52 | Section 5
IP Centrex and Hosted PBX Services
Just as it shouldn’t matter whether the PSTN gateway is on your premises or not, it also
shouldn’t matter where your IP PBX is located. This is the principle that allows IP Centrex
services or hosted PBX services to happen. These locate the hardware at the service provider’s
site and remove the need for phone system equipment at your location. In the pure case, you
would have only IP phones at your site, which would be plugged into an Ethernet switch,
which would connect to the Internet via a router. The main advantage is that someone else is
responsible for installation and maintenance of the back-end equipment. It might also be that a
vendor of these services has invented a suite of applications that would be difficult to replicate
at individual business sites.
Number portability in the US and Canada.
Number portability regulations allow you to move your existing numbers to the service pro-
vider of your choice. Unfortunately, our experience is that “choke” exchange numbers cannot
be ported. We’ve tried with various carriers - all with the same disappointing result: a call on
the day of the port explaining that it can’t be done.
Circuit-Switched Interfaces
When you order or configure a gateway, you need to know what kinds of interfaces you will be
using to connect to the Telco network.
FXS/FXO
These are designations for the two ends of a standard analog POTS line. Most often these
lines are used for basic home telephones. But they can also be used to link a PBX with a Telco
central office.
♦ An FXS (Foreign Exchange Station) interface emulates a circuit supplied by a Telco
Central Office. An FXS supplies talk battery and detects an off-hook condition. It gener-
ates 100Vac for a ringing indication. It provides dialtone and other call progress signals
such as ringback and busy. It responds to DTMF (Dual Tone MultiFrequency) tones for
dialing and may send caller ID information in modem-encoded audio.
♦ A telephone, and anything that looks like a telephone, is an FXO (Foreign Exchange
Office) device. An FXO device signals an off-hook condition by drawing loop current.
It responds to ringing voltage. It provides dialing, either by old-fashioned pulsed loop-
interruption or by DTMF. It may detect caller ID.
An interesting limitation of FXS/FXO interfaces is that signaling from the FXS that a call has
ended is sometimes not signaled, and the type of signaling varies around the world. In the
USA, most Telco central offices interrupt the loop current when the call has ended, but some
do not. And many PBXs do not. Eventually, dialtone will return, though, and this can be used
as a disconnect signal, but there will be a many-second delay. This could cause glare, the
condition where there is confusion between the CO and the PBX as to whether the line is free.
A call could ring-in just when the PBX attempts to access the line for an outgoing call. In many
other countries, an audio tone is sent on the line to indicate the end of the call. This lack of
disconnect supervision results from the idea that there was no reason for a central office to
hang up a phone by remote. This was a physical, human action performed by someone who
knew that the conversation had ended and put reacted by replacing the receiver back in its
cradle. The design was never intended to work for machine-to-machine connections.
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