedinterconnection between sites (10 MBPS or so). d. MAN can supportboth data and voice. e. Cable television networks are best examplesof MANs that distribute television signals.

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Network Models Anything that receives the data is called aclient. Anything that gives data is called a Host. A Server is thatfrom which work stations (clients) access date.

Client -Server Model

Organizations of individual computers on a network decides thathow they will interact with other computer on the network. One ofthe ways of organizing network of computers is the Client servertechnology.

In this kind of technology one host computer handles the otherconnected computer (nodes) on the network. This host computerserves the storage needs and sometimes the processing needs of allthe network nodes. This host computer is also called the server andthe connected nodes as clients.

The most common types of client server arrangement is a LANcomposed of micro computers connected to a network server whichserves to all the clients of the LAN.

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A Client program running on one of the micro computers canrequest specific data from server. The server program retrievesdata from its database and returns it to the client. This way a LANpermits all the computers connected to it to share hardware,software and data. Disk storage and printers are the most commonlyshared devices.

Characteristics of Client server technology:

1. Client/ server architecture consists of a client process anda server process that can be distinguished from each other.

2. The client portion and the server portions can operate onseparate platforms. 3. Either the client platform or the serverplatform can be upgraded without having to upgrade

the other platform. 4. The server is able to service multipleclients concurrently. 5. In some client/ server systems clients canaccess multiple servers. 6. The client server system includes somesort of networking capability 7. A vital portion of the applicationlogic resides at the client end. 8. The action is usually initiatedat the client end, not the server end. 9. The database servershould provide protection and security. 10. The GUI resides at theclient end.

Implementation examples of Client /Server technology:

Online banking application

Internet call centre application

Applications for end- users those are stored in the server.

E- Commerce online shopping page

Intranet applications

Financial, Inventory applications based on the client server

Tele communication based on Internet technologies. Benefits ofClient/ Server Technology

People in the field of information systems can use client/server computing to make their jobs easier.

Reduce the total cost of ownership

Increased productivity

End user productivity

Developer productivity

Takes less people to maintain a client/ server application thana mainframe.

The expenses of hardware and network in the client/ serverenvironment are less than those in the mainframe environment.

Can implement vendor software tools for each application.

Long term cost benefits for development and support.

The management control over the organization would beincreased.

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Components of Client server architecture 1. Client: Generallyclients are the personal computers. They are the users of theservices offered by the servers. Basically, there are 3 types ofclients. - Non-Graphical User Interface (GUI) clients: They requireminimum amount of human interaction. E.g. ATMs, cell phones, faxmachines, robots, etc. -GUI-Clients: They are human interactionmodels. They involve object/action models like pull-down menus inWindows. -Object-Oriented User Interface (OOUI) Clients: They havefurther expanded visual formats, multiple workplaces and objectinteraction rather than application interaction. 2. Server: Serversare the systems which provide required data to the clients. Serversreceive requests from the client and regulate access to sharedresources. Following are different types of servers. File servershelp to share files across a network by maintaining a sharedlibrary of documents,

data, and images. Database servers can execute Structured QueryLanguage (SQL) requests from clients. Transaction servers execute aseries of SQL commands, Web servers allow clients and servers tocommunicate with a universal language called HTTP.

3. Middleware: The network system implemented within theclient/server technology is known as middleware. Middleware is thesoftware needed to allow clients and servers to interact.Middleware allows for Communication, directory services, queuing,distributed file sharing and printing. Generally, middlewareconsists of four layers - Service, Back-end Processing, NetworkOperating System and Transport Stacks. 4. Fat-client or Fat-server:Fat-client and fat-server are popular terms in computer literature.In a fat-client system, most of the processing takes place in theclient. E.g. file server or database server. Fat-servers place moreemphasis on the server and try to minimize the processing done byclients. Examples of fat servers are transaction, GroupWare, andweb servers. Fat-clients are also known as 2-Tier systems andfat-servers are known as 3-Tier systems. 5. Network hardware: Thenetwork hardware is the communication cords and the devices thatlink the server and the clients. The communication and data flowover the network is managed and maintained by network software.Two- tier Architecture

