Data Communications Networking Devices - 4th Ed.pdf

276 ________________________________________________________ LOCAL AREA NETWORKSthe headend, feeder cables radiate outward to nodes, which in turn provide workstationswith access to the network. There may also be a feeder cable route toadditional nodes, from which workstations gain access to the network.Mixed topologiesSome network are a mixture of topologies. For example, as previously discussed, atree structure can be viewed as a series of interconnected buses. Another example ofthe mixture of topologies is a type of Ethernet known as 10BASE-T. That networkcan actually be considered a star±bus topology, in which workstations are ®rstconnected to a common device known as a hub, which in turn can be connected toother hubs to expand the network.Comparison of topologiesAlthough there are close relationships among the topology of the network, itstransmission media, and the method used to access the network, we can examinetopology as a separate entity and make several generalized observations. First, in astar network, the failure of the network controller will render the entire networkinoperative. This is because all data ¯ow on the network must pass through thenetwork controller. On the positive side, the star topology normally consists oftelephone wires routed to a switchboard. A local area network that can use in-placetwisted-pair telephone wires in this way is simple to implement and usually veryeconomical.In a ring network, the failure of any node connected to the ring normally inhibitsdata ¯ow around the ring. Due to the fact that data travels in a circular path on aring network, any cable break has the same effect as the failure of the networkcontroller in a star-structured network. Since each network station is connected tothe next network station, it is usually easy to install the cable for a ring network. Incomparison, a star network may require cabling each section to the networkcontroller if existing telephone wires are not available, and this can result in theinstallation of very long cable runs.In a bus-structured network, data is normally transmitted from a single station toall other stations located on the network, with a destination address appended toeach transmitted data block. As part of the access protocol, only the station with thedestination address in the transmitted data block will respond to the data block.This transmission concept means that a break in the bus may be limited to affectingonly network stations on one side of the break that wish to communicate withstations on the other side of the break. Thus, unless a network station functioning asthe primary network storage device becomes inoperative, a failure in a busstructurednetwork is usually less serious than a failure in a ring network. However,some local area networks, such as Token-Ring and FDDI, were designed toovercome the effect of certain types of cable failures. Token-Ring networks includea backup path which, when manually placed into operation, may be able toovercome the effect of a cable failure between hubs referred to as multistation access