
The main attributes of present day LAN:- 1. Inexpensive transmission media 2. Inexpensive media (modems, repeaters and Trans- receiver) to interface to the media. 3. Easy physical connection of devices to the media. 4. High data transmission rates 5. A high degree of interconnection between devices 6. There is seldom central controlling processor which polls the attached devices on the

network.

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

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

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

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

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

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

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

7. Software cost and up- gradation Since a single server is used hence the software is to be purchased only once instead of buying multiple copies, hence, resulting in reduced cost of software for every machine in organization. Also up gradation is much easier.

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


Wireless LAN (WLANs)

Provides network connectivity over wireless media

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

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

WAN:

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

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

telephone lines, satellite transmission and even under sea cables. This type of communication network may be developed to operate nationwide or worldwide.

It is a commercial data network that provides data communication services for business and

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

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

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

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

various inter connected communication lines to sub-ordinate computers. Communication channels between machines are usually furnished (= provided by) by a third

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

Channels are of relatively low capacity.

Channels are relatively error prone (a bit error rate of 1 in 105 bits transmitted)


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 any organization.

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

3 Communication mode

Direct cabling is the commonly used mode of communication in LAN

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

4 Cost Transmitting data is cheaper Transmitting data is costlier

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

WAN can be set within the departments or division which are dispersed 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 a LAN and a WAN. b. Sometimes MAN refers to a network that connects systems or local area networks within a metropolitan area (roughly 40 kms in length from one point to another). c. MANs are based on fiber optic transmission technology and provide high speed interconnection between sites (10 MBPS or so). d. MAN can support both data and voice. e. Cable television networks are best examples of MANs that distribute television signals.


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

Client -Server Model

Organizations of individual computers on a network decides that how they will interact with other computer on the network. One of the ways of organizing network of computers is the Client server technology.

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

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


A Client program running on one of the micro computers can request specific data from server. The server program retrieves data from its database and returns it to the client. This way a LAN permits all the computers connected to it to share hardware, software and data. Disk storage and printers are the most commonly shared devices.

Characteristics of Client server technology:

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

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

the other platform. 4. The server is able to service multiple clients concurrently. 5. In some client/ server systems clients can access multiple servers. 6. The client server system includes some sort of networking capability 7. A vital portion of the application logic resides at the client end. 8. The action is usually initiated at the client end, not the server end. 9. The database server should provide protection and security. 10. The GUI resides at the client 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 of Client/ 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 than a mainframe.

The expenses of hardware and network in the client/ server environment 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 be increased.


Components of Client server architecture 1. Client: Generally clients are the personal computers. They are the users of the services offered by the servers. Basically, there are 3 types of clients. - Non-Graphical User Interface (GUI) clients: They require minimum amount of human interaction. E.g. ATMs, cell phones, fax machines, robots, etc. -GUI-Clients: They are human interaction models. They involve object/action models like pull-down menus in Windows. -Object-Oriented User Interface (OOUI) Clients: They have further expanded visual formats, multiple workplaces and object interaction rather than application interaction. 2. Server: Servers are the systems which provide required data to the clients. Servers receive requests from the client and regulate access to shared resources. Following are different types of servers. File servers help to share files across a network by maintaining a shared library of documents,

data, and images. Database servers can execute Structured Query Language (SQL) requests from clients. Transaction servers execute a series of SQL commands, Web servers allow clients and servers to communicate with a universal language called HTTP.

3. Middleware: The network system implemented within the client/server technology is known as middleware. Middleware is the software needed to allow clients and servers to interact. Middleware allows for Communication, directory services, queuing, distributed file sharing and printing. Generally, middleware consists of four layers - Service, Back-end Processing, Network Operating 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 the client. E.g. file server or database server. Fat-servers place more emphasis on the server and try to minimize the processing done by clients. Examples of fat servers are transaction, GroupWare, and web servers. Fat-clients are also known as 2-Tier systems and fat-servers are known as 3-Tier systems. 5. Network hardware: The network hardware is the communication cords and the devices that link the server and the clients. The communication and data flow over the network is managed and maintained by network software. Two- tier Architecture


