Friday, April 4, 2008

COMPUTER NETWORK

INTRODUCTION

If we look by decade, we find that the market was proliferated large computers. A large no. of minicomputer were installed and operated as stand alone systems. The concept then was to centralize the central of all EDP (Electronic Data Processing) operations and users department would only give their requirements and interact with the systems analysts of programmers. As the demands of the users departments grew both in terms of volume and complexity an increasing need of computing power was felt necessary.

The falling prices and the increasingly powerful microcomputer made this possible and offered cost effective solution. The easy availability of the microcomputer with standardized hardware and software made them an extremely viable solution. These Microcomputer also provided a user friendly menu-driven environment where the end users could interact directly with the computer and full comfortable the availability of a large number of applications software for every requirement of area of specialization offered the end users with the tools they were looking for. This reduced their dependence on systems personnel increased the personnel productivity by about 10-15 percent with researchers scientists, technocrats. Students, decision makers and managers with the increasing use it was felt that though there microcomputer offered users computation power of their disk. They could not suffice all their power at disk. They could not suffice all their needs because applications often demanded exchange of data between several such users. The need for communication between these microcomputer was clear and Network offered itself as viable and attractive area solutions.
In the ever-growing requirements of computer users there cluster of microcomputer in a LAN had their own limitations. In fact most applications of there end users required a shared database, which could not be distributed among several workstations in the LAN. Also there users required peripherals like fast printers backup devices etc. This kind of a totally decentralized and distributed environment often forced replication of data.

There was a backward shift and the need was felt for a powerful central facility offering high capacity hard disk storage sharable printers magnetic tapes etc. in the existing distributed. This compromise offered them the most cost effective solutions as.
1. It provided total computing facility at each users end.
2. It provided communications facilities amongst several such users.
3. It offered access to mass storage systems high speed printers and other service computers.
4. It was very economical because these resources were shared by all the users in the LAN.

As an outcome of these powerful features. The overhead on the network was enormous and various techniques started was required for the network management consequent to this certain supplementary.



DEFINITION


A computer network is composed of multiple computers that communicate over a wired of wireless medium to share files and a printers using the network. The size and scalability of any computer network are determined both by the physical medium of communication and by the software controlling the communication.

Experts in the field of networking debate whether two computers that are connected together using some form of communications medium constitute a network. Therefore, some works state that a network require three connected computers. One such source, Telecommunications: Glossary of telecommunication Terms “states that a computer networks is “ A network of data processing nodes that are interconnected for the purpose of data communication”. The term “network” being defined in the same document as “ An interconnection of three of more communicating entities”. A computer connected to a non-computing device may also represent a computer network, although this article does not address this configuration.

This article uses the definition, which require two of more computers to be connected together to form a network. The same basic functions are generally present in this case as with larger no of connected computers. In order for a network to function, it must meet three basic requirements, it most provide connections, communications and services. Connections refers to the hardware, communications is the way in which the devices talk to each other, and services are the things which are shared with the rest of the network.


THE ADVANTAGES OF NETWORKS…

§ You can open previous work from any computer If you have done any work on particular system then you can login on any computer. And can open your work.
§ You can share printers or other resources printer like laser, which can really print on high speed.
§ You can access the Internet throughout the school.
§ You can send instant messages to other computers
§ You can share software across the school.
§ Store copies of programs in locked, read-only spaces on the server.
§ Protect program files from unauthorized copying.
§ Prevent tampering with programs.
§ Save personal work in private spaces on hard disk
§ Password protected from inadvertent or malicious copying and/or deletion.
§ No loss of information from damaged or lost floppy disks..
§ Floppy disks are needed only for archiving old files.
§ Boot disks may be needed to start up workstations (Remote booting removes the need for boot disks.)
§ Share data and program files to all network users.
§ Publicly accessible area to store assignments, tests, clipart, and other files.
§ No copying of entire class sets of data disks.
§ Share the cost of printers among all users .
§ Gain access to different types of printers.
§ Current networking software has built-in print spooling capabilities.
§ Share peripheral devices.
§ Access powerful information resources (CD-ROM drives)
§ Network modems to access outside resources
o Library card catalogues
o Databases from all over the world
o Wire services (newspapers, radio stations, television stations)
§ Well-planned network meets needs for several years.
§ Single major task of maintenance of boot disks for workstations.


