Computer Network Assignment A133222 & A134388 & A131166

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.It is a scientific journal of computer and telecommunications networking published by Elsevier.

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 first computer network in the world. Development of the network began in 1969, based on designs developed during the 1960s.

 

BACKGROUND
Prior to the widespread inter-networking that led to the Internet, most communication networks were limited by their nature to only allow communications between the stations on the network. Some networks had gateways or bridges between them, but these bridges were often limited or built specifically for a single use. One prevalent computer networking method was based on the central mainframe method, simply allowing its terminals to be connected via long leased lines. This method was used in the 1950s by Project RAND to support researchers such as Herbert Simon, in Pittsburgh, Pennsylvania, when collaborating across the continent with researchers in Santa Monica, California, on automated theorem proving and artificial intelligence[12].

INTERNER ANCESTORS
At the core of the inter-networking problem lay the issue of connecting separate physical networks to form one logical network. During the 1960s, several groups worked on and implemented packet switching. Donald Davies (NPL), Paul Baran (RAND Corporation) and Leonard Kleinrock (MIT) are credited with the simultaneous invention. The notion that the Internet was developed to survive a nuclear attack has its roots in the early theories developed by RAND. Baran's research had approached packet switching from studies of decentralisation to avoid combat damage compromising the entire network[13].

ARPANET

 * Promoted to the head of the information processing office at ARPA, Robert Taylor intended to realize Licklider's ideas of an interconnected networking system. Bringing in Larry Roberts from MIT, he initiated a project to build such a network. The first ARPANET link was established between the University of California, Los Angeles and the Stanford Research Institute on 21 November 1969. By 5 December 1969, a 4-node network was connected by adding the University of Utah and the University of California, Santa Barbara. Building on ideas developed in ALOHAnet, the ARPANET started in 1972 and was growing rapidly by 1981. The number of hosts had grown to 213, with a new host being added approximately every twenty days[14].


 * ARPANET became the technical core of what would become the Internet, and a primary tool in developing the technologies used. ARPANET development was centred around the Request for Comments (RFC) process, still used today for proposing and distributing Internet Protocols and Systems. RFC 1, entitled "Host Software", was written by Steve Crocker from the University of California, Los Angeles, and published on April 7, 1969. International collaborations on ARPANET were sparse. For various political reasons, European developers were concerned with developing the X.25 networks. Notable exceptions were the Norwegian Seismic Array (NORSAR) in 1972, followed in 1973 by Sweden with satellite links to the Tanum Earth Station and University College London. These early years were documented in the 1972 film Computer Networks: The Heralds of Resource Sharing[14].

X.25 and Public Access

 * Following on from DARPA's research, packet switching networks were developed by the International Telecommunication Union (ITU) in the form of X.25 networks. In 1974, X.25 formed the basis for the SERCnet network between British academic and research sites, which would later become JANET. The initial ITU Standard on X.25 was approved in March 1976.


 * The British Post Office, Western Union International and Tymnet collaborated to create the first international packet switched network, referred to as the International Packet Switched Service (IPSS), in 1978. This network grew from Europe and the US to cover Canada, Hong Kong and Australia by 1981. By the 1990s it provided a worldwide networking infrastructure.


 * Unlike ARPAnet, X.25 was also commonly available for business use. X.25 would be used for the first dial-in public access networks, such as Compuserve and Tymnet. In 1979, CompuServe became the first service to offer electronic mail capabilities and technical support to personal computer users. The company broke new ground again in 1980 as the first to offer real-time chat with its CB Simulator. There were also the America Online (AOL) and Prodigy dial in networks and many bulletin board system (BBS) networks such as The WELL and FidoNet. FidoNet in particular was popular amongst hobbyist computer users, many of them hackers and radio amateurs[12].

UUCP

 * In 1979, two students at Duke University, Tom Truscott and Jim Ellis, came up with the idea of using simple Bourne shell scripts to transfer news and messages on a serial line with nearby University of North Carolina at Chapel Hill. Following public release of the software, the mesh of UUCP hosts forwarding on the Usenet news rapidly expanded. UUCPnet, as it would later be named, also created gateways and links between FidoNet and dial-up BBS hosts. UUCP networks spread quickly due to the lower costs involved, and ability to use existing leased lines, X.25 links or even ARPANET connections. By 1983 the number of UUCP hosts had grown to 550, nearly doubling to 940 in 1984[12].

