FXS vs FXO: Understanding Analog Phone Line Interfaces

This article breaks down the difference between FXS and FXO, two terms crucial to understanding analog telephone systems.

The acronym FXS stands for Foreign eXchange Subscriber, while FXO stands for Foreign eXchange Office. In the world of analog telephone systems, the connection between the central office (CO) and a subscriber’s location traditionally involves two copper wires. These wires are twisted together to minimize electromagnetic interference (EMI), hence the term “twisted pair.” This type of system is commonly referred to as POTS, short for Plain Old Telephone Service.

Both FXS and FXO are terms directly related to this analog POTS system. They essentially define the interfaces between different components within the analog telephone network, as illustrated below.

FXS-Foreign eXchange Subscriber Interface

This interface provides the actual POTS service originating from the telephone company’s local central office. It must connect to the subscriber’s side of the connection. Think of it as the point where telephones, fax machines, modems, and other similar devices connect to the telephone network. These devices are subscriber equipment.

The FXS interface effectively “points” towards the subscriber. It’s responsible for delivering the following services to the connected device:

• Ring Voltage
• Dial Tone
• Battery Current

FXO-Foreign eXchange Office Interface

This interface is on the opposite end. It receives the POTS service, typically from the CO within the PSTN (Public Switched Telephone Network).

The FXO interface “points” towards the telco (telephone company) office, as depicted in the figure above. The FXO interface ports provide “on-hook” and “off-hook” indications (also known as loop closure) to the local telco office.

By understanding the roles of each, you can easily compare FXS vs FXO and grasp the fundamental difference between them. One provides the service to the customer, and the other receives the service from the telephone company.