The Bus Network

shown in figure 1-2. Only one cable is required from the central computer to each PC’s network interface card to tie that workstation to the LAN. The star is one of the earliest types of network topologies. It uses the same approach to sending and receiving messages as our phone system. Just as a telephone call from one person to another is handled by a central switching station, all messages must go through the central computer or network server that controls the flow of data. You can easily add new workstations to the network without interrupting other nodes. This is one of the advantages of the star topology. Another advantage of star topology is that the network administrator can give selected nodes a higher priority status than others. The central computer looks for signals from these higher priority workstations before recognizing other nodes. Also, the star topology permits centralized diagnostics (troubleshooting) of all functions. It can do this because all messages must first go through the central computer. This can prove invaluable for ensuring network security has not been breached. So much for the good news; now for the bad news, or the disadvantages of the star network. Of all the topologies, the star is the least reliable because it has a single point of failure. The network relies mainly on the central computer for all functions. If it fails, all nodes also stop functioning, resulting in failure of the entire network. This is precisely the same weakness multi-user computer systems have that rely on a central processor. The Bus Network The bus topology is like a data highway. That is, all components or nodes are connected to the same cable, Figure 1-2.—A star network topology. and the far ends of this cable never meet (see figure 1-3). Bus LANs are best suited to applications involving relatively low usage of the bus coupled with the need to pass relatively short messages from one node to another. In many such networks, the workstations check whether a message is coming down the highway before sending their messages. Since all nodes share the bus, all messages must pass through the other workstations on the way to their destinations. Each node checks the address attached to the message to see if it matches its own address. Bus topologies allow individual nodes to be out of service or to be moved to new locations without disrupting service to the remaining nodes. Unlike the star topology, where dozens of cables come together at the central computer causing logistical problems, bus cabling is simple. The bus topology is very reliable, because if any node on the bus network fails, the bus itself is NOT affected, and the remaining nodes can continue to operate without interruption. Many of the low-cost LANs use a bus topology and twisted-pair wire cabling. A disadvantage of the bus topology is that generally there must be a minimum distance between workstations to avoid signal interference. Another disadvantage is that nodes must contend with each other for the use of the bus. Simultaneous transmissions by more than one node are NOT permitted. This problem, however, can be solved by using one of several types of systems designed to control access to the bus. They are collision detection, collision avoidance, and token passing, which we will discuss shortly. Also, there is no easy way for the network administrator to run diagnostics on the entire network. Finally, the bus network can be easily compromised by an unauthorized network user, since all messages are sent along a common data highway. For this reason, it is difficult to maintain network security. Figure 1-3.—A bus network topology. 1-10