What is a PSTN gateway, and what do FXS and FXO ports do?

A gateway is the device where a telephone circuit meets an IP network. RFC 3435, an IETF document on controlling gateways, describes a media gateway as typically a network element that converts between the audio on telephone circuits and the data packets on an IP network. The conversion runs both ways. Speech arriving on a telephone line leaves as packets, and packets arriving from the IP side leave as a telephone signal.

The RFC sorts gateways by what they connect to. A trunking gateway sits between the telephone network and a voice over IP network and typically manages a large number of digital circuits. A residential gateway offers "a traditional analog (RJ11) interface" for ordinary phones. An access gateway can offer an analog interface or a digital PBX interface. So a PSTN gateway does not need analog ports at all. One that connects only digital trunks, such as a PRI, needs neither FXS nor FXO for that job. Whether a given model also has analog ports depends on how it is equipped. Our answer on how the PSTN works covers the carrier-scale version.

A gateway's job is also different from a phone system's. In the design RFC 3435 describes, a call agent outside the gateway decides where a call goes, and the gateway does the converting. Other gateways handle more of the call setup themselves, and some products put a gateway and a phone system in one box. Either way, the gateway's job is the conversion. Extensions and call queues come from the phone system it works with.

An analog phone line has a side that supplies and a side that uses. Cisco's guide to analog signaling describes the common loop-start arrangement. The central office or FXS side supplies battery power to the line and has a ring generator. The telephone or FXO side leaves the loop open while idle and closes it when someone picks up. Current then flows, and the FXS side detects it and sends dial tone.

FXS stands for foreign exchange station, and FXO stands for foreign exchange office. Each name tells you what belongs on the other end of the cable. A station, meaning a telephone, plugs into an FXS port. A line from the phone company's office plugs into an FXO port.

The two types only work in pairs. Patton's technical note on the two interfaces states the rule: "a telecommunications line from an FXO port must connect to an FXS port in order for the connection to work." Join two FXO ports and neither side supplies dial tone. The jacks look the same, so read the label on the port before plugging anything in.

An analog phone plugs into an FXS port. When someone lifts the handset, the gateway detects it, plays dial tone and collects the digits. It then passes the call to the IP phone system, which decides where the call goes. To that system, the old phone is one more extension.

An analog carrier line goes the other way. Suppose an office with an IP phone system keeps one analog line from the local carrier and plugs it into an FXO port. When a call arrives on that line, the gateway answers it as a telephone would and hands it to the phone system. For an outside call, the gateway takes the line off hook and dials.

An existing PBX can stand in the carrier's place. A PBX's analog extension ports are FXS ports, since analog phones plug into them. A gateway that connects to one of those ports therefore needs FXO. The PBX's analog line ports are the reverse. They are FXO ports made to take a carrier line, so a gateway that feeds them needs FXS.

The mix of ports is a buying decision, and Cisco's VG400 data sheet shows the range within one product family. The VG400 connects analog phones, fax machines and modems to a Cisco phone system. One model has two FXS and two FXO ports. Another has eight FXS ports and no FXO, which suits a site with analog devices to keep and no analog carrier line to connect.

So count connections before choosing a gateway. Each connection that expects to be given dial tone needs an FXS port. That covers the line to each analog device you are keeping and each analog line port on an older PBX the gateway will feed. Each connection that supplies dial tone needs an FXO port. That covers each analog carrier line you are keeping and each analog PBX extension port the gateway will connect to. If the outside connection is a SIP trunk or a PRI and no PBX extension ports are involved, the FXO count is zero. We cover the rest of that setup in what equipment you need for SIP trunking.

FXO ports can also serve as a fallback. The VG400 data sheet describes a way to use designated analog phone ports "to make phone calls through the PSTN during a power outage." That only helps while a working analog line is still attached, and not every gateway has the feature.

The port type settles which side supplies dial tone. It does not settle whether a fax machine or alarm panel will get its data through an IP network. Fax is the usual example. The VG400 documents two methods, T.38 fax relay and fax pass-through. The POTS replacement service from Atlantech Online, a business phone and fiber provider, uses T.38 relay or G.711 pass-through for fax lines. Whichever method is used, the gateway has to use the same one as the service on the other side.

Atlantech's service also shows how the pieces fit together in a building. It replaces copper phone lines with a managed appliance, and its cutover description says "each analog line is moved to an FXS port on the new appliance." That appliance is doing a gateway's FXS job for elevator phones, fire panels and fax machines, the kind of legacy devices at risk when copper lines are retired. On the other side it connects over broadband, with 4G LTE as a second path.

The port is one of three things to check, along with transport and power. The port role is FXS, which tells you the device will be given an analog line. The transport is broadband or LTE, which tells you how the call leaves the building. A copper line was powered from the phone company's office, and an appliance needs power on site. Atlantech lists up to 24 hours of battery backup.

None of the three shows that a particular alarm panel will report correctly, and Atlantech's process does not assume it will. Fire and elevator lines are "tested end-to-end with the monitoring center after each cut." That live test with the party on the far end is the step to copy with any gateway, whoever supplies it.