rally frowned upon in the IP networking world, I've seen it used in traditional telephony quite often. 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 out- side or central office (hence the O in FXO). 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 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 stands for Packet Voice Digital S