a. With the appearance of LAN, PCs are being connected to servers also. b. Servers are mainly database servers which offer data on the server. c. Thus the application intelligence is implemented on the client. d. Since there are only tiered data-server and client, this is called 2-tier architecture. e. This model is still predominant today, f. This is actually opposite to the terminal based system where the entire intelligence lies on the host system. Suitability: 2-tier architecture is suitable in the following situations a. Tools and middleware implemented in client are _ Relatively inexpensive and well integrated PC-tools and _ Good Rapid Application Development (RAD) qualities i.e. simple applications 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 in systems technology, so that they can optimize scarce resources. c. Since the application logic is executed on the PC, in case of a new software release, all personal computers have to be updated. This is very costly, time consuming, complicated and error prone. d. Once the software reaches users desktop it should be installed and then tested for correct execution. Due to distributed character of such a procedure, there is no guarantee that all clients work on the correct copy of the program.

Three Tier Architecture

In 3-tier architecture, application logic is transferred from the 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. _ The actual business logic is moved to application server. Today, Java applets offer an alternative to traditionally written PC applications.


b. Application Server tier: This tier is new, i.e., it is not present in 2-tier architecture. Business objects that implement the business rules live here, and are available to the client-tier. This level is the central key to solve 2-tier problems. It protects the data from direct access by the clients. Furthermore, the term component is also found here. Today the term describes visual Components on the client-side. Components on the server-side can be defined as configurable objects, which can be put together to form new application processes. c. Data-server-tier: This tier is responsible for data storage: Besides the widespread relational database systems, existing legacy systems databases are also used here. Advantages: It solves number of problems that are inherent in 2-tier architecture. a. Clear separation of user-interface-control and data presentation from application-logic: With this separation more clients are able to have access to a wide variety of server applications. The two main advantages for client-application are _ Quicker development _ Shorter test phase. b. Dynamic load balancing: If bottlenecks occur in terms of performance then the server process can be moved to other servers at runtime. c. Change management: _ It is easy and faster to exchange components on the server than to provide number of PCs with new program versions. In addition such components require high standard of quality control.

Multi- tier Architecture

Meaning:

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


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


Peer to Peer Model

P2P refers to point-to-point communication among computers. In a P2P network, each computer acts as a peer, functions as both client and server to the other peers on the network. The P2P model of network connection differs from the client-server model where data is transferred between clients and a central server. Examples of services 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 called servers that are specially dedicated to provide various services to the other computers called clients.

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

2 Arrangement The most common type of client server management is LAN. It permits all the computers connected to it to share hardware, software and data.

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

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

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


Communication software Management of flow of data across a network is done by communication software. This software is written in a wide variety of protocols which are rules and procedures of exchanging data. Functions of a communication SW:

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

2. Also restricts access to authorized users only.

Network management 1. Determines system priorities, route messages, 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 of transmission error caused by distortions in the communication channel such as noise, power surge, etc

Data security Protects from unauthorized access. Encryption is also used.

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

Switches: A Device that filters and forwards packets between LAN segments.


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

Repeaters: A repeater connects two segments of your network cable. It retimes and regenerates the signals to proper amplitudes and sends them to the other segments. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row.


Gateways: A node on the network that serves as an entrance to another network. In enterprises the gateway is the computer that routes the traffic from a work station to the outside network that is serving the web pages. In home, the gateway is the ISP (Internet service provides say, BSNL, Airtel, etc) MODEM: The word "modem" is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line. The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems convert digital data into radio signals and back.

Multiplexing: is the sharing of bandwidth between multiple users. Front end communication processor: These are programmable devices which control the functions of communication system. They support the operations of a mainframe computer by performing functions, which it would otherwise be required to perform itself. These functions include code conversion, editing and verification of data, terminal recognition and control of transmission lines. Protocol converters: Dissimilar devices cannot communicate with each other unless a strict set of communication standards is followed. Such standards are commonly referred to as protocols. A protocol is a set of rules required to initiate and maintain communication between a sender and receiver device. Remote access devices: Are modem banks that serve as gateways to the internet or to private corporate networks. Their function is to properly route all incoming and outgoing connections.

