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# Network topology

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• 1. Network topologyNetwork topology is the layout pattern of interconnections of the various elements (links, nodes, etc.) of acomputer network. Network topologies may be physical or logical. Physical topology means the physical designof a network including the devices, location and cable installation. Logical topology refers to how data is actuallytransferred 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 theactual physical design of the devices on the computer network. The computers on a home network can bearranged in a circle but it does not necessarily mean that it represents a ring topology.A local area network (LAN) is one example of a network that exhibits both a physical topology and a logicaltopology. Any given node in the LAN has one or more links to one or more nodes in the network and the mappingof these links and nodes in a graph results in a geometric shape that may be used to describe the physical topologyof the network. Likewise, the mapping of the data flow between the nodes in the network determines the logicaltopology of the network. The physical and logical topologies may or may not be identical in any particularnetwork.Basic topology typesB Bus topologyB Star topologyB Ring topologyB Tree topologyB Mesh topologyNote: The physical topology of a network is determined by the capabilities of the network access devices andmedia, the level of control or fault tolerance desired, and the cost associated with cabling or telecommunicationscircuits.Networks can also be classified according to their physical span as follows: • LANs (Local Area Networks) • Building or campus inter-networks • Wide area inter-networks
• 2. Classification of network topologiesThere are also two basic categories of network topologies: • Physical topologies • Logical topologiesThe shape of the cabling layout used to link devices is called the physical topology of the network. This refers tohow the cables are laid out to connect many computers to one network. The physical topology you choose foryour network influences and is influenced by several factors: • Office Layout • Troubleshooting Techniques • Cost of Installation • Type of cable usedLogical topology describes the way in which a network transmits information from network/computer to anotherand not the way the network looks or how it is laid out. The logical layout also describes the different speeds ofthe cables being used from one network to another.
• 3. Physical topologiesThe mapping of the nodes of a network and the physical connections between them – the layout of wiring, cables,the locations of nodes, and the interconnections between the nodes and the cabling or wiring system.Classification of physical topologies:Bus TopologyIn local area networks where bus topology is used, eachmachine is connected to a single cable. Each computer orserver is connected to the single bus cable through somekind of connector. A terminator is required at each end ofthe bus cable to prevent the signal from bouncing back andforth on the bus cable. A signal from the source travels inboth directions to all machines connected on the bus cableuntil it finds the MAC address or IP address on the networkthat is the intended recipient. If the machine address does not match the intended address for the data, the machineignores the data. Alternatively, if the data does match the machine address, the data is accepted. Since the bustopology consists of only one wire, it is rather inexpensive to implement when compared to other topologies.However, the low cost of implementing the technology is offset by the high cost of managing the network.Additionally, since only one cable is utilized, it can be the single point of failure. If the network cable breaks, theentire network will be down.Notes:1.) All of the endpoints of the common transmission medium are normally terminated with a device called aterminator (see the note under linear bus).2.) The physical linear bus topology is sometimes considered to be a special case of the physical distributed bustopology – i.e., a distributed bus with no branching segments.3.) The physical distributed bus topology is sometimes incorrectly referred to as a physical tree topology –however, although the physical distributed bus topology resembles the physical tree topology, it differs from thephysical tree topology in that there is no central node to which any other nodes are connected, since thishierarchical functionality is replaced by the common bus.