COMPUTER NETWORKS

Definition..A computer network is a collection of autonomous computers interconnected by a single technology.i.e Two computers are said to be interconnected if they are able to exchange information.The connection can be via a copper wire; fiber optics, microwaves, infrared and communication satellites.A network consists of multiple computers connected using some type of interface, each having one or more interface devices such as a Network Interface Card (NIC) and/or a serial device

Normally each computer has its own operating system, a network operating system that allows a user to manage the resources of a computer network. Then the user is aware of the network and the different computers.In a network operating system a user must explicitly connect to other computers in order to communicate with them.

TYPES OF NETWORKS

LANMANWAN

LAN

A local area network (LAN) is a privately owned communications network that covers a limited geographic area such as a company computer laboratory, an executives office,one building, or a group of buildings close together. The range is typically within a mile or so.

MICROCOMPUTER (WORKSTATION)A LAN consists of many workstations, each of which canbe used as a server, a terminal, or a microcomputer. A serverin a LAN stores network management software and performsmanagement functions. There are different types ofLAN servers depending on the major functions assigned ina client/server environment. A printer server provides printingservices to other workstations, while a file server storesnetwork and application software for the network. Otherdevices may include a fax machine, a scanner, or a printer.Components of LAN

ContNETWORK INTERFACE CARDA network interface card is the interface between the networkand the computer. This card provides the communicationprotocol recognized by the network architecture. Forexample, an ethernet card is used to connect a PC with thenetwork in a BUS network.

NETWORK OPERATING SYSTEMA network operating system (NOS) is software that allows auser to manage the resources of a computer network. TheNOS runs on the server computer in addition to the clientoperating systems, such as Windows or OS/2. The functionsprovided by NOS are: Administration: To add, delete, or organize client usersand perform maintenance tasks such as backup. File management: To store and transfer software to clientcomputers. Printer management: To prioritize printing jobs and direct reports to specific printers on the network. Network security: To control the access and usage ofthe network.

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Types of LANs/Network models1. Client-server model - It is widely used and forms the basis of much network usage. In this model, the data are stored on powerful computers called servers. Often these are centrally housed and maintained by a system administrator. The users have computers called clients on their desks, with which they access remote data. The client and server machines are connected by a network as illustrated below:

Client-server model

2. Peer-to-peer communication model Its another type of person-to-person communication.In this form, individuals who form a loose group can communicate with others in the group. Every person can, in principle, communicate with one or more other people; there is no fixed division into clients and servers.

However, peer-to peer systems eliminates the central database by having each user maintain his own database locally, as well as providing a list of other nearby people who are members of the system as shown below

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Metropolitan Area Networks (MANs) are networks that connect LANs together within a city. From The Big Picture, we see that telecommunication services provide the connection (storm clouds) between networks. Local telecommunication services provides the external connection for joining networks across cities. The main criteria for a MAN is that the connection between the LANs is through a local exchange carrier (the local phone company). Location: Separate buildings distributed throughout a city. TYPES OF NETWORKS

Examples of companies that use MANs are universities, colleges,department stores and banks. Examples of MAN protocols are: RS-232, V.35 X.25 (56kbps) Frame Relay (up to 45 Mbps), Asynchronous Transfer Mode (ATM) ISDN (Integrated Services Digital Network) Cont

Wide area networks are communication networks covering a large geographical area. A WAN uses telephone lines, microwaves, satellites, or a combination of communication channels to transmit signals.Wide Area Networks (WAN) are a communication system linking LANs between cities, countries and continents. The main difference between a MAN and a WAN is that the WAN uses Long Distance Carriers rather than Local Exchange carriers. Otherwise the same protocols and equipment are used as a MAN. TYPES OF NETWORKS

NETWORK TOPOLOGIES

Network topologiesIntroductionIn networking, the term topology refers to the layout of connected devices on a network. Each topology has its advantages and disadvantages: usually related to cost, complexity, reliability and traffic "bottlenecks".

Types of topologiesBus topology

Bus topology like broadcasting radio or Ethernet network. It is essentially a single multidrop line shared by many nodes. A message to be transmitted is placed on the common path and is broadcast to all the nodes.

Obviously, all messages should be included with senders and receivers addresses. Failure of any nodes in the bus does not affect the other nodes.Meanwhile new nodes can be added easily by tapping into the bus .Messages which are not addressed to nodes in the network are ignored by them. Many devices connect to a single cable "backbone". If the backbone is broken, the entire segment fails. Bus topologies are relatively easy to install and don't require much cabling compared to the alternatives.

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Ring Topology

In a ring network, every device has exactly two neighbours for communication purposes. A device can communicate with any other devices in the ring. The data flow in such topology is unidirectional. All messages must be addressed.

