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Table 26: Endpoints in a three-site system
Endpoints
IP stations
Circuit-switched
stations
PSTN trunks
First consider the 32 Erlangs of intercom CUR between Site 1 stations
matrix for the Uniform Distribution model in Example 2: Uniform Distribution model
page 153). 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.
Atlanta
1107
(1080 IP
Telephones + 27
Road Warriors)
648
(540 DCP stations
+ 108 analog
stations)
120 (DS0) PSTN
Trunks
(5 T1 facilities)
Boston
450
(450 IP
Telephones)
198
(180 DCP stations
+ 18 analog
stations)
48 (DS0) PSTN
Trunks
(2 T1 facilities)
Issue 3.4.1 June 2005
Call usage rates
Cleveland
270
(270 IP
Telephones)
36
(36 analog stations)
24 (DS0) PSTN
Trunks
(1 T1 facility)
(Table 24: Intercom COI
on
157
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