Repeater

Weak

inbound

digital signal

Strong

outbound

digital signal


Communication channels Guided media: Fiber optic cables

Fiber optics (optical fibers) are long, thin strands of very pure glass about the diameter of a human hair. They are arranged in bundles called optical cables and used to transmit light signals over 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 travels Cladding - Outer optical material surrounding the core that reflects the light back into the

core Buffer coating - Plastic coating that protects the fiber from damage and moisture

Hundreds or thousands of these optical fibers are arranged in bundles in optical cables. The bundles are protected by the cable's outer covering, called a jacket. Twisted pair It consists of pairs of cables that transmit data. The pairs of cables are twisted so as to prevent the cross talk (noise generated by other pairs of cables). Each pair of copper wire is twisted together and is encased in a unique color-coded plastic jacket for insulation. An outer jacket is put encasing all the twisted pair into one single bundle that consists of all the pairs for effective transmission of data.

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


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 the geometrical arrangement of computer resources, remote devices and communication facilities. It comprises of nodes and links. A node is the end point of any branch in a computer, a terminal device, a workstation or an inter connected equipment facility. A Link is a communication path between two nodes. Basic network topologies are discussed as under:

1. Star Network: In this network the communication channel emanates from a central computer system. It means the processing nodes are directly connected with a central system. To transmit information from one node to another, it should be necessarily routed through the central computer.

Appropriate for: Banking companies.


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

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

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

linked along with this communication line with two end points called the bus. Most appropriate for: LAN Advantages: Reliable in very small networks as well as easy to use and understand. Highly reliable since any line break down does not affect the communication between

two computers. Requires least amount of cable to connect and therefore less expensive. Easy to extend by joining cable with connector or repeater.

Disadvantages:

Heavy network traffic slows down the transmission rate. only a single message can travel at a time Each connection between two cables weakens the electrical signal. Troubleshooting is very difficult in bus configuration.

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

connected in the form of loop or ring. Suitable for: LAN Advantages: Offers high performance for small number of work stations or for longer networks where

each station has a similar workload. Can span longer distance than other type of networks. Reliable since communication between two computers is not dependent on a single

computer like the host computer. Ring networks are easily extendable. Disadvantages: Relatively expensive and difficult to install Failure of one computer on the network can affect the entire network. Difficult to troubleshoot a ring network Adding or removing computer can disrupt the entire network.


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

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

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

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

network traffic can be redirected to another node. Disadvantages:

The cost of installation and maintenance is high If there are few cables in the network, the loss of even one cable or device may

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 after another.

In this, the data bits are transmitted simultaneously.

2 Data is transmitted over a single wire. Data is transmitted over 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 uses parallel path, so cross talk may occur.

5 Relatively slower Relatively faster.


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 accompanied by 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 of transfer is slower

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

It does not require special smart devices for differentiation of actual 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.


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. The path is a connected sequence of links between nodes. On each physical link, a channel is dedicated to the connection. Message in text is not transferable. Example: Telephone network for continuous flow of data.

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

path, and store retrieve textual information. No need to establish a dedicated path between two stations. Under this type of switching the computer receives data, stores it and when communication channel is available transfers the information to the destination. Example: Used in companies with huge volume of data transmission as telegrams, electronic mails, computer files and transaction queries and responses.

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

chunks called packets. Each packet is has the source address and destination place available. Each packet has synchronizing error correction and control bits. Packet switching maximizes transmission capacity of networks. Transmission cost is by packet and not by message, route of distance.


Communication protocols Communication protocols are a set of rules for inter- computer communication that have been agreed upon and implemented by many vendors, user and standard bodies. Ideally, a protocol, standard allows heterogeneous computer to talk to each other. Protocols are software that performs a variety of actions necessary for data transmission between computers. At the most basic level, protocols define the physical aspects of communication, such as how the system components will be interfaced and at what voltage levels will be transmitted. In data transmission establishment and termination of sessions between computers and the synchronization of those transmission takes place. A protocol defines the following three aspects of digital communication.