NETWORK PROTOCOLS, TCP/IP
With so many different network methods, something needed to unify them. Robert E. Kahn of DARPA and ARPANET recruited Vint Cerf of Stanford University to work with him on the problem. By 1973, they had soon worked out a fundamental reformulation, where the differences between network protocols were hidden by using a common internetwork protocol, and instead of the network being responsible for reliability, as in the ARPANET, the hosts became responsible. Cerf credits Hubert Zimmerman and Louis Pouzin (designer of the CYCLADES network) with important work on this design[15].

With the role of the network reduced to the bare minimum, it became possible to join almost any networks together, no matter what their characteristics were, thereby solving Kahn's initial problem. DARPA agreed to fund development of prototype software, and after several years of work, the first somewhat crude demonstration of what had by then become TCP/IP occurred in July 1977. This new method quickly spread across the networks, and on January 1, 1983, TCP/IP protocols became the only approved protocol on the ARPANET, replacing the earlier NCP protocol.

After the ARPANET had been up and running for several years, ARPA looked for another agency to hand off the network to; ARPA's primary business was funding cutting-edge research and development, not running a communications utility. Eventually, in July 1975, the network had been turned over to the Defense Communications Agency, also part of the Department of Defense. In 1984, the U.S. military portion of the ARPANET was broken off as a separate network, the MILNET.

The networks based around the ARPANET were government funded and therefore restricted to noncommercial uses such as research; unrelated commercial use was strictly forbidden. This initially restricted connections to military sites and universities. During the 1980s, the connections expanded to more educational institutions, and even to a growing number of companies such as Digital Equipment Corporation and Hewlett-Packard, which were participating in research projects or providing services to those who were[14].

Another branch of the U.S. government, the National Science Foundation (NSF), became heavily involved in internet research and started development of a successor to ARPANET. In 1984 this resulted in the first Wide Area Network designed specifically to use TCP/IP. This grew into the NSFNet backbone, established in 1986, and intended to connect and provide access to a number of supercomputing centres established by the NSF. It was around the time when ARPANET began to merge with NSFNet, that the term Internet originated,[10] with "an internet" meaning any network using TCP/IP. "The Internet" came to mean a global and large network using TCP/IP, which at the time meant NSFNet and ARPANET. Previously ``internet" and ``internetwork" had been used interchangeably, and "internet protocol" had been used to refer to other networking systems such as Xerox Network Services.

As interest in wide spread networking grew and new applications for it arrived, the Internet's technologies spread throughout the rest of the world. TCP/IP's network-agnostic approach meant that it was easy to use any existing network infrastructure, such as the IPSS X.25 network, to carry Internet traffic. In 1984, University College London replaced its transatlantic satellite links with TCP/IP over IPSS.

Many sites unable to link directly to the Internet started to create simple gateways to allow transfer of e-mail, at that time the most important application. Sites which only had intermittent connections used UUCP or FidoNet and relied on the gateways between these networks and the Internet. Some gateway services went beyond simple e-mail peering, such as allowing access to FTP sites via UUCP or e-mail.

The first ARPANet connection outside the US was established to NORSAR in Norway in 1973, just ahead of the connection to Great Britain. These links were all converted to TCP/IP in 1982, at the same time as the rest of the Arpanet[14].

BENEFITS
Following are some of the advantages of computer networks.


 * File Sharing:  Networks offer a quick and easy way to share files directly. Instead of using a disk or USB key to carry files from one computer or office to another, you can share files directly using a network.
 * Software Cost and Management:  Many popular software products are available for networks at a substantial savings in comparison to buying individually licensed copied for all of your computers. You can also load software on only the file server which saves time compared to installing and tracking files on independent computers. Upgrades are also easier because changes only have to be done once on the file server instead of on individual workstations.
 * Security: Specific directories can be password protected to limit access to authorized users. Also, files and programs on a network can be designated as "copy inhibit" so you don’t have to worry about the illegal copying of programs.
 * Resource Sharing: All computers in the network can share resources such as printers, fax machines, modems, and scanners.
 * Communication:  Even outside of the internet, those on the network can communicate with each other via electronic mail over the network system. When connected to the internet, network users can communicate with people around the world via the network.
 * Flexible Access: Networks allow their users to access files from computers throughout the network. This means that a user can begin work on a project on one computer and finish up on another. Multiple users can also collaborate on the same project through the network.
 * Workgroup Computing:  Workgroup software like Microsoft BackOffice enables many users to contribute to a document concurrently. This allows for interactive teamwork.

