he facts are that the PSTN approaches total capacity utilization for only several hours during the working day. After 10 P.M. and before 7 A.M., capacity utilization may be 2% or 3%. The network is dimensioned (sized) to meet the period of maximum usage demand. This period is called the busy hour (BH). There are Tandem Exchange Exchange Exchange Direct route Figure 1.6 Direct route and tandem connectivities. The erlang is a unit of traffic intensity. One erlang represents one hour of line (circuit) occupancy. INTRODUCTORY CONCEPTS Residential subscribers Business subscribers Figure 1.7 The busy hour. two periods where traffic demand on the PSTN is maximum: one in the morning and one in the afternoon. This is illustrated in Figure 1.7. Note the two traffic peaks in Figure 1.7. These are caused by business subscribers. If the residential and business curves were combined, the peaks would be much sharper. Also note that the morning peak is somewhat more intense than the afternoon busy hour. North America (i.e., north of the Rio Grande), the busy hour (BH) is between 9:30 A.M. and 10:30 A.M. Because it is more intense than the afternoon high-traffic period, it is called the busy hour. There are at least four distinct definitions of the busy hour. The IEEE (Ref. 2) gives several definitions. We quote only one: "That uninterrupted period of 60 minutes during the day when the traffic offered is maximum." Other definitions may be found in Ref. 4. BH traffic intensities are used to dimension the number of trunks required on a connec- tivity as well as the size of (a) switch(es) involved. Now a PSTN company (administration) can improve its revenue versus expenditures by cutting back on the number of trunks required and making switches "smaller." Of course, network users will do a lot of com- plaining about poor service. Let's just suppose the PSTN does just that, cuts back on the number of circuits. Now, during the BH period, a user may dial a number and receive either a voice announcement or a rapid-cadence tone telling t