11 37.00 39.32 43.54 of servicing channels or trunks. In this case we would opt for the next highest integer because we cannot install a fraction of a trunk. For instance, if calculations show that a trunk route should have 31.4 trunks, it would be designed for 32 trunks. The Erlang B formula, based on lost calls cleared, has been standardized by the CCITT (CCITT Rec. Q.87) and has been generally accepted outside the United States. In the United States the Poisson formula is favored. This formula is often called the Molina 4.2 TRAFFIC INTENSITY DEFINES THE SIZE OF SWITCHES AND THE CAPACITY OF TRANSMISSION LINKS formula. It is based on the LCH concept. Table 4.2 provides trunking sizes for various grades of service deriving from the P formula; such tables are sometimes called "P" tables (Poisson) and assume full availability. We must remember that the Poisson equation also assumes that traffic originates from a large (infinite) number of independent subscribers or sources (random traffic input), with a limited number of trunks or servicing channels and LCH (Ref. 3). 4.2.3 Waiting Systems (Queueing) The North American PSTN became entirely digital by the year 2000. Nearly all digital switches operate under some form of queueing discipline, which many call waiting systems because an incoming call is placed in queue and waits its turn for service. These systems are based on our third assumption, namely, lost calls delayed (LCD). Of course, a queue in this case is a pool of callers waiting to be served by a switch. The term serving time is the time a call takes to be served from the moment of arrival in the queue to the moment of being served by the switch. For traffic calculations in most telecommunication queueing systems, the mathematics is based on the assumption that call arrivals are random and Possonian. The traffic engineer is given the parameters of offered traffic, the size of the queue, and a specified grade of service and will determine the number of serving circuits or trunks that are required. The method 