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

2. Semantics: For reliable and error free transmission, type and order of messages used. 3. Timing: Defines data rate selection and correct 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 Model All People Seem To Need Data Processing Please Do Not Throw Sweet Pongal Away


Physical Layer: The physical layer is at the bottom of this data networking model. It deals with crude data that is in the form of electrical signals. The data bits are sent as 0's and 1's. 0's correspond to low voltage signals and 1's correspond to high voltage signals. The mechanical aspects of communication, such as wires or connectors come under this layer. The physical layer also deals with how these wires, connectors, and voltage electrical signals work. Also, the process that is required for these physical aspects are taken into account in this layer itself. Read more on Ethernet cable wiring. The Data Link Layer: The transmission of the data over the communication medium is the responsibility of this layer. The 0's and 1's that are used in the communication are grouped into logical encapsulation. This encapsulation is called frames. The data is transported in frames. The responsibility of these frames is that of the data link layer. Network Layer: All over the world, there are many different types of Ethernets. These networks are connected to each other through various media. When a data packet wants to reach a particular destination, it has to traverse through these networks. Essentially, there are lot of operations that are taking place between the connected networks. Also, the packet data which is traversing has to choose an optimum route, and the addressing of these packets has to be proper. The various operations between the networks, packet data issues, addressing and routing are handled by this network layer. Transport Layer: The transport layer ensures quality and reliability of the communication. The data packet switching is entirely handled by the transport layer. There are basically two types of packet switching. They are connectionless packet switching and connection oriented packet switching. In connectionless packet switching, the packet data is allowed to choose the route in which it is going to reach the destination. Obviously, the packet in itself cant do this. Physical devices like routers are mainly responsible for the behavior of packets, but the packets formed from the same datum can reach their destination in different ways. Whereas, in connection oriented packet switching, once the route is decided, then all the packets have to follow the same route. Examples of connectionless packet switching are text messages in mobile phones, and the example of connection oriented switching is a direct voice call. The Sessions Layer: The sessions layer is mainly responsible for creating, maintaining and destroying the communication link. PDU (Protocol Data Unit), in which various protocols are defined, that have to be followed during communication, are the responsibility of the sessions layer. The applications that use RPC's (remote procedure calls) are taken care of by the sessions layer. Presentation Layer: There are various techniques of data compression which are used to send and receive the optimized data. For example, if certain data is repeating itself for a number of times, then it is logical to send the data only once, and specify the number of times it is repeated. This bundling of the repeated data is one of the techniques of compressions. The compression and decompression of the data is handled by the presentation layer. Also, encryption and decryption techniques used to thwart malicious attacks on data are handled by the presentation layer. Application Layer: This is the topmost layer of the OSI reference model. This layer comes into picture when there is a process to process communication. Whenever a user invokes any application, all the


associated processes are run. Many a times, when an application wants to communicate with another application, then there has to be communication between these associated processes. The application layer 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 distinguished before

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 used whenever a message passes from or to a

user. The lower three layers are used when any message passes through 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 to another host.

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

application header and then passes the frame to the presentation layer.

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

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


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

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

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

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

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

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

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

finally reaches the receiving process.

Virtual Private Network


A VPN is a private network that uses a public network (usually Internet) to connect remote sites or users together. It will not use a dedicated, real-world connection such as leased line. It uses "virtual" connections routed through the Internet from the company's private network to the remote site or employee. Types of VPN: Following are the 2 most common types of VPN 1. Remote-access: Meaning: This is a user-to-LAN connection used by a company that needs to connect to the private network from various remote locations. It is also called as Virtual Private Dial-Up Network (VPDN). Working: Generally, a company that wishes to set up a large remote-access VPN will outsource it to an Enterprise Service Provider (ESP). The ESP sets up a Network Access Server (NAS) and provides desktop client software to all the computers of remote users. Employees can dial a toll-free number to reach the NAS and use their VPN client software to access the corporate network. They provide secure, encrypted connections between a company's private network and remote users through a third-party service provider. Suitability: It is suitable for a company with large number of employees spread over wide area. For example a company with hundreds of sales people in the field needs a remote-access VPN. 2. Site-to-Site VPN: Through the use of dedicated equipment and large-scale encryption, a company can connect multiple fixed sites over a public network such as Internet. Site-to-site VPNs can be one of the following two types: a. Intranet based: If a company has one or more remote locations that they wish to join in a single private network, they can create an intranet VPN to connect LAN to LAN. b. Extranet based: When a company has a close relationship with another company (for example, a partner, supplier or customer), 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. Integrated Services Digital Network (ISDN) is a circuit switched telephone network system, designed to allow digital transmission of voice and data over ordinary telephone copper wires, resulting in better quality and higher speeds. b. ISDN is a set of protocols for establishing and breaking circuit switched connections and for advanced call features for the user. 2. Types of channel: a. Bearer channels (B channels): Data and voice are carried by these channels having a bandwidth of


