Monday, December 21, 2015

Computer Network

Computer Networks
Computer network is a collection of computers, Peripherals devices (I/O Devices ) and terminals connected to gether by a communication system. The set of computers may include large- scale computers mini computers and micro processors .
Before getting in to the details on types and working of computer networks, we-have to understand why we use the computer networks.
Advantages of networks:
1.File sharing : It provides sharing and grouping of data files over the network
2.Print sharing : It provides sharing of computer resources such as hard disk,
                    printers etc
3.E-Mail: E-mail can send or receive out side of the organization in faster, cheaper
         and easier way
4.Remote Access : Network allows users to access remote data
Classification of network:
Local area network (LAN) : A typical LAN connects as many as hundred or so microcomputers that are located in a relatively small area, such as building or several adjacent building organizations having their own LAN enable is multiple users to share software, data, and devices
Metropolitan Area Network (MAN) : It is same where between a LAN and a WAN. The term (MAN) is some times used to refers to networks which connect. Systems or local area networks with in a metropolitan area
Wide area network : Covers a large geographic area with various communication facilities such as long distance telephone services, salellite transmission and under- sea cables Examples of WANs are interstate banking networks and airline reservation systems
WAN effectively provides a slow speed communication because communication channels used in WAN are ultimately shared among users for data communication.
Examples of wan banking network, mobile phone network, railway network, etc
 ( Internet is a/s a wan)
Network architecture: Network consisting of the hardware, software, connectivity, communication protocols and mode of transmission
A supports two basic networks
1.Client server 2) peer to peer
Cline server : Clinet- server consists of servers and client. Servers are typically powerful computers running advanced network operating systems and user workstations (clients) which access data or run applications located on the servers
Advantages :
Disadvantages :
Function Based Classification

Data networks
This networks transmits data

Voice networks
This networks transmits voice

Multimedia network
This network transmits voice, images and
Area coverage Based

Network with limited coverage

Network with unlimited coverage

Network within a city
Data forward based

Switched network
Provide point- to – point data communication i.e data is switched to required user

Shared network
Provide broadcasting communication by using a HUB i.e data can be seen by every user on network

It is mix of switched and shared network
Owner ship Based

Public network
Any used can use these network e.g internet

Private network
Used by particular organization, not available to outside world user

Vitual private Network (VPN)
VPN uses public network but for secured private network access form any where i.e it provides secured private communication
Class V
 Media Based classification

Wired network
Network uses psychical channels for communication like gable

Wireless network
Network uses wireless channels for communication

Peer- to- peer models
In this architecture, no dedicated servers are used all the computers are at equal levels, and therefore termed a peers normally, each computer can function both as client and server. The main advantages of this architecture is simplicity of its design and maintenance. This is a low cost and easy to maintain architecture and linking computers in peer to peer network is significantly easier than client server mode
Communication Devices/communication modes:
These are the devices  which help in communication of data. Communication devices are mainly used for connecting sender and receiver with communication channels for data communication

