FXO vs FXS: Meaning, Differences & Ports Explained

Switching to a VoIP phone system has become a popular option for many small and medium businesses, and it’s easy to see why. VoIP offers better call quality, more features, and the ability to make and receive calls both locally and internationally.

Before VoIP, businesses used analog phone systems that relied on FXS and FXO Gateway ports to connect calls. In this article, we’ll explain the difference between FXO and FXS and how you can use them to link analog phones and PBX systems to VoIP, creating a modern, hybrid communication setup for your business.

What is FXO?

FXO, or Foreign Exchange Office, refers to the port or interface used in telecommunication devices like analog phones, fax machines, or PBX provider (Private Branch Exchange) systems.

An FXO device connects to the Public Switched Telephone Network (PSTN) and acts as the recipient of analog line signals. When a phone line rings, the FXO gateway port detects the ring signal, enabling communication between the caller and the recipient.

The term FXO is primarily associated with analog telephony and is used to indicate a line side connection that requires the provision of services from an external source, such as a telephone company.

What is FXS?

FXS, or Foreign Exchange Subscriber, is the port or interface that delivers the analog line signal to a device such as a phone or fax machine. It provides the dial tone, generates ringing voltage, and transmits other services to the connected device.

In essence, the FXS gateway port is the source of the communication signal, and it connects the service provider to the end-user equipment. FXS devices are usually found in telephone exchanges or PBX systems, and they serve the customer-side equipment such as telephones by providing the necessary communication services.

FXO vs FXS: What’s the Difference?

The key difference between FXO and FXS lies in their respective roles in the analog telecommunication system:

• FXO (Foreign Exchange Office): Receives the analog line signal. It’s responsible for interpreting the incoming signal, such as detecting a ring or converting the signal into sound.
• FXS (Foreign Exchange Subscriber): Sends the analog line signal to devices like phones or fax machines. It provides the necessary dial tone, power, and ringing signal.

In simple terms, FXS is the sender of the line, and FXO is the receiver. The two work together to establish a connection between the telephone network and the communication device.

How Do FXS and FXO Ports Work?

An FXS port delivers the dial tone, the battery current, and the ring voltage that powers the analog phone, while the FXO port acts as the interface on the phone that receives this line signal. When someone places a call, the FXS port signals the FXO port, which in turn responds by connecting the call.

For example, in a typical office setup:

• The FXS port on the PBX connects to the FXO port on the analog phone.
• The FXS port sends power and signal to the FXO device (phone), enabling it to make and receive calls.

When a call is received, the FXS port sends a ringing voltage to the FXO port, signaling the phone to ring. Once the phone is answered, the FXO port detects this and the communication begins.

What Are FXO and FXS Ports Used For?

FXO and FXS ports are crucial in traditional telecommunication systems and are commonly found in:

• PBX systems: These systems often have FXS ports to connect internal analog devices and FXO ports to interface with the external phone line.
• Analog phones and fax machines: These devices usually have FXO ports to receive line signals from the FXS ports of a PBX or telephone exchange.
• VoIP Gateways FXO: In hybrid systems, VoIP gateways often include FXO and FXS ports to bridge the gap between digital IP telephony and traditional analog lines.

FXO and FXS ports are used when a company wants to add another phone network, like Voice over IP (VoIP) or a Private Branch Exchange (PBX). Setting this up can be a bit more complicated because it requires new lines from a different network provider.

What is a VoIP (Voice Over IP) Phone System?

A VoIP system allows companies to make phone calls over the internet instead of traditional phone lines. It offers more advanced features compared to analog phone systems. While traditional phones use FXO and FXS ports, VoIP systems need additional devices, called gateways, to connect.

FXS and FXO Ports with VoIP Systems

FXS Gateway

An FXS gateway is used when you want to connect regular analog devices, like phones and fax machines, to a VoIP system. This gateway converts the signals from these devices so they can work with the VoIP provider.

At Dinstar, we offer a cost-effective FXS VoIP Gateway, which helps small and medium businesses switch to VoIP easily. This gateway allows companies to keep using their existing analog phones while accessing modern VoIP features, saving money on new equipment.

FXO Gateway

An FXO gateway is needed to connect a VoIP system to traditional analog phone lines. It converts the analog signals to VoIP so calls can be made over the internet.

The Dinstar FXO VoIP Gateway provides an affordable way to link analog lines to SIP trunks, enabling internet-based calls. It can also act as a backup, connecting your PBX to traditional phone lines if the network goes down, ensuring you can still make calls during outages.

When linked to a company’s IP PBX system, FXO gateways can use local phone lines at remote sites to avoid long-distance call charges.

What Are the FXS/FXO Procedures?

The typical procedure for setting up FXS and FXO ports involves configuring them in such a way that they can effectively communicate with each other:

1. Determine the number of required ports: Assess how many analog devices (phones, fax machines) need to be connected and how many external phone lines will be used.
2. Connect the devices: Use an FXS port to connect to analog devices like phones, and use FXO ports to interface with external phone lines.
3. Test the connection: Ensure that the FXS port is providing the necessary signals (ringing voltage, dial tone) and that the FXO device responds correctly (rings, makes calls).

Conclusion

FXS and FXO are foundational concepts in analog telephony, with each serving a distinct role in the communication process. The FXS port sends signals to the device, while the FXO port receives them, enabling seamless communication over analog lines.

While newer technologies like VoIP have transformed the landscape, FXS and FXO ports remain relevant, particularly in hybrid systems that still rely on analog components. Understanding the difference between FXS and FXO is key to setting up efficient and functional telecommunication systems.