64 kilobits per second. b. Delta channels (D channels): It carries signals and controls. But it can also be used for carrying data. 3. Types of ISDN services: a. Basic Rate Interface (BRI): It consists of two 64 kbps B channels and one 16 kbps D channel to form a total of 144 kbps. It is suitable for individual users. b. Primary Rate Interface (PRI): It consists of 23 B channels and one 64 kbps D channel to form a total of 1536 kbps. It is suitable for users with higher capacity requirements. 4. Advantages: a. Multiple channel operation in same cable: ISDN allows multiple digital channels to be operated simultaneously through the same regular phone cable which is meant for analog signals. However, this is possible only if the telephone companys switches can support digital connections. b. Combines different digital sources: With ISDN, it is possible to combine several digital data sources and send the information to the proper destination. In a digital line it is easy to reduce noise and interference even after combining these signals. Data Centre

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

Types of data centres

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

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

Features of Data Centres

Size

Data security

Availability of data


Electrical and power systems

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

Database monitoring

Web monitoring

Backup and restore

Intrusion detection system

Storage on demand. Disaster Recovery Plan Data centres need to be equipped with appropriate disaster recovery systems, this is a plan made by the organization before purchasing the computer. This plan ensures the management that, in case of disasters, the recovery is possible through certain procedures and plans. When disaster occurs, the IT organizations cannot stop their processors for a long time, and cannot tolerate too much downtime. Hence every organization goes for its own disaster recovery sites. There are three types of sites available: Hot site- all the facilities, like key sites are available, very expensive, hardware, software, equipment are available. Warm site- Partially equipped, with some more facilities the processing can be resumed. Cold site- only the space is available with air- conditioning, hardware, software, equipments are not available, making the required facilities, and the processing can be resumed. Disaster Events

1. There is a potential for significantly interrupting normal business processing. 2. Business is associated with natural disasters like earthquake, flood, tornadoes,

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

period of time. 4. Catastrophes are disruptions resulting from disruption of processing facility.

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

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

cover all operations. 4. Commitment to maintain plan to keep up with changes.


There are different phases in BCP: Phase I: Risk analysis, identifying the critical and non critical areas, determining the critical time period, coverage of insurance. Phase II: Determination of minimum resources necessary, reviewing the operations between current policies and back up procedures. Review of the entire system like file backups, operation and documentations. Phase III: Identifying the alternative sites, reciprocal arrangements, preparing a list of alternatives, visits and reviews. Phase IV: Plan preparation, provision for manual processes, plan development, team building, developing a general plan. Phase V: Testing the plans, structured walk- through (paper test), localized test, fully operational test, test review procedures, etc. Virus: A virus is a malicious program that attaches itself to a legitimate program and penetrates into the operating system. A virus is a program that instructs the operating system to append it to other programs and thus propagates to other programs via files containing macros. A virus can replicate itself over and over and the replicated modules can grow independent of the initial virus. Sometimes a virus can be benevolent and it may cause minor disruptions by printing laughing message and sometimes a virus can be malignant and it may delete files or corrupt other programs. Following are the controls to safeguard against the viruses: a. Preventive controls like: _ using only clean and licensed copies of software, _ cutting the use of pubic domain software / shareware, _ downloading files or software only from reliable websites, _ implementing read-only access to software. _ checking new files / software with anti-virus software before installation, _ imparting education and training programs to end users b. Detective controls like: _ regularly running antivirus software, _ file size comparison to observe whether the size of programs has changed, _ date / time comparisons to detect any unauthorized modifications. c. Corrective controls like: _ maintaining a clean backup, _ having a recovery plan from virus infections, _ regularly running antivirus software (which is useful for both detection & removal of virus)


