Fax over IP and T.38, explained
Why a technology from the analogue 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 analogue 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.
Sensible machine settings to begin with: V.29 at 9,600 bps, Super G3 (V.34) turned off, and ECM turned off unless the line genuinely uses T.38.
A single successful page proves little. Test with a document of several pages, at various times of day.
Why faxes fail on an IP line
On a traditional copper line, the fax machine had a dedicated circuit with constant delay. Behind a box or an adapter, the signal is digitised, cut into packets of 10 to 30 ms and sent over a network it shares with other traffic. T.30 was never designed for that; it expects specific tones at precise moments.
Most failures have three causes. Compressed codecs such as G.729, Opus or AMR are tuned for speech and discard the very detail a fax modem uses to carry its data. Jitter, meaning packets that arrive at irregular intervals, makes the device stretch or trim the stream. And a single 20 ms packet lost at the wrong instant can corrupt a line of the image.
Over 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 hangs up. With ECM off, a lost packet generally leaves at worst a faint streak and the page stays legible.
T.38 and G.711 passthrough compared
At call setup, the adapter, router or gateway decides which method to use.
| 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 | Adapter, provider and receiving gateway all have to agree to switch to T.38 (SIP re-INVITE). | G.711 from end to end (A-law in Europe, mu-law in North America) with no transcoding. |
| Speed to use | V.17 at 14,400 bps often works as well; stay away from 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 tends to produce more failures over IP.
- 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 analogue port of the box or adapter, with no splitter or old DSL filter in between. If the interface offers it, declare the port as a fax port.
- 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
- Choose a fixed buffer between 60 and 100 ms over an adaptive one, which resizes itself during the call.
Adapter, gateway or fax server: which equipment
Many internet boxes and business routers have an analogue phone socket, 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 box, then ask your provider, before buying new hardware.
An ATA (analogue telephone adapter) converts an analogue port into a SIP account. It helps when your company uses a VoIP provider or an IP phone system instead of the box's own line. Choose a model with T.38 support and confirm the SIP trunk handles it as well, or the call reverts to G.711.
A multi-port FXS gateway lets several machines keep working at one location. A fax server takes over from the machines entirely: incoming faxes arrive as PDFs in mailboxes and people send from their desks, although it requires a T.38 trunk and a person 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.
Next, send a document of five to ten pages mixing fine print and dark areas to somebody who can report what came out. Do this at three different times of day, including a period when your connection is under the heaviest load, typically the evening, and request a fax back to test reception.
Hold on to the transmission reports. A machine that regularly drops to 4,800 bps reveals a weak line, even if the pages arrive. If failures persist, raise a ticket with your provider asking whether your line supports T.38.
When an online fax service is simpler
A fax machine on an IP line pays off when faxes regularly arrive on a number your contacts already know. For occasional sending, a handful of times a month, the cost of setting it up and maintaining it soon outweighs the odd send.
An online service removes the IP chain on your side from the picture: you upload the document in your browser and our platform dials the recipient's fax machine. CaptainFax works like this: no account, landline recipients in seven countries. It provides no number and receives no faxes, so it complements a receiving line instead of replacing it.
Fax over IP: questions and answers
- Does a fax machine work on a fibre connection?
- Often yes. Fibre is not the obstacle; the box 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 necessarily. ECM is mostly a problem with G.711 passthrough. If your line really negotiates T.38, keep it enabled, run a few tests and compare them with a few runs with it disabled.
- How can I tell whether my box supports T.38?
- Check the interface's telephony settings for a fax or T.38 setting. If nothing is there, ask your provider's support, since results vary with the device model and its firmware.
Keep exploring
A few pages that go well with the one you just read.
- A page goes in one machine and comes out of another. Here is what happens in between.
- What reception takes, why it is not free, and why CaptainFax stays on the sending side.
- Where your document is safe, where it is exposed, and your options.
- Brief, straightforward answers to what people wonder before they send a fax, while it goes out, and once it is done.
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