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Sizing the PSTN/Gateway Trunks
If you are sizing the number of PSTN/gateway trunks for a new deployment or are planning to estimate the number of PSTN/gateway trunks required, use the Erlang-B calculator, as discussed in Chapter 6. The information requested in Table E-11 is required to estimate the number of trunks. In an existing network, you can obtain this information from the statistics available in the PBX.
| No. | Question | Answer |
|---|---|---|
| 1 | What is the BHCA? | BHCA: |
| 2 | What is the AHT? | AHT: |
| 3 | What is the BHT? | BHT = (BHCA x AHT / 3600): |
| 4 | What is the GoS? | GoS: |
Notes/Comments
The following list explains the terms used in Table E-11:Busy hour call attempts (BHCA) Number of calls attempted during the busiest interval. This could be any interval, such as the "busy hour," a half-hour interval, or any other interval.Average handle time (AHT) The average duration (talk time) of the call.Busy hour traffic (BHT) Measure of the traffic load during your network's busiest hour, which represents the maximum traffic load that your network must support.Grade of service (GoS) The percent of calls that will receive busy tone (no trunks available on gateway/PSTN) during the busy hour (also called percent blockage). A 1-percent blockage means 99 percent of all calls from the PSTN attempted during the interval will have a trunk port available on the gateway/PSTN to reach the user.