1. Modem : Modem is an acronym for modulator / demodulator. Modern is coding and decoding device i.e when data is transmitted from one computer to another computer over a distance through a telephone line or other communication channel then modem converts the computer digital signals into analog signals which can be transmitted on telephone line and vice a versa
2) Multiplexer : multplexer is a device that enables several devo to share common communication channel. A multiplexer scans multiple devices to collect and transmit data one a single  communication line to the appropriate destination the multiple devices are polled together and periodically asked whether  devices are polled together and periodically asked whether there is any data to transmit
3) Front –end communication processor : these are programmable devices which contol the functions of communication system. The devices support the operation of a mainframe computer by perfoming fucnitons which it would other wise be required to perform itself. These functions include authenticaiton of user by checking his user id and password, coding & decoding of data and control of transmission lines. (Eg: stock exchange network) front end processor is like an assistant to main frame computer
4) Protocol converters : Dissimilar devices cannot communicate with each other unless a strict set of communication standards is followed. For example, if you want to connect an IBM data communication network to AT & T data communication network then the protocol converter the message formats one network system to another so that these are compatible for communication with each other
5) Bridge: This device is used to connect two networks with each other that are using the same protocols
6.Router: This device also helps to select the best route (shortest and most reliable roule) for data communication when there are multiple paths between them
7.Gatway: A device that connects two or more dissimilar network i.e the networks having different protocols
8.Repeates: Repeater is a device, which simply repeats the incoming signal for outgoing communication on channel but with increased strength repeator is also known as amplifer it amplify the incoming signal for long distance communication
9.Remite access device: this device contains a bank of modems i.e a collection of modems to serve as central point for access of data from various distant or remote location
Network structures or network topology
The geanetrical arrangement of computers, devices and communcation facilities is unown as network structures of network topology . A network is comprised of  nodes and links. A node is the end point of any branch in a network
Four basic network structures are discussed below
Star topology: the most common structure or topology known as star network is characterized by communication channels emanating from centralized control the processing nodes in a star network topology internals directly with a central system. Each terminal can communicate only with the central computer
1. Easy to add new nodes to this star network
2. If one node fail does not bring down the entire network
3. Easy to diagnese the network problem
1. If central system is fail, the whole network will network
2. The cable cost is more
2. Bus topology :
This structure is very popular for local area networks. A single network cable run in the building and all nodes are linked along with this cimmunication the with two endpoints called the bus or backbone two ends of the cable are terminated with terminators
1. reliable in very small networks as well as easy to use and understand
2. Easy to extend
3. A repeater can also be used to extend a bus configuration
1. Heavy network traffic can slow the transmission
2. Each connection between two cables weaker the electrical signal
3. It is difficult to troubleshoot
3.Ring topology: This network cable passes form one node to another until all nodes are connected in the form of a loop or ring

