SMISSION LINKS Figure 4.3 A normal distribution curve showing the mean and standard deviation, O. The standard deviation is usually expressed by the Greek letter sigma (a). For example, 1 o either side of the mean in Figure 4.3 will contain about 68% of the population or measurements, 2 o will contain about 95% of the measurements, and 3 o will contain around 99% of the subject, population, or whatever is being measured. The curve shown in Figure 4.3 is a normal distribution curve. 4.2.2 Discussion of the Erlang and Poisson Traffic Formulas When dimensioning a route, we want to find the optimum number of circuits to serve the route. There are several formulas at our disposal to determine that number of circuits based on the BH traffic load. In Section 4.2.1.2, four factors were discussed that will help us to determine which traffic formula to use given a particular set of circumstances. These factors primarily dealt with (1) call arrivals and holding-time distributions, (2) number of traffic sources, (3) availability (full or limited), and (4) handling of lost calls. The Erlang B loss formula was/is very widely used outside of the United States. Loss in this context means the probability of encountering blockage at the switch due congestion or to "all trunks busy" (ATB). The formula expresses grade of service or the probability of finding X channels busy. The other two factors in the Erlang B formula are the mean of the offered traffic and the number of trunks or servicing channels available. The formula assumes the following: Traffic originates from an infinite number of sources. Lost calls are cleared assuming a zero holding time. The number of trunks or servicing channels is limited. Full availability exists. The actual Erlang B formula is out of the scope of this text. For more detailed information, it is recommended that the reader consult Ref. 3, Section 1. It is far less involved to use traffic tables as found in Table 4.1, which gives trunk-dimensioning information for some specific grades of service, fro