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a. With the appearance of LAN, PCs are being connected toservers also. b. Servers are mainly database servers which offerdata on the server. c. Thus the application intelligence isimplemented on the client. d. Since there are only tiereddata-server and client, this is called 2-tier architecture. e. Thismodel is still predominant today, f. This is actually opposite tothe terminal based system where the entire intelligence lies on thehost system. Suitability: 2-tier architecture is suitable in thefollowing situations a. Tools and middleware implemented in clientare _ Relatively inexpensive and well integrated PC-tools and _Good Rapid Application Development (RAD) qualities i.e. simpleapplications can be produced in a comparatively short time. b.Server side uses relatively expensive tools. Limitations: a.Massive applications implemented on PC are expensive to maintain.b. Windows 3.X and Mac systems have tough resource restrictions.For this reason application programmers should be well trained insystems technology, so that they can optimize scarce resources. c.Since the application logic is executed on the PC, in case of a newsoftware release, all personal computers have to be updated. Thisis very costly, time consuming, complicated and error prone. d.Once the software reaches users desktop it should be installed andthen tested for correct execution. Due to distributed character ofsuch a procedure, there is no guarantee that all clients work onthe correct copy of the program.

Three Tier Architecture

In 3-tier architecture, application logic is transferred fromthe client back to the server. Tiers: It has the following tiers -a. Client tier: It is responsible for the presentation of data,receiving user events and controlling the user interface. _ Theactual business logic is moved to application server. Today, Javaapplets offer an alternative to traditionally written PCapplications.

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b. Application Server tier: This tier is new, i.e., it is notpresent in 2-tier architecture. Business objects that implement thebusiness rules live here, and are available to the client-tier.This level is the central key to solve 2-tier problems. It protectsthe data from direct access by the clients. Furthermore, the termcomponent is also found here. Today the term describes visualComponents on the client-side. Components on the server-side can bedefined as configurable objects, which can be put together to formnew application processes. c. Data-server-tier: This tier isresponsible for data storage: Besides the widespread relationaldatabase systems, existing legacy systems databases are also usedhere. Advantages: It solves number of problems that are inherent in2-tier architecture. a. Clear separation of user-interface-controland data presentation from application-logic: With this separationmore clients are able to have access to a wide variety of serverapplications. The two main advantages for client-application are _Quicker development _ Shorter test phase. b. Dynamic loadbalancing: If bottlenecks occur in terms of performance then theserver process can be moved to other servers at runtime. c. Changemanagement: _ It is easy and faster to exchange components on theserver than to provide number of PCs with new program versions. Inaddition such components require high standard of qualitycontrol.

Multi- tier Architecture

Meaning:

The client program has only UI code. The UI code talks to themiddle tier on which the business and database logic sits. In turnthe middle tier talks to the database. If necessary the middle tiercan be placed on the same machine as the database. In either casethe data traffic is highest between database logic and database.So, the network infrastructure that connects the database logicwith the database server needs have Very high bandwidth i.e.expensive. Advantages of Multi-tier architecture: The advantages ofa multi-tier architecture are: Forced separation of UI and businesslogic. Low bandwidth network. Business logic sits on a small numberof centralized machines.

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Comparison of Architectures

Architecture Advantage Disadvantage

One tier Simple

Very high performance

Self-contained

No networking can't access remote services

Potential for spaghetti code

Two tiers Clean, modular design

Less network traffic

Secure algorithms

Can separate UI form business logic

Must design/implement protocol

Must design/implement reliable data storage

Three tiers Can separate UI, logic, and storage

Reliable, replicable data

Concurrent data access via transactions

Efficient data access

Need to buy DB product

Need to hire DBA

Need to learn SQL

Object-relational mapping is difficult

N tiers Support multiple applications more easily

Common protocol/API

Less efficient

Must learn API (CORBA, RMI, etc.)

Expensive products

More complex, more faults

Load balancing is hard

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Peer to Peer Model

P2P refers to point-to-point communication among computers. In aP2P network, each computer acts as a peer, functions as both clientand server to the other peers on the network. The P2P model ofnetwork connection differs from the client-server model where datais transferred between clients and a central server. Examples ofservices that might be provided are file storage, printers,Internet access, etc

S.No Basis Client Server Peer- to- Peer

1 Meaning The network has certain machines and devices calledservers that are specially dedicated to provide various services tothe other computers called clients.

In this network, there are no dedicated servers. All computersare equal, and therefore termed as peer.

2 Arrangement The most common type of client server managementis LAN. It permits all the computers connected to it to sharehardware, software and data.

In peer- to- peer network, there is no central server to whichall the computers have to be connected. Linking computer in thisnetwork is significantly more straightforward.

