Fax over IP and T.38, explained
Why a technology from the analog era struggles on internet telephony, and how to give it the best chance.
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A fax machine speaks T.30, a protocol made for a steady analog line. On an IP network, the call is cut into packets, and two methods let a fax cross it: T.38, which converts the fax signal into data packets, and G.711 passthrough, which sends the audio as it is. T.38 tolerates packet loss better. G.711 only works on a clean connection with no compression. Whichever you use, slow the machine down and test with more than one page.
Key points
During a transmission two modems talk to each other continuously. Anything that alters the audio, such as compression, lost packets or uneven timing, can break the handshake or damage the page being sent.
T.38 was built with IP networks in mind. G.711 passthrough may work too, but only if nothing along the path compresses or re-encodes the audio.
For the machine itself, a sensible first configuration is V.29 at 9,600 bps, Super G3 (V.34) off, and ECM off unless the line really runs T.38.
A single page that goes through proves little. Test with a multi-page document and at different hours of the day.
Why faxes fail on an IP line
On a conventional copper line, a fax machine had a dedicated circuit and a steady delay. Behind a modem or adapter, the signal is digitized, chopped into 10 to 30 millisecond packets and carried over a network that also handles other traffic. T.30 was never designed for that, since it expects particular tones at precise moments.
Three causes explain most failures. Compressed codecs such as G.729, Opus or AMR are tuned for speech and throw away the very detail a fax modem uses to carry its data. Jitter, meaning packets arriving unevenly spaced, makes the device stretch or trim the stream. Finally, one lost 20 ms packet at the wrong instant can corrupt a line of the image.
With G.711, error correction mode (ECM) often does more harm than good: damaged blocks are requested again and again, retransmissions pile up, the session times out and the machine gives up. If ECM is off, a lost packet at worst leaves a faint streak, and the page remains readable.
T.38 and G.711 passthrough compared
The adapter, router or gateway decides on the method as the call is being set up.
| Criterion | T.38 (fax relay) | G.711 passthrough |
|---|---|---|
| Principle | The fax signal is decoded at the edge of the IP network, carried as data (UDPTL or TCP) and rebuilt at the other end. | The modem audio is carried unchanged, like an uncompressed voice call. |
| Packet loss | Well tolerated: packets repeat earlier data (redundancy). | Poorly tolerated: a loss can corrupt a line or abort a page. |
| Jitter | Little impact, because the modem timing is regenerated locally. | Strong impact; a fixed jitter buffer helps a lot. |
| Bandwidth | Low, increasing with the level of redundancy. | About 64 kbps of audio in each direction plus headers, roughly 80 to 90 kbps. |
| Prerequisites | For T.38 to be used, the adapter, the provider and the gateway at the receiving end all have to agree to the switch (SIP re-INVITE). | G.711 from end to end (mu-law in North America, A-law elsewhere) with no transcoding. |
| Speed to use | Speeds up to 14,400 bps (V.17) usually hold up; steer clear of V.34. | 9,600 bps (V.29) with ECM off. |
Settings on the machine and on the adapter
Menus differ between models, so treat these values as a starting point rather than a rule.
- Transmission speed
- Limit the machine to V.29, i.e. 9,600 bps. Beginning at 14,400 bps usually leads to more failures on an IP line.
- Super G3 and V.34
- Switch them off. At 33,600 bps the connection must be almost perfect.
- ECM
- Off on G.711. If T.38 works well, turn it on again and compare the results.
- Telephone port
- Connect the machine directly to the analog port on the modem or adapter, with no splitter and no leftover DSL filter in between. If the interface lets you mark the port as a fax port, do so.
- Codec and voice features
- If you can choose, put G.711 first, and turn off silence suppression, comfort noise and echo cancellation for the fax line.
- Jitter buffer
- A fixed buffer between 60 and 100 ms is preferable to an adaptive one, which resizes itself as the call goes on.
Adapter, gateway or fax server: which equipment
Many cable and fiber modems and business routers have an analog phone jack, but how they treat a fax call (passthrough, T.38, adjustable or fixed settings) depends on the model and the firmware. Read the settings of your modem, then ask your provider, before buying new hardware.
An ATA (analog telephone adapter) converts an analog port into a SIP account. It comes in handy when your business relies on a VoIP provider or an IP phone system rather than the modem’s own line. Pick a model with T.38 support and confirm the SIP trunk allows it as well; if not, the call drops back to G.711.
With a multi-port FXS gateway, several machines can keep running at the same location. A fax server takes over from the machines completely, delivering incoming faxes as PDFs to mailboxes and letting staff send from their desks, though it requires a T.38 trunk and somebody to maintain it.
Test before you rely on it
Start by plugging an ordinary telephone into the same port: it should give a dial tone and be able to call a landline. If it cannot, the problem is the line and not the fax.
Then send a document of five to ten pages mixing fine print and dark areas to someone who can tell you what came out. Do it at three different times of day, including an evening hour when your connection is busiest, and request a fax back to test reception.
Keep the transmission reports. When a machine regularly drops to 4,800 bps, the line is fragile, even if the pages go through. If trouble persists, contact your provider’s support and find out whether T.38 works on your line.
When an online fax service is simpler
Keeping a fax machine on an IP line pays off when you often receive faxes at a number people already know. If you send only a handful per month, installing and maintaining it will soon cost more than those occasional sends.
An online service removes the IP chain at your end from the equation: you upload the file in your browser and the service dials the recipient’s fax machine. CaptainFax operates this way, needs no account, and reaches landlines in seven countries. It supplies no number and can’t take incoming faxes, so it complements a receiving line instead of replacing one.
Fax over IP: questions and answers
- Does a fax machine work on a fiber connection?
- Often yes. Fiber is not the obstacle; the modem or adapter is. With clean G.711 passthrough or with T.38, and the machine limited to 9,600 bps, most transmissions go through.
- Should ECM be switched off with T.38?
- Not always. ECM chiefly causes trouble with G.711 passthrough. If the line truly negotiates T.38, keep it enabled, run several tests, and compare against a similar set with it disabled.
- How can I tell whether my modem supports T.38?
- Check the telephony settings in its interface for a fax or T.38 option. If there isn’t one, contact your provider’s support, as the answer frequently hinges on the model and firmware.
Keep exploring
A few pages that go well with the one you just read.
- Feed a page into one machine and it prints out on another. This is the story of the call in between.
- What it takes to receive, why it isn’t free, and why CaptainFax sticks to sending.
- Which parts of the journey are protected, which are not, and what you can do about the gaps.
- Quick, plain answers to what people wonder before they send a fax, while it goes out and once it is done.
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