and switches) to have a high efficiency and still keep our customers relatively happy. The goal of our previous exercises in traffic engineering was just that. The grade of service is one measure of subscriber satisfaction. As an example, let us assume that between cities X and Y TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Table 4.2 Trunk-Loading Capacity, Based on Poisson Formula, Full Availability Grade of Grade of Grade of Grade of Grade of Service Service Service Service Service 1 in 1000 1 in 100 1 in 50 1 in 20 1 in 10 Trunks 0.003 10.30 10.05 11.15 10.85 11.95 10.40 11.65 12.85 10.35 11.15 12.45 13.65 11.10 11.90 13.25 14.55 11.85 12.70 14.10 15.40 10.30 12.60 13.50 14.90 16.25 10.95 13.35 14.30 15.70 17.10 11.65 14.10 15.05 16.55 17.95 12.35 14.85 15.90 17.40 18.85 13.05 15.60 16.65 18.20 19.70 13.75 16.40 17.40 19.05 20.60 14.45 17.15 18.20 19.90 21.45 15.15 17.95 19.05 20.75 22.35 15.85 18.75 19.85 21.60 23.20 16.60 19.55 20.65 22.45 24.10 17.35 20.35 21.45 23.35 25.00 18.05 21.10 22.30 24.20 25.90 18.80 21.90 23.10 25.05 26.80 19.55 22.70 23.95 25.90 27.65 20.25 23.55 24.80 26.80 28.55 21.00 24.35 25.60 27.65 29.45 21.75 25.15 26.40 28.50 30.35 22.50 25.95 27.30 29.35 31.25 23.25 26.80 28.10 30.20 32.14 24.05 27.60 28.95 31.10 33.05 24.80 28.40 29.80 31.95 33.95 25.55 29.20 30.65 32.85 34.85 26.30 30.05 31.45 33.70 35.75 27.10 30.90 32.35 34.60 36.65 27.85 31.70 33.15 35.45 37.55 28.60 32.55 34.05 36.35 38.45 29.40 33.35 34.85 37.20 39.35 30.15 34.20 35.70 38.10 40.30 30.95 35.05 36.60 38.95 41.15 4.2 TRAFFIC INTENSITY DEFINES THE SIZE OF SWITCHES AND THE CAPACITY OF TRANSMISSION LINKS there were 47 trunks on the interconnecting telephone route. The tariffs, from which the telephone company derives revenue, are a function of the erlangs of carried traffic. Suppose we allow $1.00 per erlang-hour. The very upper limit of service on the route is 47 erlangs, and the telephone company would earn $47 for the busy hour (much less for all other hours) for that trunk route and the portion of the switc