e also unpredictable peaks caused by stock market or money market activity, weather, natural disaster, international events, sporting events, and SO on. Normal traffic growth must also be taken into account. Hour of day Figure 4.1 Bar chart of traffic intensity over a typical working day. (US, mixed business and residential). 4.2 TRAFFIC INTENSITY DEFINES THE SIZE OF SWITCHES AND THE CAPACITY OF TRANSMISSION LINKS Nevertheless, suitable forecasts of BH traffic can be made. However, before proceeding further in this discussion, consider the following definitions of the busy hour. 1. Busy Hour. The busy hour refers to the traffic volume or number of call attempts, and is that continuous one-hour period being wholly in the time interval concerned for which this quantity (i.e., traffic volume or call attempts) is greatest. The Average Busy Season Busy Hour (ABSBH). This is used for trunk groups and always has a grade of service2 criterion applied. For example, for the ABSBH load, a call requiring a circuit in a trunk group should encounter all trunks busy (ATB) no more than 1% of the time. Other definitions of the busy hour may be found in Ref. 1. When dimensioning telephone exchanges and transmission routes, we shall be working with BH traffic levels and care must be used in the definition of the busy hour. Peak traffic loads are of greater concern than average loads for the system planner when dimensioning switching equipment. Another concern in modern digital switching systems is call attempts. We could say that call attempts is synonymous with offered traffic. Even though a call is not carried and is turned away, the switch's processor or computer is still exercised. In many instances a switch's capability to route traffic is limited by the peak number of call attempts its processor can handle. 4.2.1.1 Measurement of Telephone Traffic. If we define telephone traffic as the aggregate of telephone calls over a group of circuits or trunks with regard to the duration of calls as well as their number, we can say that 