Table of Contents

hairpinning is generally frowned upon in the IP networking world, I’ve seen it used
in traditional telephony quite often.
FXO
Foreign Exchange Office. An FXO port connects the phone system to an outside
line via the plain POTS. We’ll connect our IP phone system to an outside POTS
line later in this chapter. Think of FXO as a POTS line that connects to the outside
or central office (hence the O in FXO).
FXS
Foreign Exchange Station. An FXS port is one to which a traditional analog phone
is connected. FXS ports supply battery and dial tone, and generate ringing signals
to the phone endpoint. If you have a POTS line in your house, the jack on the wall
is an FXS port. Think of FXS as a POTS line for connecting a station (hence the S
in FXS).
If you’re building an IP telephony lab that will connect to POTS
lines and analog phones, you may be asking, “What type of ports
do I need” Phones need FXS ports. Outside POTS lines need FXO
ports. You may also think of it this way: analog phones require
voltage, and traditionally, the central office supplied that voltage.
FXS ports supply voltage (S for supply), while FXO ports do not.
Softphone
An IP phone that exists only as software on another device. Softphones may exist
on desktop or laptop computers, tablets, or even on smartphones.
Survivable Remote Site Telephony (SRST)
Imagine that you have a main office with 10 remote offices. Each remote office has
10 IP phones, all of which register and use the IP PBX in the main office. Should
the main IP PBX become unavailable, all those branch phones will also become
unavailable. By configuring a smaller-scale IP PBX at the branch, we can make each
remote branch capable of surviving the failure. Thus, the remote site’s telephony
survives.
PLAR
PLAR stands for Private Line Automatic Ringdown. The common description for
a PLAR line is the “bat phone.” If Commissioner Gordon picks up the bat phone,
the matching phone rings in the Batcave. No numbers need be dialed; if you need
Batman, you need him now.
Cisco Telephony Terms
PVDM
PVDM stands for Packet Voice Digital Signal Processor. There are multiple
versions—PVDM, PVDM2, and PVDM3, for example. PVDMs are Digital Signal
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