4. Trunk group sizing procedures. The procedure to determine the trunk group sizing has five steps.
To better understand the process, we will work together an example. The switchboard in the example is
an SES. Use a clean piece of paper to work the example.
PART B - SMALL EXTENSION SWITCH (EXAMPLE)
NUMBER OF SUBSCRIBERS
30
CALL RATIO:
AVERAGE HOLD TIME
4
LOCAL
.25
AVERAGE CALL RATE
3
INCOMING
.40
OUTGOING
.35
1. Determine the busy hour call-minutes. The call-minutes (CM) equal the amount of call-minutes
during the busiest hour. It refers to how many minutes the subscribers use a trunk.
a. To determine the call-minutes, we multiply the number of subscribers by the ACR.
__ (# of subs) x __(ACR) = __
30 (# of subs) x 3 (ACR) = 90
b. Then we take the results and multiply it by the average hold time (AHT).
90 x 4 (AHT) = 360 CM
2. Determine the total amount of trunk traffic offered. The total percentage of calls that use a TGC is
computed by adding the percentages of incoming and outgoing together and multiplying it by the total
CM results from the previous step to get an overall percentage. Local traffic will not be considered
when calculating the total traffic. Local traffic does not go over any trunk.
#CM x (incoming + outgoing) = offered trunk traffic
360 (CM) x (.40 + .35) = offered trunk traffic
360 (CM) x .75 = 270 CM
3. Determine the distribution of offered trunk traffic. Remember the SES has only one trunk path.
The traffic offered to the trunk group equals the amount of traffic going over the trunk. Since the SES
has only one path, the total traffic going over the trunk equals 100 percent or 1.00.
SS 0029
3-4
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