3 Request of data In this network, a client program running onone of the micro computers can request specific data from theserver. The server program retrieves the requested data from itsdatabase and returns it to the client.

Since there is no server, all nodes are fully employed. Each ofthese machines functions both as a client and as a server.

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Communication software Management of flow of data across anetwork is done by communication software. This software is writtenin a wide variety of protocols which are rules and procedures ofexchanging data. Functions of a communication SW:

Access Control 1. Linking and disconnecting a connection byautomatic dialing, answering telephones, redialing, etc.

2. Also restricts access to authorized users only.

Network management 1. Determines system priorities, routemessages, queuing input and output.

Data and file Transmission control

Allows computers to send and receive messages, commands, data,files, etc. Provides mechanism of error checking.

Error detection and Control Involves detection and correction oftransmission error caused by distortions in the communicationchannel such as noise, power surge, etc

Data security Protects from unauthorized access. Encryption isalso used.

Communication Interface Devices NIC cards: A network interfacecard is needed for any type of network connection. This includes aconnection to the Internet. Even if a small network of twocomputers is created, the administrator is required to installnetwork interface cards onto the computers. The card then attachesto cabling that is also connected to a router. This is the centralpoint for network connectivity, and it allows the two computers toshare files and data. MAC address: Media access control address isa HW that addresses that uniquely identifies each node of anetwork. Routers: A device that forwards data packets along anetwork. A Router is connected to at least two networks. Routersare located at gateways, the places where two or more networkconnect.

Switches: A Device that filters and forwards packets between LANsegments.

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Hubs: A common connection point for devices in a network. Hubsare used to connect segments of a LAN. A hub contains multipleports. When a packet arrives at one port it is copied to otherports so that all the other segments of LAN can see all packets.Bridges: Bridges determine the physical (MAC) address of thedestination of a packet and will then rebroadcast the signal onlyif it resides on the other segment, there by reducing overallnetwork traffic.

Repeaters: A repeater connects two segments of your networkcable. It retimes and regenerates the signals to proper amplitudesand sends them to the other segments. Repeaters require a smallamount of time to regenerate the signal. This can cause apropagation delay which can affect network communication when thereare several repeaters in a row. Many network architectures limitthe number of repeaters that can be used in a row.

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Gateways: A node on the network that serves as an entrance toanother network. In enterprises the gateway is the computer thatroutes the traffic from a work station to the outside network thatis serving the web pages. In home, the gateway is the ISP (Internetservice provides say, BSNL, Airtel, etc) MODEM: The word "modem" isa contraction of the words modulator-demodulator. A modem istypically used to send digital data over a phone line. The sendingmodem modulates the data into a signal that is compatible with thephone line, and the receiving modem demodulates the signal backinto digital data. Wireless modems convert digital data into radiosignals and back.

Multiplexing: is the sharing of bandwidth between multipleusers. Front end communication processor: These are programmabledevices which control the functions of communication system. Theysupport the operations of a mainframe computer by performingfunctions, which it would otherwise be required to perform itself.These functions include code conversion, editing and verificationof data, terminal recognition and control of transmission lines.Protocol converters: Dissimilar devices cannot communicate witheach other unless a strict set of communication standards isfollowed. Such standards are commonly referred to as protocols. Aprotocol is a set of rules required to initiate and maintaincommunication between a sender and receiver device. Remote accessdevices: Are modem banks that serve as gateways to the internet orto private corporate networks. Their function is to properly routeall incoming and outgoing connections.

Repeater

Weak

inbound

digital signal

Strong

outbound

digital signal

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Communication channels Guided media: Fiber optic cables

Fiber optics (optical fibers) are long, thin strands of verypure glass about the diameter of a human hair. They are arranged inbundles called optical cables and used to transmit light signalsover long distances. If you look closely at a single optical fiber,you will see that it has the following parts:

Core - Thin glass center of the fiber where the light travelsCladding - Outer optical material surrounding the core thatreflects the light back into the

core Buffer coating - Plastic coating that protects the fiberfrom damage and moisture

Hundreds or thousands of these optical fibers are arranged inbundles in optical cables. The bundles are protected by the cable'souter covering, called a jacket. Twisted pair It consists of pairsof cables that transmit data. The pairs of cables are twisted so asto prevent the cross talk (noise generated by other pairs ofcables). Each pair of copper wire is twisted together and isencased in a unique color-coded plastic jacket for insulation. Anouter jacket is put encasing all the twisted pair into one singlebundle that consists of all the pairs for effective transmission ofdata.

