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

A PSTN gateway is hardware or software that sits between the public switched telephone network and a packet network and converts calls from one to the other.[1] The formal name for that function is a media gateway, which the IETF defines as a network element that provides conversion between the audio signals carried on telephone circuits and data packets carried over the Internet or over other packet networks.[1] In practice you will also see the same box called a VoIP gateway, a voice gateway, an FXS or FXO gateway, or, in its smallest form, an analog telephone adapter.[2][3] Whatever the label, the task is the same: take a call that arrives as a continuous electrical signal on a phone line and turn it into a stream of IP packets, then do the reverse for calls going the other way.[2]

That conversion has two halves. The gateway digitizes and encodes the voice itself, sampling the audio and packing it into Real-time Transport Protocol packets with a codec such as G.711 or G.729.[2][3] It also translates the signaling, meaning the on-hook, off-hook, dialing, and ringing information that tells the network when a call starts, what number to reach, and when to hang up.[1] On the telephone side that signaling might be analog loop current or a digital protocol carried on a T1 or E1 circuit, and on the IP side it becomes a VoIP control protocol such as SIP, MGCP, or H.248.[1][8][9] Because those two worlds encode a call so differently, the gateway is what lets an analog handset, a fax machine, or an older PBX keep working while the calls behind them travel over the internet.[2]

Why a gateway is useful during the move to IP

Many organizations still run analog devices or a TDM PBX that would be costly to replace all at once, and a gateway lets that equipment connect to a SIP trunk or a hosted phone system while the rest of the network moves to IP.[5][7] A small residential-scale gateway gives a single analog line an IP path, while a trunking gateway manages a large number of digital circuits for a carrier or a large site.[1]

An FXS port is the interface a gateway uses to power and control an analog end device, and it behaves the way a phone company's central office does toward a home phone.[4] Foreign Exchange Subscriber is the full name, and the S for subscriber is the reminder that this port faces the subscriber's equipment: a telephone, a fax machine, a modem, or the trunk port of an older PBX that expects to see a phone line.[3][4] The port supplies the three things an analog phone needs: dial tone so you know the line is ready, battery current so the handset has power over the copper pair, and ring voltage so it physically rings on an incoming call.[4] Because the FXS port generates that ring voltage, which is what makes the attached device ring, it is the active side of the pair.[4][11]

On a gateway, the FXS ports are where you plug in the analog hardware you want to keep. A small analog telephone adapter might have one or two FXS ports for a single phone or fax, while a larger FXS gateway can present dozens of them so an office full of analog handsets registers to an IP PBX.[3][5] Each port usually maps to a SIP account or profile on the call server, which makes an old two-wire phone look like a modern IP endpoint to the system.[6] Connecting a fax machine this way normally means turning on T.38 so the fax tones survive the packet network, a point the trade-offs section returns to.[10]

An FXO port is the interface a gateway uses to connect to a live telephone line, and it behaves the way a telephone does toward the network.[4] Foreign Exchange Office is the full name, and the O for office is the reminder that this port faces the telephone company's central office, or a PBX that is acting as one toward its extensions.[4][5] Rather than supplying power, the FXO port receives dial tone and line power from whatever it plugs into, then signals its state by opening and closing the electrical loop: closing the loop takes the line off-hook to seize it, and reopening the loop puts it back on-hook to release the call.[4][11] To dial, the port sends the digits as DTMF tones, the same tones a push-button phone produces.[11]

On a gateway, the FXO ports are where the outside lines come in. An FXO gateway lets a VoIP phone system reach the PSTN over ordinary analog lines, which suits a site that keeps a few copper lines for local calling or for reliability.[5] The same design gives you a fallback path, because if the internet or the SIP trunk goes down, calls can route out through the FXO ports to the physical phone lines instead.[5] Some adapters combine both port types, so a single unit might offer one FXS port for a phone and one FXO port for a PSTN line, with the FXS side hard-relayed to the FXO side during a power outage so the phone still reaches the emergency network.[6]

The gateway translates between analog line signals and IP call control, so a physical event on the copper pair becomes a message the softswitch understands, and the reverse.[1] When an analog phone on an FXS port goes off-hook, the gateway detects the loop current, plays dial tone, and collects the DTMF digits the caller dials.[3][11] It then builds a call setup request, usually in SIP, and sends it to the IP PBX or trunk that will route the call.[8] For a call coming the other way, the gateway receives the SIP invitation, applies ring voltage to the FXS port so the attached phone rings, and connects the audio once someone answers.[3][4]

Underneath the signaling, the gateway is also converting the voice. It encodes the analog audio into digital samples and packs them into RTP packets with a codec, most often G.711 for full-quality calls or G.729 where bandwidth is tight, then decodes the packets back into sound for the analog side.[2][3] Two families of control protocol are common. In a decomposed design the gateway is a relatively simple device driven by an external call agent over MGCP or H.248, which keeps the call logic centralized.[1][9] In the design more common today, the gateway runs SIP itself and behaves like an intelligent endpoint that registers to the phone system on its own.[2][8] The gateway also handles the small but real details of an analog line, such as echo cancellation and passing DTMF reliably, so the IP side hears a clean call.[2]

PSTN gateways come in a few shapes depending on how many lines you have and whether they are analog or digital.[1][7] The right one depends on the equipment you are connecting and the scale, which can run from a single fax machine to a carrier trunk carrying hundreds of calls.

