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Call usage rates
Issue 6 January 2008 193
First consider the 32 Erlangs of intercom CUR between Site 1 stations (Table 28). Site 1
(Atlanta) has a total of 1755 stations, 1107 of which are IP stations, and 648 of which are
circuit-switched stations. So, 63% of the stations in Site 1 are IP, and 37% are circuit switched.
Therefore, 63% of Site 1 intercom calls are generated by IP stations, and 63% of those calls are
terminated by IP stations. Since 63% of 63% is 39.7%, 39.7% of Site 1 intercom calls are IP
station to IP station. Similarly, 37% of the Site 1 intercom calls that are generated by IP stations
are terminated by circuit-switched stations. Since 37% of 63% is 23.3%, 23.3% of Site 1
intercom calls are IP station to circuit-switched station.
Also, 37% of Site 1 intercom calls are generated by circuit-switched stations, and 63% of those
calls are terminated by IP stations. Since 63% of 37% is 23.3%, 23.3% of Site 1 intercom calls
are circuit-switched station to IP station. Finally, 37% of the Site 1 intercom calls that are
generated by circuit-switched stations are terminated by circuit-switched stations. Since 37% of
37% is 13.7%, 13.7% of Site 1 intercom calls are circuit-switched station to circuit-switched
station.
So, since 39.7% of Site 1 intercom calls are IP station to IP station, IP station to IP station call
usage is 39.7% of the 32 Erlangs of overall Site 1 intercom CUR, or 12.7 Erlangs. Similarly, both
the Site 1 IP station to circuit-switched station CUR and the Site 1 circuit-switched station to IP
station CUR are equal to 23.3% of 32 Erlangs, or 7.5 Erlangs. Finally, the Site 1 circuit-switched
station to circuit-switched station CUR is 13.7% of 32 Erlangs, or 4.4 Erlangs.
A similar process is used to break down the 12 Erlangs of intercom CUR into its components.
There are a total of 648 stations in Site 2 (Atlanta), 450 of which are IP stations, and 198 of
which are circuit-switched stations. So, 69% of the stations in Site 2 are IP, and 31% are
circuit-switched. We have already determined that 63% of intercom calls that are generated in
Site 1 are generated by IP stations. Similarly, 69% of intercom calls that are terminated in Site 2
are terminated by IP stations. Since 69% of 63% is 43.5%, 43.5% of Site 1 to Site 2 intercom
calls are IP station to IP station. Also, 31% of intercom calls that are terminated in Site 2 are
terminated by circuit-switched stations. Since 31% of 63% is 19.5%, 19.5% of Site 1 to Site 2
intercom calls are IP station to circuit-switched station.
Table 30: Endpoints in a three-site system
Endpoints Atlanta Boston Cleveland
IP stations 1107
(1080 IP
Telephones + 27
Road Warriors)
450
(450 IP
Telephones)
270
(270 IP
Telephones)
Circuit-switched
stations
648
(540 DCP stations
+ 108 analog
stations)
198
(180 DCP stations
+ 18 analog
stations)
36
(36 analog stations)
PSTN trunks 120 (DS0) PSTN
Trunks
(5 T1 facilities)
48 (DS0) PSTN
Trunks
(2 T1 facilities)
24 (DS0) PSTN
Trunks
(1 T1 facility)