Coaxial cables A coaxial cable is one that consists of twoconductors that share a common axis. The inner conductor istypically a straight wire, either solid or stranded and the outerconductor is typically a shield that might be braided or afoil.

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Media Network type

Cost Transmission distance

Security Error rates

Speed

Twisted pair

LAN Low Short Good Low Low- high

Coaxial LAN Moderate Short Good Low Low- High

Fiber Optics

Any High Moderate- long

Very good Very low High- very high

Network structure or topology Network topology is thegeometrical arrangement of computer resources, remote devices andcommunication facilities. It comprises of nodes and links. A nodeis the end point of any branch in a computer, a terminal device, aworkstation or an inter connected equipment facility. A Link is acommunication path between two nodes. Basic network topologies arediscussed as under:

1. Star Network: In this network the communication channelemanates from a central computer system. It means the processingnodes are directly connected with a central system. To transmitinformation from one node to another, it should be necessarilyrouted through the central computer.

Appropriate for: Banking companies.

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Advantages: Easily expandable and shortened as it is easy to addor remove devices. Any single node failure does not bring down theentire network. Connection of additional computers does notincrease the communication time between any

computers. It is easier to diagnose network problems through acentral hub. Disadvantages: Processing problem in the centralcomputer (o r hub) can paralyze the entire system. It costs more tocable a star configuration than other topologies. Maximum number ofnodes expected depends upon the limits of expansion permitted bythe

hub. 2. Bus Network: In this structure a single cable runs inthe building or campus and all nodes are

linked along with this communication line with two end pointscalled the bus. Most appropriate for: LAN Advantages: Reliable invery small networks as well as easy to use and understand. Highlyreliable since any line break down does not affect thecommunication between

two computers. Requires least amount of cable to connect andtherefore less expensive. Easy to extend by joining cable withconnector or repeater.

Disadvantages:

Heavy network traffic slows down the transmission rate. only asingle message can travel at a time Each connection between twocables weakens the electrical signal. Troubleshooting is verydifficult in bus configuration.

3. Ring Network: In this network cable passes from one node toanother node until all nodes are

connected in the form of loop or ring. Suitable for: LANAdvantages: Offers high performance for small number of workstations or for longer networks where

each station has a similar workload. Can span longer distancethan other type of networks. Reliable since communication betweentwo computers is not dependent on a single

computer like the host computer. Ring networks are easilyextendable. Disadvantages: Relatively expensive and difficult toinstall Failure of one computer on the network can affect theentire network. Difficult to troubleshoot a ring network Adding orremoving computer can disrupt the entire network.

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4. Mesh Network: Nodes under this network are randomly connectedusing communication

lines. Mesh network can be fully connected or partiallyconnected. In fully connected topology, each node is connected by adedicated point to point link to every node. In a partiallyconnected topology , the computers are widely scattered.

Advantages: Reliability is very high since there exist analternative path if the direct link between

two nodes is down. It is easier to diagnose the network problemsYields greater amount of redundancy in the event that one of thenodes fails where

network traffic can be redirected to another node.Disadvantages:

The cost of installation and maintenance is high If there arefew cables in the network, the loss of even one cable or devicemay

damage the network seriously. Transmission technologies

I. Serial and Parallel transmission

S.No Serial Parallel

1 In this, the data bits are transmitted serially one afteranother.

In this, the data bits are transmitted simultaneously.

2 Data is transmitted over a single wire. Data is transmittedover 8 different wires.

3 It is cheaper mode of transferring data.

Relatively expensive

4 Applicable for long distance data transmissions.

Not practical for long distance communications as it usesparallel path, so cross talk may occur.

5 Relatively slower Relatively faster.

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II. Asynchronous transmission and Synchronous transmission

S.No Basis Synchronous transmission

Asynchronous transmission

1 Transmission of bits

In this transmission bits are transferred at fixed rate

In this, transmission takes place in form of words accompaniedby stop and start bits

2 Character transmission

It allows characters to be sent down the line without start/stop bits

In this words are send with a start bit and a stop bit

3 Transfer rate Rate of transfer is high or faster Rate oftransfer is slower

4 Smart devices Smart devices are required to differentiatebetween the actual date and special synchronous characters

It does not require special smart devices for differentiation ofactual data and start/ stop bits.