Advantages :
1. High performance for a small number of workstations
2. Ring networks can span longer distance than other type of networks
3. ring networks are easily extendable
1. Relatively expensive and difficult to install
2. failure of one computer on the network can effect the whole network
3. It is difficult to trouble shoot a ring network
Mesh topology
In this structure, random connection of nodes using communication links. In real life, however, network connections are not made randomly. It is crated form highly reliable and security sensitive application like defense allocations, and telecom network if one link files between nodes then alternate links would allow the data communication
1. network problems are easier to diagnose
2. If node fad where network traffic can be redirected to another node
1. Cost of installation and Maintaince is high
Communication channel / transmission media :
Different types of cables and wireless techniques that are used to connect network devices in a local area network (LAN, communication media is divided into two groups )
1. Guided media   2. Unguided media
1. Guided media: A cabling system that guides the data signals along a specific path. The guided media is also known as Bound media. Some of the common examples of guided media are twisted pair, coaxial and optical fiber
Twisted pair cables
                                    Unshielded             Shielded                        
                                    Twisted pair             twisted pair
Twisted pair contain pairs of insulated copper wires twisted together Twisting reduces the impact of interferences
The trusted paire can carry data at a speed of  10 mbps, 1000 mbps and 1000 mbps and can transmit data upto 100 meters
2. Co-axial cables (cable TV wire) also called as coax, it contain central copper wire as its core that it surrounded by two byers of protective shielders. This shielding reduces electro magnetic interference coax can transmit data at a maximum speed of 10mbps to 500 meters with using repeaters
1.Optical fiber: Is made up of plastic or glaes that carries light along its length. Optical fiber are widely used in fiber-optic communications which permits transmission over longer distances and at higher data rate specially designed fibers are used for a verity of other applications including as sensors and fiber lasers
2.Ungiaded media:
If consists of a means for the data singles to travel but nothing to guide them along a specific path she data signal are not bound to a cabling meda as such are often called unbounded media
Ex: raida wave, microwave, and infrared wave
Radio waves:
Radio waves are an invisible form of electromagnetic radiation the varies in wave length from around a millimeter ot 100,000 km, making it one of the widest ranges in the electromagnetic spectrum
Micro waves: These waves with wavelength ranging from as long as one meter to as short as one millimeter, or equivalently are used for communication, radar systems, radio as from navigation and spectroscopy
Infrared wavers: Infrared light is used in industrial , scientific, and medical applications. Night- vision devices using infrared illumination allow people or animals to be observed with out the observer being defected
Transmission Technologies
 Serial transmission
In serial transmission, the bits of each byte are sent along a path one after another. Ex: Modem/ Mouse
Parallel Transmission :
There are separale, parallel path corresponding to each bit of the byte so that all character bits are transmitted simultaneously Ex: Printer
Asynchronous transmission
Asynchronous transmission is done without any handshaking i.e transmitter and receiver does not care for the speed of each other or doesnot  use the same clock for data transmission. There fore for reliability of data transmission in this transmission, each data word is accompanied by stop(1) and start (0) bits that identify the beginning and ending of the words
Synchronous transmission 
Bits are transmitted of fixed rate. The sender and receiver used the same clock speed. For synchronization and data communication this allows data to be send as a multi-word blocks synchronous transmission uses a group of synchronization bits which are placed at the beginning and at the end of each block to maintain synchronization
Transmission modes :
Transmission modes indicate the direction of data communication on communication channels i.e one way or unidirectional and two-ways or bi-directional there are three different types of transmission modes
1)simplex: In is connection in which the data flows in only one direction form the sender to the receives
Ex: computer to printer / mouse to computer/ Fm radio
2.Halfdupplex : Dat flows in both the direction but not at the same time ex: walkie tallie or internet
Full Duplex : Two way communication same time which means that the bandwidth is divided in two each direction of data transmission
Ex: Telephone/Mobile phones
Transmission Techniques
A communication network consists of a collection of devices that wish to communicate and inter connect together, based on the techniques used to transfer data communicates networks can be categorized into two
1) Broad cost 2) Switched networks
1. I broad cost networks data transmitted by one node is received by many
2. Switched networks: Data transferred form source to definition is routed through the switch nodes. The way in which the nodes switch data from one link to another as it is transmitted from source to destination node is referred to as a switched technique – there are three common switching techniques
1. Circuit switching   2. message switching  3. packet switching
Circuit switching : A circuit switching network is one that establishes a fixed band width circuit between nodes and terminals before the users may communicate; as if the nodes were physically connected with an electrical circuit. Iam circuit switching, this path is decided upon before the data transmission starts
Message switching : The message switching, end users communicate by sending each other a message, which contains the entire data being delivered from the source to destination node. A message is routed form its source to its destination, each intermediate switch within the network stores the entire message, providing a very valuable service when congestion occurs, the message switched network will store and delay the traffic until sufficient resources are available for successful delivery of the message there is no direct connection between source to destnit this characteristic is offen referred to as store- and –forward-Email and voice mail are also examples of message switching systems
Packet switching: Packet switching refers to protocols in which messages are broken up into small transmission units called packets before they are sent. Each packet is transmitted individually across the net the packets may even flow different routes to the destination, depends on the types of packet switching. A the destination the packets are reassembled into the original message. The maximum length of the packet is determined by the network and varies from network to network
Protocol : Protocols are a set of rules for inter-computer communication that have been agreed upon on implemented by many vendors, users and standards bodies  
A set of rules that Governs a packet
Source address
Destination address

At the most bask 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. By this we can say that network protocols when are essentially software are sets of rules for communications timings, sequencing, formatting and error checking for data transmission. A protocol defines the following three aspects of digital communication
a) syntax b) semantics     c) timing
a) syntax : the format of data being exchanged, character set used, type of error correction used, type of encoding scheme
b) semantics : type and order of messages used to ensure reliable and error free information transfer
c) Timing : Defines data rate selection and correct timing for various during data transfer
Network interface card: Every computer in a network has a special card called NIC which provides the connector to attach the network cable to a server or a workstation. An NIC card has additional memory for buffering incoming and outgoing data packets, these improving the network throughout characteristic of NIC’s include the following :
®NIC constructs, transunits , receives, and processes data to and from a host to network
Each WIC has 8 bytes permanent and unique MAC (Meda Access Control) address provided by manufacturer.  This address is called physical address and IP address is called logical address.
OPEN SYSTEM INTER CONNECTION MODEL :-   This standard model is designed by (ISO) to facilitate communication of heterogeneous hardware or software plat forms with each other.  In its most basic form, A durdes network architecture into seven lager, cohere in a layer is a collection of layers which from top to bottom, are the application, presentation, session, transport, network, data line & physical layers.  Each layer as its own functionally that provide services to the layer above it and receives services from the layer below it.
Application layer
Telent, http, SMPT
Presentation layer
Encryption data conversion
BCD to Binary etc
Session layer
Starts, stops section
Maintains order
Transport layer
Save & destination adding
Network layer
Data cable(MAC)
One node to another
Electrical Signals.