Worms _ A computer worm is a self-replicating computer program. _ Worms are similar to viruses but they exist as separate and independent programs. _ It uses a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. _ Unlike a virus, worms do not need to attach itself to an existing program. _ Worms may cause harm to the network. For e.g. they may consume bandwidth. _ They exploit security weaknesses / bugs in the operating system to penetrate into other systems. _ Exposures that arise from worms are more difficult to control than that arise from virus. _ Generally anti virus / anti spyware software can prevent the attacks from Worms. Hackers _ Hackers attempt to gain unauthorized entry into a system by circumventing the access control mechanism of the system. They can do this either with good or bad intention. _ Some hackers may just trespass and read the files without making any changes to them. _ Some hackers may cause destruction by deleting critical files, disrupting / suspending operations, stealing sensitive data and / or programs. _ They can be avoided only through robust logical access controls and / or Cyber Laws of the Land. Intrusion Detection Systems (IDS) Meaning: The goal of intrusion detection system is to monitor the network assets, to detect anomalous behavior and misuse. This concept is there for nearly twenty years. But in recent years there is a dramatic rise in its popularity and it is being incorporated into the overall information security infrastructure. Following are different components of IDS: Components of IDS: 1. Network Intrusion Detection (NID): _ _ Network intrusion detection deals with information passing in the wire between hosts. _ Typically referred to as "packet-sniffers," _ Network intrusion detection devices intercept packets traveling in different communication channels. Once captured, the packets are analyzed in a number of different ways. _ Some NID devices will simply compare the packet with a database consisting of known attacks and malicious packet "fingerprints", _ While others will look for anomalous (= abnormal) packet activity that might indicate malicious behavior. _ In either case, network intrusion detection should be treated as a perimeter defense. 2. Host-based Intrusion Detection (HID): _ _ Host-based intrusion detection systems are designed to monitor, detect and respond to user and system activity and attacks on a given host. _ Some more robust tools also offer audit policy management and centralization, supply data forensics, statistical analysis and evidentiary support, and in some cases provide some measure of access control. _ The difference between host-based and network-based intrusion detection is that NID deals with data transmitted from host to host while HID is concerned with what occurs on the hosts themselves.


_ Host-based intrusion detection is best suited to combat (= fight against) internal threats. _ Majority of computer threats come from within organization. For example, disgruntled employees, corporate spies, etc. 3. Hybrid Intrusion Detection: _ Hybrid intrusion detection systems offer management of and alert notification from both network and host-based intrusion detection devices. _ Hybrid solutions provide the logical complement to NID and HID i.e. Central Intrusion Detection Management. 4. Network-Node Intrusion Detection (NNID): _ _ Network-node intrusion detection was developed to work around the inherent flaws in traditional NID. _ Network-node pulls the packet-intercepting technology from the wire and puts it on the host. _ With NNID, the "packet-sniffer" is positioned in such a way that it captures packets after they reach their final destination i.e. host. _ The packet is then analyzed just as if it were traveling along the network through a conventional "packet-sniffer." _ In this approach, network-node is simply another module that can attach to the HID agent. The major disadvantage is that it only evaluates packets addressed to the host on which it resides. _ On the other hand, traditional network intrusion detection can monitor packets on an entire subnet. _ "packet-sniffers" are also incapable of viewing a complete subnet when the network uses high-speed communications, encryption or switches since they are essentially "without a sense of smell". _ But NNID can defend the specific hosts against packet-based attacks in these complex environments where conventional NID is ineffective.

computer networks and network secuirty 3

Documents

network security network

local area network lan

local area networks

growth of lan

set of computers

scope of networks

device sharing

communication system