5 Synchronization bit

Uses a group of synchronization bits

No special synchronization bits required.

6 Word block It allows data to be sent as multi word block

Data transfer takes place in the form of words.

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Transmission modes

a) Simplex b) Half duplex c) Full duplex

Data Transmission Techniques

Circuit Switching: In a communication using circuit switching,there is a dedicated communication path between two stations. Thepath is a connected sequence of links between nodes. On eachphysical link, a channel is dedicated to the connection. Message intext is not transferable. Example: Telephone network for continuousflow of data.

Message Switching: It is a system which uses computer techniquesto transmit and receive

path, and store retrieve textual information. No need toestablish a dedicated path between two stations. Under this type ofswitching the computer receives data, stores it and whencommunication channel is available transfers the information to thedestination. Example: Used in companies with huge volume of datatransmission as telegrams, electronic mails, computer files andtransaction queries and responses.

Packet switching: In this type of switching the message to betransmitted is broken into

chunks called packets. Each packet is has the source address anddestination place available. Each packet has synchronizing errorcorrection and control bits. Packet switching maximizestransmission capacity of networks. Transmission cost is by packetand not by message, route of distance.

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Communication protocols Communication protocols are a set ofrules for inter- computer communication that have been agreed uponand implemented by many vendors, user and standard bodies. Ideally,a protocol, standard allows heterogeneous computer to talk to eachother. Protocols are software that performs a variety of actionsnecessary for data transmission between computers. At the mostbasic level, protocols define the physical aspects ofcommunication, such as how the system components will be interfacedand at what voltage levels will be transmitted. In datatransmission establishment and termination of sessions betweencomputers and the synchronization of those transmission takesplace. A protocol defines the following three aspects of digitalcommunication.

1. syntax: The format of data being exchanged, character setused, types of error correction used, type of encoding schemaused.

2. Semantics: For reliable and error free transmission, type andorder of messages used. 3. Timing: Defines data rate selection andcorrect timing for various events during data

transfer. A number of different protocols are sued commonly.E.g.,X.12, X.75, TCP/IP, etc OSI: Open System Interconnection ModelAll People Seem To Need Data Processing Please Do Not Throw SweetPongal Away

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Physical Layer: The physical layer is at the bottom of this datanetworking model. It deals with crude data that is in the form ofelectrical signals. The data bits are sent as 0's and 1's. 0'scorrespond to low voltage signals and 1's correspond to highvoltage signals. The mechanical aspects of communication, such aswires or connectors come under this layer. The physical layer alsodeals with how these wires, connectors, and voltage electricalsignals work. Also, the process that is required for these physicalaspects are taken into account in this layer itself. Read more onEthernet cable wiring. The Data Link Layer: The transmission of thedata over the communication medium is the responsibility of thislayer. The 0's and 1's that are used in the communication aregrouped into logical encapsulation. This encapsulation is calledframes. The data is transported in frames. The responsibility ofthese frames is that of the data link layer. Network Layer: Allover the world, there are many different types of Ethernets. Thesenetworks are connected to each other through various media. When adata packet wants to reach a particular destination, it has totraverse through these networks. Essentially, there are lot ofoperations that are taking place between the connected networks.Also, the packet data which is traversing has to choose an optimumroute, and the addressing of these packets has to be proper. Thevarious operations between the networks, packet data issues,addressing and routing are handled by this network layer. TransportLayer: The transport layer ensures quality and reliability of thecommunication. The data packet switching is entirely handled by thetransport layer. There are basically two types of packet switching.They are connectionless packet switching and connection orientedpacket switching. In connectionless packet switching, the packetdata is allowed to choose the route in which it is going to reachthe destination. Obviously, the packet in itself cant do this.Physical devices like routers are mainly responsible for thebehavior of packets, but the packets formed from the same datum canreach their destination in different ways. Whereas, in connectionoriented packet switching, once the route is decided, then all thepackets have to follow the same route. Examples of connectionlesspacket switching are text messages in mobile phones, and theexample of connection oriented switching is a direct voice call.The Sessions Layer: The sessions layer is mainly responsible forcreating, maintaining and destroying the communication link. PDU(Protocol Data Unit), in which various protocols are defined, thathave to be followed during communication, are the responsibility ofthe sessions layer. The applications that use RPC's (remoteprocedure calls) are taken care of by the sessions layer.Presentation Layer: There are various techniques of datacompression which are used to send and receive the optimized data.For example, if certain data is repeating itself for a number oftimes, then it is logical to send the data only once, and specifythe number of times it is repeated. This bundling of the repeateddata is one of the techniques of compressions. The compression anddecompression of the data is handled by the presentation layer.Also, encryption and decryption techniques used to thwart maliciousattacks on data are handled by the presentation layer. ApplicationLayer: This is the topmost layer of the OSI reference model. Thislayer comes into picture when there is a process to processcommunication. Whenever a user invokes any application, all the

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associated processes are run. Many a times, when an applicationwants to communicate with another application, then there has to becommunication between these associated processes. The applicationlayer is responsible for this inter process communication.