Application Layer:- This layer mterads with software appheations  and prardes user services by the transfer, file sharing etc.
Presentation Layer:- Encryption, data compression our take place at this layer.
Session Layer:- If provides for pull duplex, half duplex, or simplex operation and establishes check painting, adjournment, termination, and restart procedures.
Transport layer:- This layer ensures reliable and transparent transfer of data between user processes, assembles and disassembles message packets, and provides coin recovery and flow control.
Network Layer: -This layer provides data sequences from a scarce to a destination via one or more networks.  Data transmission and switching, establishes, maintains, terminals connections between the nodes and ensure proper rating of data.
Data line layers:- The Data line layer is the protocol layer which transfers data between adjacent networks nodes in a wide area network.  A provides the functional and procedural means do transfer data between network entities and to detect and possibly correct error that may access in the physical layer.
Physical layers:- The physical layer is a hardware layer convert the data into electromagnetic features of the connection between the devices and the transmission.
Local Area Networks:-
·      In expensive way of data communication
·      Easy to setup network
·      High data transmission rate
·      Easy to control and maintain the network
Benefits of LANS
·      LAN allows distributed data processing
·      LAN allows the extended use of personal computers by connecting then to powerful servers.
·      Sharing of data and resources, precious resources like color laser printers, costly applications,  etc can be polled among different users.
·      Lower software cost.
·      Facilities of electronic mail and broad casting  among users.
Components of LAN:-
·      File server and work station
·      Cables
·      NIC
·      Network operating system
1.     Server or file Server:- Server also helps in handling network communication and servicing the various request of users for information sharing the various request of users for information sharing an a network.  Server can be dedicated or non-dedicated server, all the network management functions are done by server and in case of non-dedicated server some of the management functions are transferred to workstations.
Workstations:- Workstations are standalone computers which acts as nodes in the network interface card.  Work stations are normally personal computers with their own processing power, and because these are connected on network therefore use the extra processing power, and because these are connected on network therefore use the extra processing power and capabilities of server that makes then more capable than standalone personal computers.
LAN Card + Personal Computer = Workstation
2.     Cables:- Cable is communication channel of network on which data communication takes place. The popular type of cables are UTP (Unshield twisted pair) cable, coaxial and optical fiber cable.
3.     NOS:-  This software which is loaded on to the server hard disk.  The job of this software is to manage and control the working of network the NOS helps in file management, security management, applications management, user management and devices management functions of a network.
Wireless LAN:-  Wireless Lan does not require any physical media like cable for data communication.  Wireless signal like radio waves or infrared signals are used for data communication. Mostly radio waves are used for data communication.  It proves a flexible network where in users can use their LAPTOP anywhere in the premises and they stay connected with their network for data communication.

The wireless lan access point contains the antenna for provide a wireless connection to the Laptop computers and devices.

·      Integrated services Digital Network
·      It is high speed digital communication network
·      It is also known as broad band communication network.
·      ISDN network helps to transfer the data at high speed.
·      It transfer both voice and data simultaneously.
·      Most of the telephone exchanges, mobile phone networks and internet are part of this ISDN networks.
·      They are two types of ISDN network
1.     Basic Rate 2. Primary Rate

Advantages of ISDN
Allow multiple digital channels and network to work an integrated form
Allow to transfer voice and data together on same channel
Provide very high speed
Virtual private Network : -A UPr is a private network that uses a public network to connect remote sites.  Instead of using a dedicated, veal world connection such as leased line, a VPN users “Virutal” connections routed through the internet from the company’s private network to the remote site or employee.  There are two types of VPN
1.     Remote-access 2. Site – to site VPN
Remote –access VPN also called a virtual private dail up network (VPDN) is a user – to Lan connection used by a company that has employees who need to connect to the private network from various remote locations.

