by which a waiting call is selected to be served from the pool of waiting calls is called queue discipline. The most common discipline is the first-come, first-served discipline, where the call waiting longest in the queue is served first. This can turn out to be costly because of the equipment required to keep order in the queue. Another type is random selection, where the time a call has waited is disregarded and those waiting are selected in random order. There is also the last-come, first-served discipline and bulk service discipline, where batches of waiting calls are admitted, and there are also priority service disciplines, which can be preemptive and nonpreemptive. In queueing systems the grade of service may be defined as the probability of delay. This is expressed as P(t), the probability that a call is not being immediately served and has to wait a period of time greater than t. The average delay on all calls is another parameter that can be used to express grade of service, and the length of queue is yet another. The probability of delay, the most common index of grade of service for waiting systems when dealing with full availability and a Poissonian call arrival process (i.e., random arrivals), is calculated using the Erlang C formula, which assumes an infinitely long queue length. A more in-depth coverage of the Erlang C formula along with Erlang C traffic tables may be found in Ref. 3, Section 1. 4.2.4 Dimensioning and Efficiency By definition, if we were to dimension a route or estimate the required number of servicing channel, where the number of trunks (or servicing channels) just equaled the erlang load, we would attain 100% efficiency. All trunks would be busy with calls all the time or at least for the entire BH. This would not even allow time for call setup (i.e., making the connection) or for switch processing time. In practice, if we sized our trunks, trunk routes, or switches this way, there would be many unhappy customers. On the other hand, we do, indeed, want to dimension our routes (