An analog FXS gateway or ATA

An analog FXS gateway, in its smallest form an analog telephone adapter, connects individual analog devices to an IP phone system.[3] You use it when you have phones, fax machines, alarm panels, or door intercoms that you want to keep, but the calls behind them should travel over VoIP.[3][5] These units run from a single-port adapter up to rack-mounted gateways with 24 or more FXS ports for an office that still uses analog handsets.

An analog FXO gateway

An analog FXO gateway connects a VoIP phone system to physical PSTN lines.[5] You reach for it when you want to keep a handful of copper lines, whether for local calling, for a dedicated fax line, or as a backup route if the internet connection fails.[5] A hybrid adapter such as the Grandstream HT813 pairs an FXO PSTN line port with an FXS port, so one box can bring an outside line into a SIP-based system and still power a local phone.[6]

A digital trunking gateway

A digital gateway connects TDM circuits rather than individual analog lines, converting voice between PRI, T1, E1, or BRI connections and VoIP.[7] A large site or a carrier uses this kind of gateway to bridge a legacy PBX or the PSTN to SIP at scale, and Sangoma's digital gateways, for instance, come in 1 to 8 ports of T1, E1, or PRI and carry up to 120 simultaneous calls per unit.[7] The standards call this a trunking gateway, one that interfaces the telephone network to a VoIP network and manages a large number of digital circuits.[1]

A gateway keeps analog and legacy gear working on an IP network, and a few practical issues are worth knowing before you depend on one.[2]

Fax needs T.38, not just any codec

Fax machines and modems send data as tones that a voice codec can distort once calls are packetized, so a plain VoIP path often fails a fax that worked fine on copper.[10] The fix is T.38, the ITU-T protocol for carrying real-time Group 3 fax over IP, which most fax-capable gateways support.[6][10] If fax matters, confirm that both the gateway and the service on the far side handle T.38 rather than assuming G.711 will carry it.

Power and lifeline behavior

An analog phone on a traditional copper line keeps working in a blackout because the line is powered from the exchange, and a gateway on IP does not have that property on its own.[3] Some adapters address this with a lifeline relay that hard-connects the FXS port to an FXO port during a power outage, so the local phone falls back to the physical PSTN line for emergency calls.[6] Where that matters, plan backup power for the gateway and the network gear it depends on.

Echo, delay, and clocking

Bridging two different networks introduces the analog-side issues a gateway has to manage, including echo and the timing differences between a packet network and a TDM circuit.[2] Media gateways include echo cancellation and DTMF handling for this reason, and on a digital gateway the clocking of the T1 or E1 has to line up with the carrier to avoid slips.[2][7] These are usually configuration details rather than dealbreakers, though they are the first things to check when call quality is off.

Security of a networked voice device

A gateway that faces the internet is a networked computer running a voice stack, so it needs the same care as any edge device.[6] Encryption of the media and signaling, current firmware, and access control all apply, and Grandstream's adapters, for example, ship with AES media encryption and a per-unit certificate.[6] Treat the gateway as part of your security perimeter rather than as a passive phone accessory.

A PSTN gateway is often mixed up with a few neighboring devices, and separating them clarifies what it does.[2]

It is not a PBX or phone system

A gateway connects phones and lines to a call-control system, but it does not run the call logic itself.[5] The IP PBX or hosted service decides how calls route, who rings, and what features apply, and the gateway simply presents analog or TDM endpoints to that system as if they were IP.[1][5]

FXS and FXO are not interchangeable

The two port types face opposite directions, so you cannot swap one for the other.[4] Plugging a device meant for an FXS port into an FXO port, or joining two FXO ports together, leaves nothing supplying dial tone and ring voltage, and the connection fails.[4] Matching the port to the equipment is the most common setup mistake to avoid.

It is not tied to analog only

Although the FXS and FXO ports people ask about are analog, a PSTN gateway can just as easily terminate digital trunks.[7] A digital gateway converts PRI, T1, or E1 circuits to SIP, which is how larger sites and carriers bridge legacy systems without any analog ports at all.[1][7]

It is not a permanent arrangement

The FXO side of a gateway depends on live PSTN lines, and those are being retired as carriers move to all-IP.[7] Gateways stay useful for connecting analog devices and legacy PBXs through that transition, though the long-term direction is native IP, which is the destination a gateway is helping older equipment reach.[5][7]