Introduction to computer networking, topologyIn computer networking,topologyrefers to the layout of connected devices. This article introduces the standardtopologiesof networking.Topology in Network DesignThink of a topology as a network's virtual shape or structure. This shape does not necessarily correspond to the actual physical layout of the devices on the network. For example, the computers on a home network may be arranged in a circlein a family room, but it would be highly unlikely to find a ring topology there.Network topologies are categorized into the following basic types: bus ring star tree meshMore complex networks can be built as hybrids of two or more of the above basic topologies.Bus TopologyBus networks (not to be confused with the system bus of a computer) use a commonbackboneto connect all devices. A single cable, the backbone functions as a shared communication medium that devices attach or tap into with an interface connector. A device wanting to communicate with another device on the network sends abroadcast messageonto the wire that all other devices see, but only the intended recipient actually accepts and processes the message.Ethernet bus topologies are relatively easy to install and don't require much cabling compared to the alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popularEthernet cabling optionsmany years ago for bus topologies. However, bus networks work best witha limited number of devices. If more than a few dozen computers are added to a network bus, performance problems will likely result. In addition, if the backbone cable fails, the entire network effectively becomes unusable.Illustration:Bus Topology DiagramRing TopologyIn a ring network, every device has exactly two neighbors for communication purposes. All messages travel through a ring in the same direction (either "clockwise" or "counterclockwise"). A failure in any cable or device breaks the loop and can take down the entire network.To implement a ring network, one typically uses FDDI,SONET, orToken Ringtechnology. Ring topologies are found in some office buildings or school campuses.Illustration:Ring Topology DiagramStar TopologyManyhome networksuse the star topology. A star network features a central connection point called a "hub node" that may be anetwork hub,switchorrouter. Devices typically connect to the hub with Un-shielded Twisted Pair (UTP) Ethernet.Compared to the bus topology, a star network generally requires more cable, but a failure in any star network cable will only take down one computer's network access and not the entireLAN. (If the hub fails, however, the entire network also fails.)Illustration:Star Topology DiagramTree TopologyA tree topology joins multiple star topologies together onto a bus. In its simplest form, only hub devices connect directly to the tree bus, and each hub functions as the root of a tree of devices. This bus/star hybrid approach supports future expansion of the network much better than a bus (limited in the number of devices due to the broadcast traffic it generates) or a star (limited by the number of hub connection points) alone.Illustration:Tree Topology DiagramMesh TopologyMesh topology introduces the concept of routes. Unlike each of the previous topologies, messages sent on a mesh network can take any of several possible paths from source to destination. (Recall that even in a ring, although two cable paths exist, messages can only travel in one direction.) SomeWANs, most notably the Internet, employ mesh routing.A mesh network in which every device connects to every other is called a full mesh. As shown in the illustration below, partialmesh networksalso exist in which some devices connect only indirectly to others.Illustration:Mesh Topology DiagramSummaryTopology remain an important part of network design theory. You can probably build a home or small business computer network without understanding the difference between a bus design and a star design, but becoming familiar with the standard topologies gives you a better understanding of important networking concepts like hubs, broadcasts, and routes.

OSI Model Reference GuideStandard network layer architecture

OSI Model - Upper and Lower Layers.Bradley MitchellTheOpen Systems Interconnection (OSI)reference model has been an essential element of computer network design since its ratification in 1984. The OSI is an abstract model of how network protocols and equipment should communicate and work together (interoperate).The OSI model is a technology standard maintained by the International Standards Organization (ISO). Although today's technologies do notfully conform to the standard, it remains a useful introduction to the study of network architecture.The OSI Model StackThe OSI model divides the complex task of computer-to-computer communications, traditionally calledinternetworking, into a series of stages known aslayers. Layers in the OSI model are ordered from lowest level to highest. Together, these layers comprise the OSI stack. The stack contains seven layers in two groups:Upper layers- 7. application6. presentation5. sessionLower layers- 4. transport3. network2. data link1. physicalMore-OSI Model LayersUpper Layers of the OSI ModelOSI designates the application, presentation, and session stages of the stack as theupper layers. Generally speaking, software in these layers performs application-specific functions like data formatting, encryption, and connection management.Examples of upper layer technologies in the OSI model areHTTP,SSLand NFS.Lower Layers of the OSI ModelThe remaininglower layersof the OSI model provide more primitive network-specific functions like routing, addressing, andflow control. Examples of lower layer technologies in the OSI model areTCP,IP, andEthernet.Benefits of the OSI ModelBy separating the network communications into logical smaller pieces, the OSI model simplifies hownetwork protocolsare designed. The OSI model was designed to ensure different types of equipment (such as networkadapters,hubs, androuters) would all be compatible even if built by different manufacturers. A product from one network equipment vendor that implements OSI Layer 2 functionality, for example, will be much more likely to interoperate with another vendor's OSI Layer 3 product because both vendors are following the same model.The OSI model also makes network designs more extensible as new protocols and other network services are generally easier to add to a layered architecture than to a monolithic one.Introduction to LANs, WANs, and Other Kinds of Area NetworksBy Bradley MitchellOne way to categorize the different types of computer network designs is by their scope or scale. For historical reasons, the networking industry refers to nearly every type of design as some kind ofarea network. Common types of area networks are: LAN -Local Area Network WAN -Wide Area Network WLAN -Wireless Local Area Network MAN - Metropolitan Area Network SAN - Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network CAN - Campus Area Network, Controller Area Network, or sometimes Cluster Area Network PAN - Personal Area NetworkLAN and WAN are the two primary and best-known categories of area networks, while the others have emerged with technology advancesNote that network types differ fromnetwork topologies(such as bus, ring and star). (See also -Introduction to Network Topologies.)LAN - Local Area NetworkALANconnects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. InTCP/IPnetworking, a LAN is often but not always implemented as a single IPsubnet.In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarilyEthernetandToken Ring.WAN - Wide Area NetworkAs the term implies, aWANspans a large physical distance. The Internet is the largest WAN, spanning the Earth.A WAN is a geographically-dispersed collection of LANs. A network device called arouterconnects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address.

A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs tend to use technology likeATM,Frame RelayandX.25for connectivity over the longer distances.LAN, WAN and Home NetworkingResidences typically employ one LAN and connect to the Internet WAN via anInternet Service Provider (ISP)using abroadband modem. The ISP provides a WANIP addressto the modem, and all of the computers on the home network use LAN (so-calledprivate) IP addresses. All computers on the home LAN can communicate directly with each other but must go through a centralnetwork gateway, typically abroadband router, to reach the ISP.Other Types of Area NetworksWhile LAN and WAN are by far the most popular network types mentioned, you may also commonly see references to these others: Wireless Local Area Network- a LAN based onWi-Fiwireless network technology Metropolitan Area Network- a network spanning a physical area larger than a LAN but smaller than a WAN, such as a city. A MAN is typically owned an operated by a single entity such as a government body or large corporation. Campus Area Network- a network spanning multiple LANs but smaller than a MAN, such as on a university or local business campus. Storage Area Network- connects servers to data storage devices through a technology likeFibre Channel. System Area Network(also known as Cluster Area Network).- links high-performance computers with high-speed connections in a cluster configuration.