TCP/IP:- This part deals with assemblers and disassembles of data packets 1-e creating data  packets from the Ales to be transferred etc.  H provides data controls and flow control.
IP:- It is an Best-effort protocol, this part handles forwarding or hating of data packets on internet channels, from source to destination.

1.     A is 7 layer model
It is a 4 layer models
2.     A uses application, presentation, session, Transport, Network, Data line or physical layer
Application, Transport, Network and data line layer
3.     This model is has model for all the protocol used for difference communication
This model is used for internet dds communication

Client/ Server Technology
A Client/ Server is hared or distributing computing where in tasks and computing powers are split between servers and clients,  servers stores and process data common to users across the enterprise, this data can be accessed by client systems. 
Traditional Business Computing Models and their limitations .
Main Frame Computing Model:- Main frame computing model is centralized processing system. In this all the users are connected to a single central computer through terminal.
Centralized system allows users to share expressive hardware resources as disk drives, printers etc.


2.     Personal Computer Computing : With the introduction of personal computer and its operating system, personal computers, computing or independent computing workstation quickly becomes very popular. This type of processing was decentralized processing.
3.     File sharing architecture: - The original PC networks were based on file sharing architectures, where the server downloads files from the shared location to the desktop environment.  The server will send the entire file over the network even through the workstation only requires a few records, in the file to satisfy the information request. 
Note: The server handles all the global tasks, while the workstation handles all the local tasks.
Characteristics of client (server Technology):-
1.     A consists of a client process and a server process that can be distinguished from each other.
2.     The client portion and the server portion can operate on separate computer plat forms.
3.     The server is able to serve multiple clients concurrently.
4.     Action is usually initiated at the client and not the server end.
5.     The graphical user interface generally resides at the client end.
6.     The database server should provide data production and security.

Components of Client Server Architecture:-
Client:  The clients are the users of the services offered by the servers described above.  This are basically three types of clients.
·      Non –Graphical user interface (GVI) clients require a minimum amount of human interaction on –GUIS include ATMs, cell phones, fax machines, and robots.
·      GUI – clients are human interaction models usually involving object /action models we the pull-down menus in windows 3-a.
·      Object-oriented user interface (OOUI) clients take GUI-clients even further with expanded visual formats, multiple workplaces, and object interaction rather than application interaction windows 95, 98, 2000, 2003, are the common OOUI clients. 
Serves:- Server, responds the client request.

                                                         Response                     servers

Transaction servers execute a series of SQL commands, an online transaction.  Processing program (OLTP) as opposed to database servers, which respond to a single client demand. 
Middle ware : middle ware is all the distributed software needed to allow clients and servers to interact.  General middleware allows for communication, directory services, queuing, distributed file sharing & printing.  The middle wares tipically composed of four layers which are
1) Service
2) Baele –end processing
3) Network operation system (NOS)
4) Transport Stale
The service layer carries coded instructions and data from software applications to the Baele- end processing layer for encapsulating network-routing instruction.  Next the transport layer transfers data packets to destinated efficiently and correctly.
FAT client: In a Fat –client system, more of the processing takes place on the client, like with a file server or database server. 
FAT server: Fat- servers place more emphasis on the server and try to minimize the processing done by clients. 
Fat-client referred to as 2-ties and fat-servers referred to as 3-Tier system.