Sr. No. OSI Reference Model TCP/IP Reference Model

1

Model was first defined before implementation takes place

Model was defined after protocols were implemented

2 Reliable delivery of packet data

Service interface and protocols were not clearly distinguishedbefore

3 Internet working not supported

TCP/IP supports Internet working

4 Strict Layered Loosely layered

Working of the OSI

The layers are in two groups. The upper four layers are usedwhenever a message passes from or to a

user. The lower three layers are used when any message passesthrough the host computer.

Messages intended for this computer pass to the upper layers.Messages destined for some other

host are not passed up to the upper layers but are forwarded toanother host.

The sending process passes data to the application layer. Theapplication layer attaches an

application header and then passes the frame to the presentationlayer.

The presentation layer can transform data in various ways, ifnecessary, such as by translating it and

adding a header. It gives the result to the session layer. Thepresentation layer is not aware of which

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portion (if any) of the data received from the application layeris the application header and which

portion is actually user data, because that information isirrelevant to the presentation layers role.

The process of adding headers is repeated from layer to layeruntil the frame reaches the data link

layer. There, in addition to a data-link header, a data-linktrailer is added. The data-link trailer

contains a checksum and padding if needed. This aids in framesynchronization. The frame is passed

down to the physical layer, where it is transmitted to thereceiving host. On the receiving host, the

various headers and the data trailer are stripped off one by oneas the frame ascends the layers and

finally reaches the receiving process.

Virtual Private Network

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A VPN is a private network that uses a public network (usuallyInternet) to connect remote sites or users together. It will notuse a dedicated, real-world connection such as leased line. It uses"virtual" connections routed through the Internet from thecompany's private network to the remote site or employee. Types ofVPN: Following are the 2 most common types of VPN 1. Remote-access:Meaning: This is a user-to-LAN connection used by a company thatneeds to connect to the private network from various remotelocations. It is also called as Virtual Private Dial-Up Network(VPDN). Working: Generally, a company that wishes to set up a largeremote-access VPN will outsource it to an Enterprise ServiceProvider (ESP). The ESP sets up a Network Access Server (NAS) andprovides desktop client software to all the computers of remoteusers. Employees can dial a toll-free number to reach the NAS anduse their VPN client software to access the corporate network. Theyprovide secure, encrypted connections between a company's privatenetwork and remote users through a third-party service provider.Suitability: It is suitable for a company with large number ofemployees spread over wide area. For example a company withhundreds of sales people in the field needs a remote-access VPN. 2.Site-to-Site VPN: Through the use of dedicated equipment andlarge-scale encryption, a company can connect multiple fixed sitesover a public network such as Internet. Site-to-site VPNs can beone of the following two types: a. Intranet based: If a company hasone or more remote locations that they wish to join in a singleprivate network, they can create an intranet VPN to connect LAN toLAN. b. Extranet based: When a company has a close relationshipwith another company (for example, a partner, supplier orcustomer), they can build an extranet VPN that connects LAN to LAN.With this, various companies can work in a shared environment.ISDN: Integrated Services Digital Network 1. Meaning: a. IntegratedServices Digital Network (ISDN) is a circuit switched telephonenetwork system, designed to allow digital transmission of voice anddata over ordinary telephone copper wires, resulting in betterquality and higher speeds. b. ISDN is a set of protocols forestablishing and breaking circuit switched connections and foradvanced call features for the user. 2. Types of channel: a. Bearerchannels (B channels): Data and voice are carried by these channelshaving a bandwidth of

