Switching to VoIP

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Chapter 12: PSTN Trunks

need as many trunks to support a centralized setup. (There are plenty of reasons not

to centralize, too; Chapter 13 describes them.)

The case for more trunks

Some telephony applications, like conference calling, bridging, and hold queues may

tie up PSTN trunks and require you to use more of them than you’d normally need.

For instance, conference calling requires one additional trunk for every outside party

participating in the call. If you’ve got three outside participants, then you need three

trunks.

Call bridging and “find-me-follow-me” applications, which ring internal and exter-

nal phones simultaneously in order to make it easier to reach the intended party, tie

up PSTN trunks, too. If your application rings a user’s IP desk phone and his cell

phone simultaneously, keep in mind that a PSTN trunk is used to ring the cell

phone. If the call causing this to occur originated from outside your network on the

PSTN, then you’ve got another PSTN trunk tied up. That’s two phone lines tied up.

In these situations, the only solution is to add additional trunk capacity. By using the

Erlang B table in Chapter 4, you can properly size each of your PSTN connect points

so that applications don’t suffer busy signals due to inadequate capacity.

Project 12.1. Make It Easier ...