Endpoint usages in a 1/3-1/3-1/3 call mix

In a general business environment, the intercom, outbound, and inbound call usages are often assumed to be equal. In other words, each of those three components represents 1/3 of the traffic.

The average duration of a general business call is typically assumed to be 200 s (20 s for call set-up, and 180 s of talk time) as a default. Furthermore, the average station is assumed to induce the following call rates during the busy hour.

Light General Business Traffic:

• originate 0.25 intercom call per hour

• originate 0.25 outbound call per hour

• receive 0.25 inbound call per hour

Moderate General Business Traffic:

• originate 0.50 intercom call per hour

• originate 0.50 outbound call per hour

• receive 0.50 inbound call per hour

Heavy General Business Traffic:

• originate 0.75 intercom call per hour

• originate 0.75 outbound call per hour

• receive 0.75 inbound call per hour

The corresponding default per-station busy-hour usages can be calculated using the preceding call rates, a 200-s average hold time, and the formulas in the Erlang and ccs definitions section.

Light General Business Traffic:

• originate 0.5 ccs = 0.014 Erlang of intercom call usage

• originate 0.5 ccs = 0.014 Erlang of outbound call usage

• receive 0.5 ccs = 0.014 Erlang of inbound call usage

Moderate General Business Traffic:

• originate 1.0 ccs = 0.028 Erlang of intercom call usage

• originate 1.0 ccs = 0.028 Erlang of outbound call usage

• receive 1.0 ccs = 0.028 Erlang of inbound call usage

Heavy General Business Traffic:

• originate 1.5 ccs = 0.042 Erlang of intercom call usage

• originate 1.5 ccs = 0.042 Erlang of outbound call usage

• receive 1.5 ccs = 0.042 Erlang of inbound call usage

Endpoint usages driven by the number of trunks

If the number of PSTN trunks is known (or is assigned some assumed value as part of the given information), then an alternate approach to the one provided in Endpoint Usages in a 1/3-1/3-1/3 Call Mix can be used. Actually, the procedure for determining the per-station intercom usage is identical to the procedure used in the 1/3-1/3-1/3 model. The difference appears in the outbound and inbound usages; specifically, the outbound and inbound components of the traffic are derived by assuming the trunks have been engineered to a P01 GOS. The results are as follows:

• The default per-station intercom usage either 0.5 ccs = 0.014 Erlang (light general business traffic), 1.0 ccs = 0.028 Erlang (moderate general business traffic), or 1.5 ccs = 0.042 Erlang (heavy general business traffic)

• The default per-station outbound usage is determined by calculating the carried load associated with the given number of outbound trunks, an assumed grade of service (P01 is standard for PSTN trunks), and the mixed Erlang B/C model

• The default per-station inbound usage is determined by calculating the carried load associated with the given number of inbound trunks, an assumed grade of service (P01 is standard for PSTN trunks), and the mixed Erlang B/C model

One drawback to using this method is that it assumes the trunks have been engineered to a P01 GOS. If the trunks are not being heavily used (for example, if a lot of extra trunks have been added solely for redundancy purposes), this model produces estimates for the outbound and inbound usages that are far greater than the actual usages.