Computer Network Assignment A134491 & A134338

Introduction
A computer network, often simply referred to as a network, is a group of computers and devices interconnected by communications channels that facilitate communications among users and allows users to share resources. Networks may be classified according to a wide variety of characteristics.A computer network allows sharing of resources and information among interconnected devices. In the 1960s, the Advanced Research Projects Agency (ARPA) started funding the design of the Advanced Research Projects Agency Network (ARPANET) for the United States Department of Defense. It was the firstcomputer network in the world. Development of the network began in 1969, based on designs developed during the 1960s.

Background
On 11 September 1940, George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at Dartmouth College in New Hampshire. This configuration of a centralized computer or mainframe computer with remote "dumb terminals" remained popular throughout the 1950s and into the 60's. However, it was not until the 1960s that researchers started to investigate packet switching - a technology that allows chunks of data to be sent between different computers without first passing through a centralized mainframe. A four-node network emerged on December 5, 1969. This network soon became the ARPANET, which by 1981 would consist of 213 nodes.

ARPANET's development centred around the Request for Comment process and on 7 April 1969, RFC was published. This process is important because ARPANET would eventually merge with other networks to form the Internet, and many of the communication protocols that the Internet relies upon today were specified through the Request for Comment process. In September 1981, RFC 791 introduced the Internet Protocol version 4 (IPv4) and RFC793 introduced the Transmission Control Protocol (TCP) - thus creating the TCP/IP protocol that much of the Internet relies upon today.

However, not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s. A patent for the token ring protocol was filed by Olot Soderblom on October 29, 1974, and a paper on the Ethernet protocol was published by Robert Metcalfe and David boggs in the July 1976 issue of Communications of ACM. The Ethernet protocol had been inspired by the ALOHAnet protocol which had been developed by electrical engineering researchers at then Universiti of Hawaii.

Benefits
A network of computer is a group of inter connected systems that provides sharing services and interacting by means of a shared communications link. Computer networking can provide services and reduce the cost of equipment of an organization. Networking Model may be server based or peer to peer. In server based network model, Server receives requests from client and provides them services, resources were managed centralized. On the other hand in peer to peer networking resources were de-centralized. Some benefits of computer network are given below. (1) Computers which are connected through a network can share resources as hard drives, printers, scanners etc with each other.

(2) They can send file from one computer to another quite easily.

(3) You can connect all the computers which are connected through a network to the internet by using a single line. So it means that you can save the connection cost for each computer but you internet connection must be fast.

(4) IF you want to access data from the other computer which is the part of network then you can access data from that computer.

(5) Users can run those programs which are not installed on their computers but are installed on any other user's computer. This is a very huge advantage, and you can get benefit from it in many circumstances. For example, suppose you have not enough space in your computer to install a program so in this case, if your desired program is available on other user's computer then you can run that. Similarly, if you have space in your computer then you can copy that program other user's computer into your PC.

Network Classification
The following list presents categories used for classifying networks.

Connection method
Computer networks can be classified according to the hardware and software technology that is used to interconnect the individual devices in the network, such as optical fibre, Ethernet, wireless LAN, HomePNA, power line communication or G.hn.

Ethernet uses physical wiring to connect devices. Frequently deployed devices include hubs, switches, bridges, or routers. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium. ITU-T G.hn technology uses existing home wiring ( coaxial cable, phone lines and power lines) to create a high-speed (up to 1 Gigabit/s) cal area network.

Wired technologies
a)Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair wires are ordinary telephone wires which consist of two insulated copper wires twisted into pairs and are used for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnatic induction. The transmission speed ranges from 2 million bits per second to 100 million bits per second.

b)Coaxial cable is widely used for cable television systems, office buildings, and other worksites for local area networks. The cables consist of copper or aluminum wire wrapped with insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer. The layers of insulation help minimize interference and distortion. Transmission speed range from 200 million to more than 500 million bits per second.

c)Optical fibre cable consists of one or more filaments of glass fiber wrapped in protective layers. It transmits light which can travel over extended distances. Fiber-optic cables are not affected by electromagnetic radiation. Transmission speed may reach trillions of bits per second. The transmission speed of fiber optics is hundreds of times faster than for coaxial cables and thousands of times faster than a twisted-pair wire.