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64 kilobits per second. b. Delta channels (D channels): Itcarries signals and controls. But it can also be used for carryingdata. 3. Types of ISDN services: a. Basic Rate Interface (BRI): Itconsists of two 64 kbps B channels and one 16 kbps D channel toform a total of 144 kbps. It is suitable for individual users. b.Primary Rate Interface (PRI): It consists of 23 B channels and one64 kbps D channel to form a total of 1536 kbps. It is suitable forusers with higher capacity requirements. 4. Advantages: a. Multiplechannel operation in same cable: ISDN allows multiple digitalchannels to be operated simultaneously through the same regularphone cable which is meant for analog signals. However, this ispossible only if the telephone companys switches can supportdigital connections. b. Combines different digital sources: WithISDN, it is possible to combine several digital data sources andsend the information to the proper destination. In a digital lineit is easy to reduce noise and interference even after combiningthese signals. Data Centre

A data centre is a centralized repository for the storage,management and dissemination of data and information. Data centrescan be defined as highly secure fault- resistant facilities,hosting customer equipment that connects to telecommunicationnetwork. The other names are Internet hotel, server farm, datawarehouse, corporate data centre, Internet service provider (ISP),or wireless application server provider (WASP). The purpose of datacentre is to provide space and bandwidth connectivity for serversin reliable, secure and scalable environment. These data centresare also referred to as public data centres, because they are opento customers. These facilities can accommodate thousands ofservers, switches, routers and racks storage array and othertelecom equipment.

Types of data centres

1. Private data centre and Public data centre Private datacentre: Also called as enterprise data centre, this is managed byorganizations own IT department and it provides applications,storages, web- hosting, e-business functions, etc. Public datacentre: also called Internet data centre, provides services rangingfrom equipment managing to managed web- hosting.

2. Tier 1 and Tier 4 -Tier 1 is the most basic and inexpensiveand Tier 4 is costly with more facilities.

Features of Data Centres

Size

Data security

Availability of data

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Electrical and power systems

System monitoring and support and storage. Data Centres can alsoprovide VALUE- ADDED services such as:

Database monitoring

Web monitoring

Backup and restore

Intrusion detection system

Storage on demand. Disaster Recovery Plan Data centres need tobe equipped with appropriate disaster recovery systems, this is aplan made by the organization before purchasing the computer. Thisplan ensures the management that, in case of disasters, therecovery is possible through certain procedures and plans. Whendisaster occurs, the IT organizations cannot stop their processorsfor a long time, and cannot tolerate too much downtime. Hence everyorganization goes for its own disaster recovery sites. There arethree types of sites available: Hot site- all the facilities, likekey sites are available, very expensive, hardware, software,equipment are available. Warm site- Partially equipped, with somemore facilities the processing can be resumed. Cold site- only thespace is available with air- conditioning, hardware, software,equipments are not available, making the required facilities, andthe processing can be resumed. Disaster Events

1. There is a potential for significantly interrupting normalbusiness processing. 2. Business is associated with naturaldisasters like earthquake, flood, tornadoes,

thunderstorm, fire, etc. 3. Disasters are disruptions causingthe entire facility to be inoperative for a lengthy

period of time. 4. Catastrophes are disruptions resulting fromdisruption of processing facility.

Business continuity Plan (BCP) is documented description ofaction, resources and procedures to be followed, before during andafter the disaster occurs. Components of BCPs

1. Define the requirements based on business needs. 2.Statements of critical resources needed and detailed planning onuse of critical resources. 3. Defined responsibilities of trainedpersonnel and written documentation and procedures to

cover all operations. 4. Commitment to maintain plan to keep upwith changes.

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There are different phases in BCP: Phase I: Risk analysis,identifying the critical and non critical areas, determining thecritical time period, coverage of insurance. Phase II:Determination of minimum resources necessary, reviewing theoperations between current policies and back up procedures. Reviewof the entire system like file backups, operation anddocumentations. Phase III: Identifying the alternative sites,reciprocal arrangements, preparing a list of alternatives, visitsand reviews. Phase IV: Plan preparation, provision for manualprocesses, plan development, team building, developing a generalplan. Phase V: Testing the plans, structured walk- through (papertest), localized test, fully operational test, test reviewprocedures, etc. Virus: A virus is a malicious program thatattaches itself to a legitimate program and penetrates into theoperating system. A virus is a program that instructs the operatingsystem to append it to other programs and thus propagates to otherprograms via files containing macros. A virus can replicate itselfover and over and the replicated modules can grow independent ofthe initial virus. Sometimes a virus can be benevolent and it maycause minor disruptions by printing laughing message and sometimesa virus can be malignant and it may delete files or corrupt otherprograms. Following are the controls to safeguard against theviruses: a. Preventive controls like: _ using only clean andlicensed copies of software, _ cutting the use of pubic domainsoftware / shareware, _ downloading files or software only fromreliable websites, _ implementing read-only access to software. _checking new files / software with anti-virus software beforeinstallation, _ imparting education and training programs to endusers b. Detective controls like: _ regularly running antivirussoftware, _ file size comparison to observe whether the size ofprograms has changed, _ date / time comparisons to detect anyunauthorized modifications. c. Corrective controls like: _maintaining a clean backup, _ having a recovery plan from virusinfections, _ regularly running antivirus software (which is usefulfor both detection & removal of virus)