All messages travel through a ring in the same direction. Like the bus topology, a failure in any cable or device breaks the loop and will take down the entire segment. A disadvantage of the ring is that if any device is added to or removed from the ring, the ring is broken and the segment fails until it is "reforged.It is also considerably more expensive than other topologies.

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Star Topology

A star network has a central connection point - like a hub or switch. While it takes more cable, the benefit is that if a cable fails, only one node will be brought down.All traffic emanates from the hub of the star. The central site is in control of all the nodes attached to it.

The central hub is usually a fast, self contained computer and is responsible for routing all traffic to other nodes. The main advantages of a star network is that one malfunctioning node does not affect the rest of the network. However this type of network can be prone to bottleneck and failure problems at the central site.

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Tree Topology/hierachical topology

The tree topology is a combination of bus and star topologies. They are very common in larger networks. A typical scenario is: a file server is connected to a backbone cable (e.g. coaxial) that runs through the building, from which switches are connected, branching out to workstations.Cont

Mesh topology

In the topologies shown above, there is only one possible path from one node to another node. If any cable in that path is broken, the nodes cannot communicate.

Mesh topology uses lots of cables to connect every node with every other node. It is very expensive to wire up, but if any cable fails, there are many other ways for two nodes to communicate. Some WANs, like the Internet, employ mesh routing. In fact the Internet was deliberately designed like this to allow sites to communicate even during a nuclear war. Cont

Network devices

Network devicesNIC Adapters:NIC is Network Interface Card; PC cards allow a mobile PC to access the network and Internet. It is also referred to as a Local area network card. Communication mediums (cables) are attached to this card to build network. This device has unique Mac address. To build network unique IP address is assign to this LAN card to begin communication. This is the most important device in building network.

Figure showing a NICIn case of developing Wireless LAN, instead of LAN card we use Wireless card. Its functionality is same as simple LAN card; it is just wireless communication device which connects to router for communication.

Routers:Router is intelligent device which routes/forwards data to destination computers. It helps in connecting two different logical and physical networks together. It performs all functions similarly without using any medium like cables etc.Router uses software known as routing table. Routing table is used to store source and destination address. In small networks, a server is connected to router along with clients for communication.

A router is a device that forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP's network. Routers are located at gateways, the places where two or more networks connect, and are the critical device that keeps data flowing between networks and keeps the networks connected to the Internet. When data is sent between locations on one network or from one network to a second network the data is always seen and directed to the correct location by the router. Cont

ControuterThe router accomplishes this by using headers and forwarding tables to determine the best path for forwarding the data packets, and they also use protocols such as ICMP to communicate with each other and configure the best route between any two hosts. Major companies which are known for manufacturing routers and wireless routers are Tp Link, Cisco systems, Nortel, D link etc.

Figure showing a routerFigure showing a hub

HubsA hub performs as a centralized device on the network. Hubs are required to build networks on larger scale. All computers are connected directly to the hub. When data is sent to the hub it broadcasts the data to all the ports of the hub and then it is sent to destination computer on the network. If hubs fails to perform its routine functions it will halt the working of the entire network until it is put back in normal condition.

Switches:Switch is another important device when we talk about computer network on broader spectrum.It is used at the same place as hub is but the only difference between the two is that switch possess switching table with in it. Switching tables store the MAC addresses of every computer it is connected to and send the data to only requested address unlike hub which broadcasts the data too all the ports. Switches can be considered advance form of hubs.

Gateways:

As name suggests it some kind of passage through to some thing. Interestingly gateways can be software or it can also be device. Gateway device connects LAN with internet. Its basic functionality is to provide security to the network. By using gateways, incoming/out going traffic can be monitored for any malicious activity within the network which can be harmful to network integrity.

Modems:

Modems can be of two types. One modem is very common in every computer which can be used to connect to internet using our telephone line by dialing to our ISP (internet service provider) and the other one is used to connect to DSL. Functions however are same for both types of modems; they are used for modulation and demodulation, they are used to convert analog signals into digital and digital signals into analog so that signals can travel on telephone lines.

Cables:Cables are obviously used to connect communication devices with each other to form network. There different types of cables, commonly used cables are 10baseT/CAT5 , coaxial cable, Ethernet and fiber optical cable. Fiber optical is the most expensive as it enables the data transfer at speed of light. It is costly solution which is mostly adopted by corporate sector.

network interface controller (NIC) is a hardware device that handles an interface to a computer network and allows a network-capable device to access that network. The NIC has a ROM chip that contains a unique number, the media access control (MAC) Address that is permanent. The MAC address identifies the device uniquely on the LAN. The NIC exists on both the 'Physical Layer' (Layer 1) and the 'Data Link Layer' (Layer 2) of the OSI model. A MAC address is a 48-bit network hardware identifier that is permanently set on a ROM chip on the NIC to identify that device on the network