Wireless technologies
a)Terrestrial microwave – Terrestrial microwaves use Earth-based transmitter and receiver. The equipment look similar to satellite dishes. Terrestrial microwaves use low-gigahertz range, which limits all communications to line-of-sight. Path between relay stations spaced approx, 30 miles apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.

b)Communications satellites– The satellites use microwave radio as their telecommunications medium which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically 22,000 miles (for geosynchronous satellites) above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.

c)Cellular and PCS systems – Use several radio communications technologies. The systems are divided to different geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.

d)Wireless LANs – Wireless local area network use a high-frequency radio technology similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. An example of open-standards wireless radio-wave technology is IEEE.

e)Infrared communication, which can transmit signals between devices within small distances not more than 10 meters peer to peer or ( face to face ) without any body in the line of transmitting.

Scale
Networks are often classified as local area network (LAN), wide area network (WAN), metropolitan area network (MAN), personal area network (PAN), virtual private network (VPN), campus area network (CAN), storage area network (SAN), and others, depending on their scale, scope and purpose, e.g., controller area network (CAN) usage, trust level, and access right often differ between these types of networks. LANs tend to be designed for internal use by an organization's internal systems and employees in individual physical locations, such as a building, while WANs may connect physically separate parts of an organization and may include connections to third parties

Network topology
Network topology is the layout pattern of interconnections of the various elements (links, nodes, etc.) of a computer network Network topologies may be physical or logical. Physical topology means the physical design of a network including the devices, location and cable installation. Logical topology refers to how data is actually transferred in a network as opposed to its physical design.

Topology can be considered as a virtual shape or structure of a network. This shape does not correspond to the actual physical design of the devices on the computer network. The computers on a home network can be arranged in a circle but it does not necessarily mean that it represents a ring topology.

Any particular network topology is determined only by the graphical mapping of the configuration of physical and/or logical connections between nodes. The study of network topology uses graph teory. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical.

A local area network (LAN) is one example of a network that exhibits both a physical topology and a logical topology. Any given node in the LAN has one or more links to one or more nodes in the network and the mapping of these links and nodes in a graph results in a geometric shape that may be used to describe the physical topology of the network. Likewise, the mapping of the data flow between the nodes in the network determines the logical topology of the network. The physical and logical topologies may or may not be identical in any particular network.

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. Network topology is the coordination by which devices in the network are arranged in their logical relations to one another, independent of physical arrangement. Even if networked computers are physically placed in a linear arrangement and are connected to 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. Networks may be classified based on the method of data used to convey the data, these include digital and analog networks.

Local area network
A local area network (LAN) is a network that connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings. Each computer or device on the network is a node. Current wired LANs are most likely to be based on Ethernet technology, although new standards like ITU-T G.hn also provide a way to create a wired LAN using existing home wires (coaxial cables, phone lines and power lines).

All interconnected devices must understand the network layer (layer 3), because they are handling multiple subnets (the different colors). Those inside the library, which have only 10/100 Mbit/s Ethernet connections to the user device and a Gigabit Ethernet connection to the central router, could be called "layer 3 switches" because they only have Ethernet interfaces and must understand IP. It would be more correct to call them access routers, where the router at the top is a distribution router that connects to the Internet and academic networks' customer access routers.

The defining characteristics of LANs, in contrast to WANs (Wide Area Networks), include their higher data transfer rates, smaller geographic range, and no need for leased telecommunication lines. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s. This is the data transfer rate. IEEE has projects investigating the standardization of 40 and 100 Gbit/s.

Personal area network
A personal area network (PAN) is a computer network used for communication among computer and different information technological devices close to one person. Some examples of devices that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs, scanners, and even video game consoles. A PAN may include wired and wireless devices. The reach of a PAN typically extends to 10 meters.A wired PAN is usually constructed with USB and Firewire connections while technologies such as Bluetooth and infrared communication typically form a wireless PAN.

Home area network
A homearea area network (HAN) is a residential LAN which is used for communication between digital devices typically deployed in the home, usually a small number of personal computers and accessories, such as printers and mobile computing devices. An important function is the sharing of Internet access, often a broadband service through a CATV or Digital subscriber line (DSL) provider. It can also be referred as an office area network (OAN).

Wide area network
A wide area network (WAN) is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances, using a communications channel that combines many types of media such as telephone lines, cables, and air waves. A WAN 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.

Campus network
A campus network is a computer network made up of an interconnection of local area networks (LAN's) within a limited geographical area. The networking equipments (switches, routers) and transmission media (optical fiber, copper plant, Cat5 cabling etc.) are almost entirely owned (by the campus tenant / owner: an enterprise, university, government etc.).

In the case of a university campus-based campus network, the network is likely to link a variety of campus buildings including; academic departments, the university library and student residence halls.