CLASSIFICATION OF COMPUTER NETWORKS
By network layer
Computer networks may be classified according to the network layer at which they operate according to some basic reference models that are considered to be standards in the industry such as the seven layer OSI reference model and the four layer Internet Protocol Suite model. In practice, the great majority of networks use the Internet Protocol (IP) as their network layer. Some networks, however, are using IP Version 6 Ipv6, usually in coexistence with Ipv4. Ipv6 use is often experimental. It is an interconnection of a group of computers in other words.
A network as simple as two computers linked with a crossover cable has several points at which the network could fail: either network interface, and the cable. Large networks, without careful design, can have many points at which a single failure could disable the network.
When networks are critical the general rule is that they should have no single point of failure. The broad factors that can bring down networks, according to the Software Engineering Institute at Carnegie-Mellon University:
Attacks: these include software attacks by various miscreants (e.g., malicious hackers, computer criminals) as well as physical destruction of facilities.
Failures: these are in no way deliberate, but range from human error in entering commands, bugs in network element executable code, failures of electronic components, and other things that involve deliberate human action or system design.
Accidents: Ranging from spilling coffee into a network element to a natural disaster or war that destroys a data center, these are largely unpredictable events. Survivability from severe accidents will require physically diverse, redundant facilities. Among the extreme protections against both accidents and attacks are airborne command posts and communications relays which either are continuously in the air, or take off on warning. In like manner, systems of communications satellites may have standby spares in space, which can be activated and brought into the constellation.
Dealing with Power Failures
One obvious form of failure is the loss of electrical power. Depending on the criticality and budget of the network, protection from power failures can range from simple filters against excessive voltage spikes, to consumer-grade Uninterruptible Power Supplies(UPS) that can protect against loss of commercial power for a few minutes, to independent generators with large battery banks. Critical installations may switch from commercial to internal power in the event of a brownout,where the voltage level is below the normal minimum level specified for the system. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage.
Some network elements operate in a manner to protect themselves and shut down gracefully in the event of a loss of power. These might include noncritical application and network management servers, but not true network elements such as routers. UPS may provide a signal called the “Power-Good” signal. Its purpose is to tell the computer all is well with the power supply and that the computer can continue to operate normally. If the Power-Good signal is not present, the computer shuts down. The Power-Good signal prevents the computer from attempting to operate on improper voltages and damaging itself
To help standardize approaches to power failures, the Advanced Configuration and Power Interface (ACPI) specification is an open industry standard first released in December 1996 developed by HP, Intel, Microsoft, Phoenix and Toshiba that defines common interfaces for hardware recognition, motherboard and device configuration and power management.

By scale
Computer networks may be classified according to the scale: Personal Area Network (PAN), Local Area Network, Campus Area Network, Metropolitan area network (MAN), or Wide area network (WAN). As Ethernet increasingly is the standard interface to networks, these distinctions are more important to the network administrator than the end user. Network administrators may have to tune the network, based on delay that derives from distance, to achieve the desired Quality of Service (QoS).
Controller Area Networks are a special niche, as in control of a vehicle’s engine, a boat’s electronics, or a set of factory robots.

By connection method
Computer networks may be classified according to the hardware technology that is used to connect the individual devices in the network such as Ethernet, Wireless LAN, HomePNA, or Power line communication.

By functional relationship
Computer networks may be classified according to the functional relationships which exist between the elements of the network, for example Active Networking, Client-server and Peer-to-peer (workgroup) architectures.

By network topology
Computer networks may be classified according to the network topology upon which the network is based, such as Bus network, Star network, Ring network, Mesh network, Star-bus network, Tree or Hierarchical topology network, etc.
Network Topology signifies the way in which intelligent devices in the network see their logical relations to one another. The use of the term “logical” here is significant. That is, network topology is independent of the “physical” layout of the network. Even if networked computers are physically placed in a linear arrangement, if they are connected via a hub, the network has a Star topology, rather than a Bus Topology. In this regard the visual and operational characteristics of a network are distinct.