Information superhighway :


Formally, networks are composed of links that connect nodes. It is inherent in the structure of a network that many components of a network are required for the provision of a typical service. Thus, network components are complementary to each other. Figure 1, represents the emerging Information Superhighway network. Clearly, services demanded by consumers are composed of many complementarycomponents. For example, interactive ordering while browsing in a "department store" as it appears in successive video frames requires a number of components: a database engine at the service provider, transmission of signals, decoding through an interface, display on a TV or computer monitor, etc. Clearly, there are close substitutes for each of these components; for example, transmission can be done through a cable TV line, a fixed telephone line, a wireless satellite, PCN, etc.; the in-home interface may be a TV-top box or an add-on to a PC, etc. It is likely that the combinations of various components will not result in identical services. Thus, the information superhighway will provide substitutes made of complements; this is a typical feature of networks.

Figure 2 shows a simple star telephone network. A phone call from A to B is composed of AS (access to the switch of customer A), BS (access to the switch of customer B), and switching services at S. Despite the fact that goods AS and BS look very similar and have the same industrial classification, they are complements and not substitutes. 3

Simple star network :


Networks where services AB and BA are distinct are named "two-way" networks in Economides and White (1994). Two-way networks include railroad, road, and many telecommunications networks. When one of AB or BA is unfeasible, or does not make economic sense, or when there is no sense of direction in the network so that AB and BA are identical, then the network is called a one-way network. In a typical one-way network, there are two types of components, and composite goods are formed only by combining a component of each type, and customers are often not identified with components but instead demand composite goods. For example, broadcasting and paging are one-way networks.4

Simple local and long distance network :


The classification in network type (one-way or two-way) is not a function of the topological structure of the network. Rather, it depends on the interpretation of the structure to represent a specific service. For example, the network of Figure 3 can be interpreted as a two-way telephone network where SA represents a local switch in city A, Ai represents a customer in city A, and similarly for SB and Bj.5 In this network, there are two types of local phone calls AiSAAk and BjSBBl, as well as long distance phone call AiSASBBj. We can also interpret the network of Figure 3 as an Automatic Teller Machine network. Then a transaction (say a withdrawal) from bank Bj from ATM Ai is AiSASBBj. Connections AiSAAk and BjSBBl may be feasible but there is no demand for them.

Pair of vertically-related markets :


We have pointed out earlier that the crucial relationship in both one-way and two-way networks is the complementarity between the pieces of the network. This crucial economic relationship is also often observed between different classes of goods in non-network industries. In fact, Economides and White (1994) point out that a pair of vertically-related industries is formally equivalent to a one-way network. Figure 4 can represent two industries of complementary goods A and B, where consumers demand combinations AiBj. Notice that this formulation is formally identical to our long-distance network of Figure 3 in the ATM interpretation.

The discussion so far was carried under the assumption of compatibility, i.e., that various links and nodes on the network are costlessly combinable to produce demanded goods. We have pointed out that links on a network are potentially complementary, but it is compatibility that makes complementarity actual. Some network goods and some vertically related goods are immediately combinable because of their inherent properties. However, for many complex products, actual comlementarity can be achieved only through the adherence to specific technical compatibility standards. Thus, many providers of network or vertically-related goods have the option of making their products partially or fully incompatible with components produced by other firms. This can be done through the creation of proprietary designs or the outright exclusion or refusal to interconnect with some firms.

Traditionally, networks were analyzed under the assumption that each network was owned by a single firm. Thus, economic research focussed on efficient use of the network structure as well as on the appropriate allocation of costs.6 In the '70s, partly prompted by the antitrust suit against AT&T, there was a considerable amount of research on economies of scope, i.e., on the efficiency gains from joint operation of complementary components of networks.7

