Avaya 555-245-600 manual Erlangs

Call usage rates

Communication Manager uses a first-site-preference algorithm for outbound trunk calls. This algorithm specifies that all outbound calls first attempt to seize a trunk within the originating station’s site, and tries to use a trunk in a different site if and only if it is blocked at its local trunks. For inbound PSTN trunk calls, the CO selects the trunk. Therefore, Communication Manager cannot use an analogous first-site-preference algorithm for inbound calls. However, such an algorithm can be effectively imposed by assigning different calling numbers for the three sites, which is typical in this example since the sites are in different area codes.

The goal of a first-site preference algorithm is to minimize intersite traffic. When this algorithm is used, there is intersite traffic if and only if it overflows to a trunk on another site after having been blocked at the trunks in its own site. Under the assumption that a first-site preference algorithm is used in this example, the trunks at the three individual sites must be sized independently, as opposed to all together. Initially, the overflow traffic is ignored, but that topic is discussed later in this example.

Since overflow traffic is ignored for the time being, intersite trunk traffic is zero, which implies that the off-diagonal entries of the inbound and outbound COI matrices will all be zero. To determine the values of the diagonal entries, which correspond to intrasite trunk usage, the Uniform Distribution model is applied. In particular, 65% (that is, 1755/2709) of the stations are in Atlanta, 24% (that is, 648/2709) of the stations are in Boston, and 11% (that is, 306/2709) of the stations are in Cleveland. Therefore, the Uniform Distribution model implies that 65% of the 75 Erlangs of inbound CUR (that is, 49 Erlangs) is assumed to terminate in Site 1 (Atlanta), 24% (that is, 18 Erlangs) is assumed to terminate in Site 2 (Boston), and 11% (that is, 8 Erlangs) is assumed to terminate in Site 3 (Cleveland). Similarly, 49 Erlangs of outbound CUR is assumed to originate in Site 1, 18 Erlangs is assumed to originate in Site 2, and 8 Erlangs is assumed to originate in Site 3.

It is instructive for this example to construct three different COI matrices rather than just one. Specifically, it is useful to construct one for intercom CUR, one for inbound CUR, and one for outbound CUR. The information from the previous paragraph can be used to populate the following inbound and outbound COI matrices (Table 26: Inbound COI matrix for the Uniform Distribution model in Example 2: Uniform Distribution model on page 187):

Table 26: Inbound COI matrix for the Uniform Distribution model in Example 2: Uniform Distribution model

Inbound CUR		To stations in Site __
		1	2		3
From trunks in Site	1	49 Erlangs	0		0
____
	2	0	18 Erlangs		0
	3	0	0	8	Erlangs

Issue 6 January 2008 187