. The Bell System technical journal . KEY O TOLL CENTERS INTERTOLL TRUNK GROUPS Fig. 1 — Principal intertoll trunk groups in Minnesota and Wisconsin. THEORIES FOR TOLL TRAFFIC ENGINEERING IN THE U. S. A. 425. 4 5 6 7 8 9 10NUMBER OF TRUNKS 30 40 50 Fig. 2 — Permitted intertoU trunk occupancy for a usage timeper message. the expected, or average, delay in seconds for calls to obtain an idletrunk in that group during the average Busy Season Busy Hour. In 1954the system average trunk speed was approximately 30 seconds, re-sulting from operating the majority of the groups at a busy-hour
. The Bell System technical journal . KEY O TOLL CENTERS INTERTOLL TRUNK GROUPS Fig. 1 — Principal intertoll trunk groups in Minnesota and Wisconsin. THEORIES FOR TOLL TRAFFIC ENGINEERING IN THE U. S. A. 425. 4 5 6 7 8 9 10NUMBER OF TRUNKS 30 40 50 Fig. 2 — Permitted intertoU trunk occupancy for a usage timeper message. the expected, or average, delay in seconds for calls to obtain an idletrunk in that group during the average Busy Season Busy Hour. In 1954the system average trunk speed was approximately 30 seconds, re-sulting from operating the majority of the groups at a busy-hour trunk-ling efficiency of 75 to 85 per cent in the busy season. The T-engineering tables show permissible call minutes of use for a 426 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 wide range of group sizes, and several selections of message holdingtimes. They were constructed following summarization of many obser-vations of load and resultant average delays on ringdown (non-dial)intertoll trunks.^ Fig. 2 shows the permissible occupancy (efficiency) ofvarious trunk group sizes for minutes of use per message, for a va-riety of T-schedules. It is perhaps of somfe interest that the best fittingcur
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