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Call usage rates
Issue 6 January 2008 195
The general formula that is used to determine the expanded intercom CUR entries in Table 31
is:
where:
● “Type t” refers to IP or circuit-switched
●
●
Now that the intercom CURs have been determined, CURs that involve trunks will be
addressed. First, because Communication Manager systems are rarely used to route PSTN
traffic, all of the COI matrix entries that correspond to PSTN-PSTN traffic are zero. Next,
Table 26
and Table 27 help us determine the entries that correspond to inbound and outbound
PSTN traffic.
According to Table 26
, the inbound PSTN usage that arrives on Site 1 trunks and terminates at
Site 1 stations is 49 Erlangs. We have already determined that 63% of the stations in Site 1 are
IP and 37% are circuit switched. Therefore, the Uniform Distribution model implies that 63% of
the 49 Erlangs (that is, 30.9 Erlangs) is inbound to Site 1 IP stations, and 37% of the 49 Erlangs
(that is, 18.1 Erlangs) is inbound to Site 1 circuit-switched stations. Similarly, the Uniform
Distribution model and Table 27
together imply that 63% of the 49 Erlangs of Site 1 outbound
PSTN usage (that is, 30.9 Erlangs) is outbound from Site 1 IP stations through Site 1 PSTN
trunks, and 37% (that is, 18.1 Erlangs) is outbound from Site 1 circuit-switched stations through
Site 1 PSTN trunks. Note that by an assumption in Example 2: Uniform Distribution model
, Site
1 inbound and outbound traffic only terminates and originates at Site 1 stations. This completes
the work for Site 1. Sites 2 and 3 are handled in a similar manner, and the resulting completed
COI matrix for Example 4: Expanded COI matrices
is provided in Table 32: Completed COI
matrix for Example 4: Expanded COI matrices on page 196.
CUR generated by stations of type t in Site i
and terminated by stations of type t in Site j
f
i
t
f
j
t
× intercom CUR from Site i to Site j ()×=
f
i
t
number of type t stations in Site i
total number of stations in Site i
----------------------------------------------------------------------------------
=
f
j
t
num
b
er o
f
type t stat
i
ons
i
n S
i
te j
total number of stations in Site j
----------------------------------------------------------------------------------
=