4. The Erlang charts found in Table 3-1 use call-seconds as a traffic measurement. If you review your
work, we have been using call-minutes to measure our traffic load. In this step, we will convert the call-
minutes into call-seconds. To make the conversion, we multiply our call-minutes by the constant Ke.
The constant Ke equals .60.
_____ CM x _____ TGC % (% of traffic) x Ke = _______ CCS
270 CM x 1 TGC % x .60 = 162 CCS
5. Determine the desired overflow. To determine the desired overflow, first we must determine how
large these trunk groups must be. After we know the size of the trunk, we can determine the GOS. Next
we multiply the total call-seconds offered by the GOS desired.
Overflow = CCS offered to a trunk x GOS
Overflow = 162 CCS x .10 = 16.2 CCS
As a rule of thumb, always remember what happened in Europe when Siemens forgot to program a
blockage rate for the switch.
6. Determine the number of trunks needed for CCS offered. The two known quantities provide us
with the traffic offered and the desired overflow. In Table 3-1, the charts have the traffic offered listed
in the extreme left and extreme right. The intent is to make it easier to read the charts. Start by finding
the traffic offered in our example--162 CCS.
a. Next, we need to find the desired overflow. In our example, the desired overflow is 16.2 CCS.
When the desired overflow falls between two values, like in our example, by rule of thumb we always
round up to the next value. In our example, we will round up to 25 CCS.
b. Once we intersect both numbers (162 CCS and 25 CCS), we read straight up to find the total
number of trunks needed. In this case, the correct answer is six trunks.
7. If you want to work another problem here is the information. You will still determine the number
of trunks needed for SES. The correct answer can be found in the Answer Key and Feedback section at
the end of this lesson.
NUMBER OF SUBSCRIBERS
AVERAGE HOLD TIME
AVERAGE CALL RATE