If the computers on the network connect via the same piece of cable, what topology is employed?11/14/2022 We will look at a more realistic detailed example in Section 13.2.6.Įxample 9.1 Consider the network shown in Figure 9.27(a), with three light-paths to be supported. Here we provide a simple example to illustrate the efficiency of mesh protection relative to ring protection. Efficiency improvements ranging from 20 to 60% have been reported for mesh protection schemes relative to ring protection schemes. In contrast, if traffic in the network is distributed, then rings are inefficient: many lightpaths will need to be partitioned into multiple rings, and multiple rings will need to be interconnected and protected to support these lightpaths. Also, if traffic in the network is primarily localized, then rings can do a good job. In general, the more dense or meshed the topology, the greater the benefit of mesh protection. The bandwidth efficiency of a mesh relative to a ring depends on several factors, including the network topology, the traffic pattern, and the type of mesh protection scheme used. For such networks, mesh protection schemes offer more bandwidth-efficient protection than rings. A typical North American long-haul carrier's backbone network may have, say, 50 nodes, with an average node having 3 to 4 adjacent nodes, with some nodes having as many as 5 to 10 adjacent nodes. Many backbone networks tend to be somewhat more densely connected than rings and are essentially meshed, with traffic being fairly distributed. Ring architectures are inherently suitable for sparse physical topologies and in situations where most of the traffic is confined within the ring. In a full mesh network, each pair of computers is directly connected. For example, a mesh topology is similar to a star with point-to-point connections between devices that are not in the center of the star. There are also variations on the basic physical topologies. The circuitry of the central device replicates the electrical activity of a logical bus or ring-signals are broadcast or passed sequentially within the central device. Networks with both broadcast and sequential logical topologies (i.e., logical buses and rings) are usually implemented today as physical stars. However, in the star, since all communication passes through the central device, if it is not working, there is no network. Problems on a ring or bus cannot be isolated as easily. If a computer on a star has trouble receiving or sending data, the problem must be between that computer and its connection to the device at the center of the star. A star is easier to troubleshoot than is a physical ring or bus. Various types of hardware, such as multiport repeaters or switches, can serve as the central device, depending on the network architecture. Therefore, any problems in a particular computer or the media connecting it to the central device do not affect the rest of the network. In a star, each node communicates directly with only the central device in a point-to-point connection. Therefore, a physical ring has the same shortcoming as a physical bus topology: a cable break or loose connection in the network could cause the whole network to stop working. In a ring each device physically removes a frame from the media and regenerates it. A drawback to a bus network is that a cable break or loose connection in the network will cause the network to stop working. Each end of the bus must be terminated to prevent signal loss and echoes. Signals propagate along the entire length of the bus. In a bus physical topology, all devices are connected in a line along a single channel. That is, the cables in a network approximate the shape of a bus, ring, or star. There are three basic physical topologies: bus, ring, and star. Judy Wynekoop, in Encyclopedia of Information Systems, 2003 III.A.2.
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