By protocol
Computer networks may be classified according to the communications protocol that is being used on the network. See the articles on List of network protocol stacks and List of network protocols for more information

Types of networks:
Below is a list of the most common types of computer networks in order of scale.

Personal Area Network (PAN)
A personal area network (PAN) is a computer network used for communication among computer devices (including telephones and personal digital assistants) close to one person. The devices may or may not belong to the person in question. The reach of a PAN is typically a few meters. PANs can be used for communication among the personal devices themselves (intrapersonal communication), or for connecting to a higher level network and the Internet (an uplink).
Personal area networks may be wired with computer buses such as USB and FireWire. A wireless personal area network (WPAN) can also be made possible with network technologies such as IrDA and Bluetooth.

Local Area Network (LAN)
A network covering a small geographic area, like a home, office, or building. Current LANs are most likely to be based on Ethernet technology. The defining characteristics of LANs, in contrast to WANs (wide area networks), include their much higher data transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines. Hosts can be made part of a specific LAN can be defined by setting their address to one within the address range of the LAN subnet This can be done by manual configuration, or by configuring DHCPDynamic Host Configuration Protocol autoconfiguration to give the host an address in the appropriate range.
Currently standardized LAN technologies operate at speeds up to 10 Gbit/s. IEEE has projects investigating the standardization of 100 Gbit/s, and possibly 40 Gbit/s. Inverse multiplexing is commonly used to build a faster aggregate from slower physical streams, such as bringing 4 Gbit/s aggregate stream into a computer or network element with four 1 Gbit/s interfaces.

Campus Area Network (CAN)
A network that connects two or more LANs but that is limited to a specific and contiguous geographical area such as a college campus, industrial complex, or a military base. A CAN, may be considered a type of MAN (metropolitan area network), but is generally limited to an area that is smaller than a typical MAN, so it is called a CAN.
This term is most often used to discuss the implementation of networks for a contiguous area. In the past, when layer 2 switching (i.e., bridging (networking) was cheaper than routing, campuses were good candidates for layer 2 networks, until they grew to very large size. Today, a campus may use a mixture of routing and bridging. The network elements used, called “campus switches”, tend to be optimized to have many Ethernet interfaces rather than an arbitrary mixture of Ethernet and WAN interfaces.

Metropolitan Area Network (MAN)
A network that connects two or more Local Area Networks or CAN together but does not extend beyond the boundaries of the immediate town, city, or metropolitan area. Multiple routers, switches & hubs are connected to create a MAN

Wide Area Network (WAN)
A WAN is a data communications network that covers a relatively broad geographic area (i.e. one country to another and one continent to another continent) and that often uses transmission facilities provided by common carriers, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI reference model: the physical layer, the data link layer, and the network layer.
The highest data rate commercially available, as a single bitstream, on WANs is 40 Gbit/s, principally used between large service providers. Wavelength Division Multiplexing, however, can put multiple 10 or 40 Gbyte/s streams onto the same optical fiber.

Global Area Network (GAN)
Global area networks (GAN) specifications are in development by several groups, and there is no common definition. In general, however, a GAN is a model for supporting mobile communications across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is “handing off” the user communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial Wireless local area networks (WLAN) is the [3]. INMARSAT has defined a satellite-based Broadband Global Area Network (BGAN).
IEEE mobility efforts focus on the data link layer and make assumptions about the media. Mobile IP is a network layer technique, developed by the IETF, which is independent of the media type and can run over different media while still keeping the connection.

Internetwork
Two or more networks or network segments connected using devices that operate at layer 3 (the ‘network’ layer) of the OSI Basic Reference Model, such as a router. Any interconnection among or between public, private, com, or governmental networks may also be defined as an internetwork.
In modern practice, the interconnected networks use the Internet Protocol. There are at least three variants of internetwork, depending on who administers and who participates in them:
Intranet
Extranet
“The” Internet
Intranets and extranets may or may not have connections to the Internet. If connected to the Internet, the intranet or extranet is normally protected from being accessed from the Internet without proper authorization. The Internet itself is not considered to be a part of the intranet or extranet, although the Internet may serve as a portal for access to portions of an extranet.