Once one of the most important networks (the AT&T telecommunications network in the US) was broken to pieces, economic research focused in the '80s and '90s on issues of interconnection and compatibility. Similar research on issues of compatibility was prompted by the reduced role of IBM in the '80s and '90s in the setting of technical standards in computer hardware and software. Significant reductions in costs also contributed and will contribute to the transformation toward fragmented ownership in the telecommunications sector in both the United States and abroad. Costs of transmission have fallen dramatically with the introduction of fiberoptic lines. Switching costs have followed the fast cost decreases of microchips and integrated circuits. These cost reductions have transformed the telecommunications industry from a natural monopoly to an oligopoly. The same cost reductions have made many new services, such as interactive video and interactive games, feasible at low cost. Technological change now allows for joint transmission of digital signals of various communications services. Thus, the monopoly of the last link closest to home is in the process of being eliminated,8 since both telephone lines and cable lines (and in some cases PCS and terrestrial satellites) will provide similar services. 9, 10

In a network where complementary as well as substitute links are owned by different firms, the questions of interconnection, compatibility, interoperability, and coordination of quality of services become of paramount importance. We will examine these issues in detail in the next few sections. We first focus on a fundamental property of networks, i.e., the fact that they exhibit network externalities.

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).

Global area network

 * A global area network (GAN) is a network used 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 LANs.[5]

Internet

 * The Internet is a global system of interconnected governmental, academic, corporate, public, and private computer networks. It is based on the networking technologies of the Internet Protocol Suite. It is the successor of the Advanced Research Projects Agency Network (ARPANET) developed by DARPA of the United States Department of Defense. The Internet is also the communications backbone underlying the World Wide Web (WWW).


 * Participants in the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet Protocol Suite and an addressing system (IP addresses) administered by the Internet Assigned Numbers Authority and address registries. Service providers and large enterprises exchange information about the reachability of their address spaces through the Border Gateway Protocol (BGP), forming a redundant worldwide mesh of transmission paths.

Intranets and extranets

 * Intranets and extranets are parts or extensions of a computer network, usually a local area network.


 * An intranet is a set of networks, using the Internet Protocol and IP-based tools such as web browsers and file transfer applications, that is under the control of a single administrative entity. That administrative entity closes the intranet to all but specific, authorized users. Most commonly, an intranet is the internal network of an organization. A large intranet will typically have at least one web server to provide users with organizational information.


 * An extranet is a network that is limited in scope to a single organization or entity and also has limited connections to the networks of one or more other usually, but not necessarily, trusted organizations or entities—a company's customers may be given access to some part of its intranet—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 an extranet cannot consist of a single LAN; it must have at least one connection with an external network.

Overlay network



 * An overlay network is a virtual computer network that is built on top of another network. Nodes in the overlay are connected by virtual or logical links, each of which corresponds to a path, perhaps through many physical links, in the underlying network.


 * For example, many peer-to-peer networks are overlay networks because they are organized as nodes of a virtual system of links run on top of the Internet. The Internet was initially built as an overlay on the telephone network .[6]


 * Overlay networks have been around since the invention of networking when computer systems were connected over telephone lines using modem, before any data network existed.


 * Nowadays the Internet is the basis for many overlaid networks that can be constructed to permit routing of messages to destinations not specified by an IP address. For example, distributed hash tables can be used to route messages to a node having a specific logical address, whose IP address is not known in advance.


 * Overlay networks have also been proposed as a way to improve Internet routing, such as through quality of service guarantees to achieve higher-quality streaming media. Previous proposals such as IntServ, DiffServ, and IP Multicast have not seen wide acceptance largely because they require modification of all routers in the network.[citation needed] On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from Internet service providers. The overlay has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes a message traverses before reaching its destination.


 * For example, Akamai Technologies manages an overlay network that provides reliable, efficient content delivery (a kind of multicast). Academic research includes End System Multicast and Overcast for multicast; RON (Resilient Overlay Network) for resilient routing; and OverQoS for quality of service guarantees, among others.

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 802.12) 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 often provides a low-level addressing system through the use of MAC addresses.

Repeaters
A repeater is an electronic device that receives a signal, cleans it of unnecessary noise, regenerates it, and retransmits 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.[7] 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:
 * 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 with routers.
 * 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.[8] 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.[9] 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
While the age-old concept of the network is foundational in virtually all areas of society, Computer Networks and Protocols have forever changed the way humans will work, play, and communicate. Forging powerfully into areas of our lives that no one had expected, digital networking is further empowering us for the future. New protocols and standards will emerge, new applications will be conceived, and our lives will be further changed and enhanced. While the new will only be better, the majority of digital networking's current technologies are not cutting-edge, but rather are protocols and standards conceived at the dawn of the digital networking age that have stood solid for over thirty years.