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Dial-Plan and PBX Design
or voice mail port, but most don’t. Fortunately, VoIP is not as geared around exten-
sion numbers as legacy PBX systems are.
Extensions just because
Some users may request a particular extension because it’s the extension number
they had on their former employer’s system or because it’s easy to remember. Other
extension numbers may be baggage from generations of phone system upgrades.
There may even be a hodgepodge of geography or legacy issues and special number
requests cluttering your dial-plan today, before you’ve even had a chance to add
VoIP to the dial-plan mix. Fortunately, VoIP networks can be eminently more pro-
grammable than the old PBX system.
Dial-Tone Trunks
Dial-tone trunks are the PSTN pathways of a PBX. Outbound or PSTN-bound traffic
flows to the dial-tone trunks, and inward traffic flows across them to the PBX. Tradi-
tional dial-tone trunks can be POTS, Centrex, BRI channels, or PRI DS0 channels.
Chapter 12 discusses the impact of voice applications on the selection
of PSTN trunk technologies and sizing.
It could make sense to have a very large dial-tone trunk group to make sure the rest
of the world never gets a busy signal when calling in to your phone system. But the
cost of phone lines—or T1 circuits—adds up quickly. There’s an easy way to figure
an adequate number of dial-tone trunks for your PBX, whether it’s a conventional
TDM system or a softPBX. Use the Erlang traffic-engineering method, described
here, to balance the cost of trunks against your system’s capacity requirements.
An Erlang is a unit of voice traffic. Each Erlang is equal to the average number of
calls to a system in an hour times the average duration of the calls in seconds,
divided by the number of seconds in an hour—3,600. The Erlang formula, there-
fore, is e=cs/3600. For a system with 120 calls per hour at 250 seconds apiece, the
formula is applied like this:
(120 cph x 250 seconds ) / 3,600 = 8.3 Erlangs
You can use this formula to calculate the Erlang rating of your phone system.
Remember, the Erlang is a unit of voice traffic, not a unit of physical capacity. You’ll
need to use something called the Erlang B table in order to correlate your system’s
Erlang rating with a physical capacity number—a trunk count. So, in a nutshell, fig-
ure out the Erlang rating and look it up in Table 4-1 to figure out how many trunks
your system requires.
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Chapter 4: Circuit-Switched Telephony
By looking up the number of Erlangs in the Erlang B table, you can tell how many
trunks are required to satisfy several levels of probability for blocking due to busy
So for a system with an Erlang rating of 8.3, 15 or 16 dial-tone trunks is enough to
support a 1% probability of blocking, while the same system would need only 12 or
13 trunks to support a 5% probability of blocking.
The blocking formula can be used to size private trunk groups, too, but tolerance for
blocking on private trunks may be higher. Because blocking of private trunks can
often be overcome using the PSTN as a go-between when the private trunks are
blocked, a higher probability percentage can be applied to save money. Ultimately,
the size of each trunk group depends on how tolerant your telephony applications,
and users, are of busy signals.
Table 4-1. Erlang B table
Trunks required Blocking 1%, Erlangs Blocking 5%, Erlangs
1 .01 .05
2 .15 .4
4 .9 1.5
5 1.4 2.2
6 1.9 3.0
7 2.5 3.7
8 3 4.5
9 3.8 5.4
10 4.5 6.2
11 5.2 7.1
12 5.9 8.0
13 6.6 8.8
14 7.4 9.7
15 8.1 10.6
16 8.9 11.5
17 9.7 12.5
18 10.4 13.4
19 11.2 14.3
20 12 15.3
21 12.8 16.2
22 13.7 17.1
23 14.5 18.1
24 15.3 19

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