n overall network we see a certain architecture. This architecture is called the topology. It defines the way in which the work stations and peripheral support devices are interconnected to form a network. Networks can be broken down into a few basic types. Star con- figurations (Fig. 6-1) have the work stations arranged around a cen- tral hub with radial links going between the hub and the work stations. In this network all communication between the work sta- tions must pass through the hub. This can provide fast response time to communication requests. However, it is more susceptible to sys- station station Radial communication links Central station station station station Fig. 6-1. A star configuration with a single dedicated hub. NETWORK STRUCTURES 83 tem failures due to the single centralized communication hub. This configuration finds its best use in networks which must communicate over long distances. The star configuration is an example of a non- contention network. In other words, work stations wishing to com- municate with others don't have to contend for a communication link as compared to a contention network, of which loop technology is an example. Loop topologies or rings are typified by work stations arranged in a ring with a communication link going from one station to the next. This topology requires the work stations to cooperate in their use of the communication link. This cooperation can be in the form of master/slave allocation schemes or token passing. Essentially, the allocation of the link must be accomplished without interfering with a station already using the link. Figure 6-2 shows an example of a multidrop master/slave topol- ogy. The master originates all communication with the slave re- sponding at the appropriate time. The slaves serve the role of sensors for the master. The system finds its weakness when the number of sensory inputs gets larger than about 25 devices. The problem is that the slave can only transfer information when asked by the master. As the number of slaves goes u