Intranet
An intranet is a set of interconnected networks, using the Internet Protocol and uses IP-based tools such as web browsers, that is under the control of a single administrative entity. That administrative entity closes the intranet to the rest of the world, and allows only specific users. Most commonly, an intranet is the internal network of a company or other enterprise.

Extranet
A extranet is network or internetwork that is limited in scope to a single organization or entity but which also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities (e.g., a company’s customers may be provided access to some part of its intranet thusly creating an extranet while at the same time the customers may not be considered ‘trusted’ from a security standpoint). Technically, an extranet may also be categorized as a CAN, MAN, WAN, or other type of network, although, by definition, an extranet cannot consist of a single LAN, because an extranet must have at least one connection with an outside network.

Internet, The
A specific internetwork, consisting of a worldwide interconnection of governmental, academic, public, and private networks based upon the Advanced Research Projects Agency Network (ARPANET) developed by ARPA of the U.S. Department of Defense – also home to the World Wide Web (WWW) and referred to as the ‘Internet’ with a capital ‘I’ to distinguish it from other generic internetworks.
Obtained from address registries that control assignments. Service providers and large enterprises also exchange information on the reachability of their address ranges through the Border Gateway Protocol.

Basic Hardware Components
All networks are made up of basic hardware building blocks to interconnect network nodes, such as Network Interface Cards (NICs), Bridges, Hubs, Switches, and Routers. In addition, some method of connecting these building blocks is required, usually in the form of galvanic cable (most commonly Category 5 cable). Less common are microwave links (as in IEEE 802.11) or optical cable (“optical fiber”).
Network Interface Cards
A network card, network adapter or NIC (network interface card) is a piece of computer hardware designed to allow computers to communicate over a computer network. It provides physical access to a networking medium and provides a low-level addressing system through the use of MAC addresses. It allows users to connect to each other either by using cables or wirelessly.

Repeaters
A Because repeaters work with the actual physical signal, and do not attempt to interpret the data being transmitted, they operate on the Physical layer, the first layer of the OSI model.BY HAYTECH

Hubs
A hub contains multiple ports. When a packet arrives at one port, it is copied the packets to all the ports of the hub. When the packets are copied, the destination address in the frame does not change to a broadcast address. It does this in a rudimentary way, it simply copies the data to all of the Nodes connected to the hub.

Bridges
A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges do not promiscuously copy traffic to all ports, as does a hub. But learns which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address only to that port. Bridges do send broadcasts to all ports except the one on which the broadcast was received.
Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.
Bridges come in three basic types:
Local bridges: Directly connect local area networks (LANs)
Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced by routers.
Wireless bridges: Can be used to join LANs or connect remote stations to LANs
Switches
Switches are a marketing term that encompasses routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier). Switches may operate at one or more OSI layers, including physical, data link, network, or transport (i.e., end-to-end). A device that operates simultaneously at more than one of these layers is called a multilayer switch.
Overemphasizing the ill-defined term “switch” often leads to confusion when first trying to understand networking. Many experienced network designers and operators recommend starting with the logic of devices dealing with only one protocol level, not all of which are covered by OSI. Multilayer device selection is an advanced topic that may lead to selecting particular implementations, but multilayer switching is simply not a real-world design concept.
Routers
Routers are the networking device that forwards data packets along networks by using headers and forwarding tables to determine the best path to forward the packets. Routers work at the network layer (layer 3) of the OSI model. Routers also provide interconnectivity between like and unlike media. This is accomplished by examining the Header of a data packet.They use routing protocols such as Open Shortest Path First (OSPF) to communicate with each other and configure the best route between any two hosts. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP’s network. Some DSL and Cable Modems have been integrated with routers for home consumers.


CONCLUSION –
We concluded that due to we network we can share any data & any resource only in few seconds from any where in this world. We can use the hardware devices like printers.

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