Network interface cards
A network card, network adapter, or NIC (network interface card) is a piece of designed to allow computers to communicate over a computer network. It provides physical access to a networking medium and often provides a low-level addressing system through the use of computer hardware MAC addresses.

Repeaters
A repeater is an electronic device that receives a signal, cleans it of unnecessary noise, regenerates it, and retransmit it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. Repeaters work on the Physical Layer of the OSI model.

Hubs
A network hub contains multiple ports. When a packet arrives at one port, it is copied unmodified to all ports of the hub for transmission. The destination address in the frame is not changed to a broadcast address.It works on the Physical Layer of the OSI model.

Bridges
A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address to that port only.

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:

a)Local bridges: Directly connect local area networks (LANs)

b)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 with routers.

c)Wireless bridges: Can be used to join LANs or connect remote stations to LANs.

Switches
A network switch is a device that forwards and filters OSI layer 2 datagrams (chunk of data communication) between ports (connected cables) based on the MAC addresses in the packets.A switch is distinct from a hub in that it only forwards the frames to the ports involved in the communication rather than all ports connected. A switch breaks the collision domain but represents itself as a broadcast domain. Switches make forwarding decisions of frames on the basis of MAC addresses. A switch normally has numerous ports, facilitating a star topology for devices, and cascading additional switches. Some switches are capable of routing based on Layer 3 addressing or additional logical levels; these are called multi-layer switches. The term switch is used loosely in marketing to encompass devices including routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier).

Routers
A router is an internetworking device that forwards packets between networks by processing information found in the datagram or packet (Internet protocol information from Layer 3 of the OSI model). In many situations, this information is processed in conjunction with the routing table (also known as forwarding table). Routers use routing tables to determine what interface to forward packets (this can include the "null" also known as the "black hole" interface because data can go into it, however, no further processing is done for said data)

Conclusion
Computer networks have been a boon for businesses because they connect different makes and models of microcomputers, minicomputers or even mainframe computers, allowing for shared communication, files and equipment. The first networks appeared in the 1960s when multi-user networks were introduced. In multi-user systems, dumb terminals, which do not have processing capabilities of their own, are connected to a central host computer, which shares processing time with these dumb terminals. Now much more popular are Local Area Networks, or LANS, which appeared in the early 1970s. A LAN is a communication network privately owned by the organization using it. LANs utilize stand-alone microcomputers rather than dumb terminals and can vary greatly in size, range and complexity. The actual distance and number of computers that can be connected is highly dependent upon the type of LAN and communication line used. Telephone lines are sometimes used and are most convenient; however, coaxial cable connections permit faster, higher quality transmissions. Fiber optics, tiny tubes of glass half the diameter of a human hair, has become the preferred technology of the 1990s, allowing faster and less expensive data transmissions than wire cabling. All networks are different in size and complexity. However, they all share one of three similar shapes. These shapes are known as network topologies. There are three basic topologies to which most networks conform: Bus, Ring, and Star. The Bus topology uses a single communication line to connect equipment, allowing contact between all stations, or nodes. Failure of a single micro-computer may interfere with the rest of the network.A Star topology consists of several microcomputers connected to a centralized networking device called a hub. This acts as a crossing guard for network traffic. Star topologies may also allow the computers to connect to a centralized host computer, called the file server, which shares its files among all users of the network. A variation of this, the Multistar network, has several file servers connected to each other, yet each also has a Star network of its own. Star networks are more costly to install due to the extra cabling involved, but are frequently used among retail stores having several branch outlets. The third topology, the Ring, is somewhat similar in design to the Bus topology. Each computer is connected to two other computers and can communicate and share files with any station in the network. However the two ends of the Bus are connected together, making the Bus into a Ring. A Ring network is slower and more expensive to install than the Star, but it is also more reliable, since alternative paths exist if a communications line is interrupted. A modification on the Ring network is the Complex or Distributed network. Communication lines connect all stations in the network, but on the downside, installation is complicated and requires high maintainance. LANs offer many benefits within companies, industrial sites, college campuses and hospitals, allowing for more efficient and convenient computer services. Future predictions call for worldwide interconnected networks resembling the telephone systems of today. Accessing these networks will likely become just as commonplace as making a telephone call. One application of networking, electronic mail (E-Mail), introduced in the early 1970s, is very popular among businesses. E-mail service allows letters to be sent across the United States for less than the cost of mail, with the added advantage of arriving within seconds. Another popular offshoot of networking are Information Networks, such as America Online, CompuServe, or Prodigy, wherein computer users may access a wide variety of computerized services, including E-Mail, investment advice, reference and travel information, shopping and much more.