3CX FAQ: Legacy Technology (ISDN)
What do the Terms FXS and FXO Mean?FXS and FXO are the name of the ports used by analog phone lines (also known as POTS – Plain Old Telephone Service) or phones. The expression “POTS” was initially intended as a joke but is now used as common expression in the telecommunications industry.
FXS (Foreign Exchange Subscriber) is the port that actually delivers the analog line to the subscriber. In other words it is the “plug in the wall” that delivers a dial tone, battery current and ring voltage. This is the jack or interface to the phone system which FXO devices can be connected to.
FXO (Foreign Exchange Office) is the port that receives the analog line. It is the plug on the phone or fax machine, or the plug(s) on your analog phone system. It delivers an on-hook/off-hook indication (loop closure). Since the FXO port is attached to a device, such as a fax or phone, the device is often called the “FXO device”. This port establishes the connection to the analog line (FXS).
FXO and FXS are always paired, i.e similar to a male / female plug. If no phone system is used the telephone (FXO) is directly connected to the FXS port. The port is provided by the telephone company.
What is a TDM PBX?EA TDM (Time Division Multiplexers) PBX is one of the most common types of voice infrastructures as it has been around the longest. A TDM PBX consists of proprietary, self-contained systems as it was designed before contemporary server technology was invented.
Involving a cabinet with numerous different boards that can perform certain functions, for example intercom functionality boards or analog extension boards, the TDM PBX is coming to the end of its life cycle. The TDM PBX boards are only compatible with systems from the same vendor as an overall architecture, locking in its users to use the same vendor for everything.
A TDM PBX requires dedicated staff to be able to manage it as well as extensive maintenance. It is mostly used by companies which have yet to update their network cabling.
The main difference between a TDM PBX and an IP PBX is that an IP PBX uses Internet Protocol to route calls whereas a TDM PBX uses physical switches. Additionally, an IP PBX is scalable, offers no vendor lock-in and can reduce telco costs drastically.