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Worms _ A computer worm is a self-replicating computer program._ Worms are similar to viruses but they exist as separate andindependent programs. _ It uses a network to send copies of itselfto other nodes (computer terminals on the network) and it may do sowithout any user intervention. _ Unlike a virus, worms do not needto attach itself to an existing program. _ Worms may cause harm tothe network. For e.g. they may consume bandwidth. _ They exploitsecurity weaknesses / bugs in the operating system to penetrateinto other systems. _ Exposures that arise from worms are moredifficult to control than that arise from virus. _ Generally antivirus / anti spyware software can prevent the attacks from Worms.Hackers _ Hackers attempt to gain unauthorized entry into a systemby circumventing the access control mechanism of the system. Theycan do this either with good or bad intention. _ Some hackers mayjust trespass and read the files without making any changes tothem. _ Some hackers may cause destruction by deleting criticalfiles, disrupting / suspending operations, stealing sensitive dataand / or programs. _ They can be avoided only through robustlogical access controls and / or Cyber Laws of the Land. IntrusionDetection Systems (IDS) Meaning: The goal of intrusion detectionsystem is to monitor the network assets, to detect anomalousbehavior and misuse. This concept is there for nearly twenty years.But in recent years there is a dramatic rise in its popularity andit is being incorporated into the overall information securityinfrastructure. Following are different components of IDS:Components of IDS: 1. Network Intrusion Detection (NID): _ _Network intrusion detection deals with information passing in thewire between hosts. _ Typically referred to as "packet-sniffers," _Network intrusion detection devices intercept packets traveling indifferent communication channels. Once captured, the packets areanalyzed in a number of different ways. _ Some NID devices willsimply compare the packet with a database consisting of knownattacks and malicious packet "fingerprints", _ While others willlook for anomalous (= abnormal) packet activity that might indicatemalicious behavior. _ In either case, network intrusion detectionshould be treated as a perimeter defense. 2. Host-based IntrusionDetection (HID): _ _ Host-based intrusion detection systems aredesigned to monitor, detect and respond to user and system activityand attacks on a given host. _ Some more robust tools also offeraudit policy management and centralization, supply data forensics,statistical analysis and evidentiary support, and in some casesprovide some measure of access control. _ The difference betweenhost-based and network-based intrusion detection is that NID dealswith data transmitted from host to host while HID is concerned withwhat occurs on the hosts themselves.

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_ Host-based intrusion detection is best suited to combat (=fight against) internal threats. _ Majority of computer threatscome from within organization. For example, disgruntled employees,corporate spies, etc. 3. Hybrid Intrusion Detection: _ Hybridintrusion detection systems offer management of and alertnotification from both network and host-based intrusion detectiondevices. _ Hybrid solutions provide the logical complement to NIDand HID i.e. Central Intrusion Detection Management. 4.Network-Node Intrusion Detection (NNID): _ _ Network-node intrusiondetection was developed to work around the inherent flaws intraditional NID. _ Network-node pulls the packet-interceptingtechnology from the wire and puts it on the host. _ With NNID, the"packet-sniffer" is positioned in such a way that it capturespackets after they reach their final destination i.e. host. _ Thepacket is then analyzed just as if it were traveling along thenetwork through a conventional "packet-sniffer." _ In thisapproach, network-node is simply another module that can attach tothe HID agent. The major disadvantage is that it only evaluatespackets addressed to the host on which it resides. _ On the otherhand, traditional network intrusion detection can monitor packetson an entire subnet. _ "packet-sniffers" are also incapable ofviewing a complete subnet when the network uses high-speedcommunications, encryption or switches since they are essentially"without a sense of smell". _ But NNID can defend the specifichosts against packet-based attacks in these complex environmentswhere conventional NID is ineffective.