June 2005
Intermediate to advanced
502 pages
21h 48m
English
Content preview from Switching to VoIP
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Chapter 12: PSTN Trunks
Dial-Tone Trunks
Choosing from the plethora of legacy voice connectivity technologies isn’t a no-
brainer. When you choose a dial-tone trunk solution to supply your voice switch with
a path to the outside world, you should consider the capacity of the solution, the
implications for quality of service, geographic availability, and, of course, the cost.
POTS and Centrex Trunks
Plain old telephone service, introduced in Chapter 1, is an analog phone line from
the CO that connects to your PBX using one copper pair. This is the kind of phone
service you most likely have in your home. The POTS line is cost effective when
fewer than 10 lines are concentrated in one location. Most POTS lines are paid for
monthly, with overage charges added when a certain level of utilization (minutes)
has been surpassed. POTS lines are available just about anywhere there’s electricity
and running water. Each POTS lines can support one phone call at a time.
Centrex phone lines are POTS lines with special, business-related calling features like
four-digit private endpoint dialing. These are the lines on which you have to “dial a 9
to get out.” Centrex has the same economics as POTS—you generally pay a monthly
fee that covers a certain amount of utilization on the line; after that,
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