Types of Servers:
1.      Database servers: DBMS provides database server to provide data related services, to large number of users.  In mutinously. Sum popular DBMS which provide database servers are ORACLE, server, DBZ etc.
2.      Application servers: Application server is program which maintains business logics for data processing.  Banks servers are combination of different types of servers and application server is one of the most important server types among those servers.  Some powerful features of applications servers are :
1.     Component management
2.     Fault tolerance
3.     Load balancing
4.     Transaction management
5.     Security
3.      Print servers: Print server provides printing related services. These servers provide shared access of printers to large numbers, of users connected in the LAN or WAN mode a single printer.  Connected with any particular computer.
4.      Transaction servers: When you start your transaction at ATM machine it is the responsibility of transaction server to maintain track of your transaction right from the beginning to end until the transaction is over.
5.      Internet Servers: We all know that internet is collection of millions of servers and there are different types of servers on internet, some popular types of internet servers are  1. Web server, DNS server, New server, Chat server, Caching servers, mail server, RTP server, file server, Gopher server, proxy servers.
Web Server : This server maintains web sites in the form of web pages which can be accessed from any where through internet e.g. yahoo web server, Google web server.
DNS server: It is an internet –wide distributed database system that documents and distributes network –specific information, such as the associated IP address for a host  and vice versa.
NEWS servers: Usenet news is a worldwide discussion system consistivity of thousands of news groups organized into hierarchies by subject.  Users read and post articles to these newsgroups using client software
Chat Serves : Provide services where in multiple users communicate with each other simultaneously in a real finis manner
Caching server: Basically a caching server sits between the client computer and the server that would normally fulfill a clients request

One the clients request is sent, it is intercepted by the caching server. The chaching server maintains a library of files that have been requested in the recent past by users on the network.
Mail server: Provides email servers to users eg yahoomail, gmail, rediffmail, hotmail etc. Mail server stores the mails and sends to receiver.
FTP: File Transfer Protocol Server Provides file uploading and downloading services.
File server: Provides services where in multiple users Command with each other simultaneously in a real time manner.
Gopler Server: Gopler is a service in which multiple servers together locate for required images file etc. It is a kind of search service.
Proxy Server: A Proxy server is designed to restrict access to information on the internet. Example if a user wants to have access to pornographic materials, a proxy server can be configured to refuse to pass the request along to the intended internet server. A proxy server operates on alist of rules given to it by a system administrator.
Network Tier Architectures:
Tier: A tier is a district part of hardware or software. The most common tier system are
1). Single tier     2). Two tier                  3). Three tier 4). Four tier
Single tier : A single computer that contains a database and a front-end (GUI) to access the database is known as single tier system. Generally, this type of system is used in small business. This is one computer which stores all of the company’s data on a single database.

