Physically, a bus describes a network in which all nodes are connected to a common single communication channel or 'bus'. This bus is sometimes called a backbone, as it provides the spine for the network. Every node can hear each message packet as it goes past.
Logically, a bus is distinguished by the fact that packets are broadcast and every node gets the message at the same time. Transmitted packets travel in both directions along the bus, and need not go through the individual nodes, as in a point-to-point system. Rather, each node checks the destination address that is included in the packet to determine whether that packet is intended for the specific node. When the signal reaches the end of the bus, an electrical terminator absorbs the energy to keep it from reflecting back again along the cable and interfering with other messages already on the bus. Both ends of a bus cable must be terminated in this way, so signals are removed from the bus when they reach the end.
In a bus topology, nodes should be far enough apart so that they do not interfere with each other. However, if the bus is too long, it may be necessary to boost the signal strength using some form of amplification or repeater. The maximum length of the bus is limited by the size of the time interval that constitutes 'simultaneous' packet reception. Figure 8.3 illustrates the bus topology.
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