Two tier system: A two tier system consists of a client and a server. The database is stored on the server and the interface used to access the database is installed on the client.
Three tier Architecture: The three-tier (middle tier server) resides between the interface (client) and the data management (server) Components.
This middle server provides process management where business logic and rules are executed and can accommodate hundreds of uers by providing functions such as quering, application execution and database staging.
Multi tier Architecture: Multi – tier architecture is a client server architecture in which an applicate is executed by more than an district software agent. For example, an application that uses middle ware to service data requests between a user and a database employs multi-tier architecture. The most wide spread use of “multi-tier architecture” refers to  three – tier architecture.
Data Centre: A data center is a centralized repository for the storage management and dissemination of data and information. Data centers can be defined as highly secure, fault-resistant facilities hostins customer equipment that connects to telecomment networks. These data centers are also referred to as public data centers because they are open to customers. The purpose of a data centers because they are open to customers. The purpose of a data center is to provide space and landwidth connectivity for servers in a reliable, secure and scalable environment. The primary ‘goal’ of a data center are to deploy the requisite state – of – the art redundant infrastructure and systems so as to maximize availability and prevent downtime for customers.
Types Private Data Center:  A Private data center is managed by the organization’s own IT department, and it provides the applications, storage, webhasting and e-busines functions needed to maintain full operations.
Public Data Center: A Public data center provides services ranging from equipment co-location to manage web-hosting.
One tier Data Center: Maintain and provide basic quality of services, provide inexpensive services. These data centers do not maintain highly reliable and secured infrastructure. Tier one data center is low cost and simple to setup, Used by general enterprise where  security and reliability is not very important.
Four Tier data Center: Maintain and provide robust quality of services. They provide expensive services. These data centers maintain highly reliable and secured infrastructure. The four tier data center are costly and complex to setup usually used by financial and banking sectors where reliability and security of services is very important.
Uses of Data Centers:
1.     Database Monitoring: Help to provide fast and reliable data access services round the clock by continuously monitoring the working and performance of data managed by data centers.
2.      Web Monitoring: Provide web hosting and also monitoring of websites for round the clock availability; for reliable and efficient access of sites to their users.
3.     Backup and Restore: Help to provide automatic backup of managed data and also automatically restore the data destroyed accidentally or intentionally, by using the backup data.
4.     Instrusion Defection System: ID stands for Intrusion detection it is an art of detection inappropriate, incorrect and unauthorized activities on network. This is primarly a security system which helps to provide data access to authorized person only and protect the data from unalicious activities.
5.     Storage on Demand: Help to provide the required storage capacity to organize to maintain their data volume in online manner, which help to provide reliable and robust data management services.
Features of Data centers:
1.Size: Data centers maintain large number of servers, large bandwidth channels and large volume of storage capacity.
2.Data Security: Data centers normally employ various techniques and software to provide high security to data.
3.Availability of Date: Data centers provides round the clock availability of data and services from anywhere.
4.Security: Data centers provides both the physical and logical security to data.
Disaster Recovery Site: Disaster means fire, flood, earthquake, etc which may bring down the data centers from working. Data centers need to be equipped with appropriate disaster. Recovery system that minimize the downtime of data centers. Every data center uses different techniques to minimize the downtime. i.e. data centers should always be available for services. Following techniques are used by data centers to minimize the down time, or from disaster recovery.
1.COLDSITE: This techniques maintains the critical equipment and resources in duplicate format some offsite location. In case of disaster, these duplicate equipment and resources start providing necessary services to the authorized users, maintaining cold-site is low cost disaster recovery technique but this does not provide 100% downtime elimination.
2.Warm site: This techniques is in between the cold site and the hot site techniques. It is better than cold side but ware than the hot site technique.
3.Hot Site: This is considered to be most robust disaster recovery techniques. In this, all most equipments and resources of a working data center are maintainedin duplicate form at some offsite location in a synchronized from with outsite resources. This is the most expensive technique, but this technique provide almost zero downtime.
Network Security:
Need for security: The basic objective for providing  network security is two – fold
1.To safeguard assets and
2.To ensure and maintain the data integrity.
There are two types of security:
1). Physical             2).Logical security
1.     Physical Security : A physical security is implemented to protect the physical systems assets of an organization like the personal, hardware, facilities, supplies and documentation.
2.     Logical Security: A logical security is intended to control
i) malicious and non-malicious threats to physical security and
ii)malicious threats to logical security itself.
Threats and vulnerabilities: A threat is anything that can distrupt the operation, functioning, integrity, or availability of a network or system network security threats can be categorized into two broad themes.
1.     Unstructured threats: These originate mostly from inexperienced individuals using easily available hacking tools from the internet.
2.     Structured threats: These originate from individuals who are highly motivated and technically competent and usually understand network systems design and the vulnerabilities of those systems. They can understand as well as create hacking scripts to penetrate those network systems. The structured threats typically targets a specific destination or group.
3.     External threats: These originate from individuals or organizations working outside an organization, which does not have authorized access to organization’s computer systems or network.
4.     Internal threats: Typically, these threats originate from individuals who have authorized access to the network. An internal threat may come from a discountented  former or current employee or contractor.
5.     IDS Components: Intrusion detection system are generally made up of following major components.
1.Sensors 2.Analyzers              3.Userinterface      4.Honeypot

Honeypot: Some administrators may choose to install a “Honeypots”, essentially a system components setup as bat or decoy for intruders. Honeypots can be used as early warms systems on a attacks, decoys from critical systems, and data collection sources for attack analysis.
Firewall: Firewall are system that control the flow of traffic between the internet and the firm’s internal LANS, and systems. Access controls are common form of controls encountered in the boundary subsystem by restricting the use of system resources to authorized users can take with these resources and ensuring that the users obtain only authentic system resources.