By David Thompson · Published November 25, 2024 · Updated June 8, 2026 · 7 min read
Quick Answer: A fax machine scans a document into an image, converts it into audio tones, and sends those tones over a phone line. The receiving machine decodes them back and prints the page. Modern online fax does the same thing — without the hardware. mFax.to lets you send a fax from your phone in under 2 minutes.
Fax technology is over 150 years old — predating the telephone. Yet it never went away. The IRS still requires fax for dozens of forms. Hospitals fax medical records millions of times per day. Courts accept faxed contracts as legally binding.
Understanding how a fax works helps you use it more confidently and explains why, for certain documents, nothing else quite replaces it. This guide explains the full picture: from the original analog machine to the modern online fax you can use from your phone.
The One-Sentence Explanation
A fax machine scans a document, converts each pixel to a sound, sends those sounds over a phone line, and a second machine on the other end decodes the sounds back into pixels and prints the result.
That's it. Every other detail is just elaboration on that core loop.
How a Traditional Fax Machine Works
A classic fax machine combines three devices into one box: a scanner, a modem, and a printer. Here's what happens at each stage when you press Send:
Scanning the Document
The machine passes a light source over your document. A row of light-sensitive sensors (called a CCD array) reads each line of the page and converts it into a stream of black-and-white pixel values — typically at 200 DPI (dots per inch) for standard quality.
Encoding the Image
The pixel data is compressed using an algorithm (most commonly Modified Huffman or T.4 coding) to reduce the amount of data that needs to be transmitted. A page of mostly white space compresses dramatically; a dense, hand-drawn document compresses less.
The Handshake
Before any document data is sent, the two fax machines negotiate. The sending machine dials the receiving machine's number. When the call connects, they exchange a burst of tones — this is the iconic screeching noise you've heard. During these 3–5 seconds they agree on: transmission speed, image resolution, compression method, and error-correction protocol (called ECM).
Transmitting Over the Phone Line
The modem converts the compressed pixel data into audio tones — typically between 300 Hz and 3,400 Hz, the range the telephone network carries. These tones travel the same path as a voice call: through copper wire, telephone exchanges, and switching centers. Transmission takes roughly 30 seconds to 2 minutes per page at standard quality.
Receiving and Printing
The receiving fax machine's modem decodes the incoming audio tones back into pixel data. A thermal printer (or laser printer on newer machines) reproduces the image on paper, line by line, from top to bottom — often printing as the data arrives in real time.
Why Fax Uses Audio Tones
The telephone network was designed to carry voice frequencies (300–3,400 Hz). Fax piggybacked on this existing infrastructure by converting digital data into those same frequencies. This is why a fax signal sounds like noise — it's data disguised as sound.
The Fax Handshake: What That Noise Actually Is
The "screech" when two fax machines connect is called the T.30 handshake — a protocol defined by the ITU (International Telecommunication Union).
During those few seconds:
| Tone | What It Does |
|---|---|
| CNG tone (calling) | Sent by the sending machine to identify itself as a fax |
| CED tone (called) | Receiving machine responds, confirming it's a fax |
| DIS signal | Receiving machine announces its capabilities (max speed, resolution, features) |
| DCS signal | Sending machine confirms the settings it will use |
| TCF signal | A short test transmission to verify the line quality |
Only after this negotiation does the actual document data begin to flow. If the line quality is too poor, the machines renegotiate at a lower speed rather than failing outright.
Fax Speeds: Why Some Faxes Take Longer
Fax modems operate at different speeds, measured in bits per second (bps):
| Standard | Max Speed | Approx. Time per Page |
|---|---|---|
| V.27ter | 4,800 bps | ~2 minutes |
| V.29 | 9,600 bps | ~60 seconds |
| V.17 | 14,400 bps | ~30 seconds |
| V.34 (Super G3) | 33,600 bps | ~8 seconds |
A noisy phone line forces the modems to drop to a slower speed. This is why faxing over VoIP (internet phone) can be unreliable — audio compression artifacts disrupt the fax tones. The T.38 protocol was developed specifically to carry fax over IP networks without this degradation.
VoIP and Fax Don't Mix Well
If you're trying to fax over a VoIP phone line and getting failures, this is why. The phone company's audio compression destroys the fax tones. Use an online fax service instead — it bypasses this problem entirely.
How Online Fax Works (The Modern Way)
Online fax services like mFax replace the hardware but keep the underlying fax infrastructure. Here's what happens when you send a fax online:
- You upload a document (PDF, Word, photo) to the app or website
- The service converts it to a standardized fax image format (TIFF/F class)
- The service's servers dial the destination fax number using a real phone line or T.38 VoIP connection
- The fax is transmitted just like a traditional fax — the recipient's machine or fax inbox receives it normally
- You receive a confirmation with timestamp, recipient number, and page count
The recipient never knows whether the fax came from a machine or an online service. It arrives identically.
What Makes Online Fax Better
No hardware. No phone line. No paper jams. You can fax from anywhere — phone, laptop, or tablet. And you get a digital confirmation receipt automatically, which is more reliable than a printed confirmation page from a machine.
Fax vs. Email: Why Fax Hasn't Died
Fax is still used in legal, medical, and government contexts for specific reasons — not nostalgia.
| Characteristic | Fax | |
|---|---|---|
| Delivery proof | ✅ Time-stamped confirmation | ❌ Read receipts are optional and unreliable |
| Legal standing | ✅ Accepted as legal submission by courts, IRS, Medicare | ⚠️ Depends on jurisdiction |
| HIPAA compliance | ✅ Dedicated line = no third-party server | ⚠️ Requires end-to-end encryption and BAA |
| Interception risk | ✅ Dedicated circuit, no server copies | ❌ Passes through multiple servers |
| Recipient verification | ✅ Sent to a specific number, not a shared inbox | ❌ Anyone with the address can receive |
| Audit trail | ✅ Built-in: date, time, sender, pages | ❌ Requires extra tooling |
For an in-depth comparison, see our guide on fax vs. email — when fax is still the better choice.
Receiving a Fax: What Happens on the Other End
When a fax arrives at the receiving end — whether that's a fax machine or an online fax inbox like mFax — the process runs in reverse:
- The modem decodes the incoming audio tones into pixel data
- The image is reconstructed line by line
- If it's a physical machine, it prints immediately as the data arrives
- If it's an online fax service, it saves the incoming image as a PDF and delivers it to your email or app
With an online fax number from a service like mFax, you receive faxes as PDFs in your inbox. No machine, no paper, no ink cartridges.
A Brief History of Fax Technology
Fax is older than most people realize:
| Year | Milestone |
|---|---|
| 1843 | Alexander Bain patents the first facsimile machine |
| 1865 | Giovanni Caselli's "Pantelegraph" transmits images commercially across France |
| 1924 | AT&T transmits the first photograph via telephone line |
| 1966 | Xerox launches the first modern office fax machine (LDX) |
| 1980s | Fax becomes standard office equipment worldwide |
| 1996 | The ITU T.37/T.38 standards enable fax over IP |
| 2000s | Online fax services emerge, eliminating the need for hardware |
| Today | Over 17 billion fax pages sent annually, driven by healthcare, legal, and government use |
How to Send a Fax Today (Without a Machine)
You don't need a fax machine to fax a document. With mFax:
- Open the mFax app or visit mFax.to in your browser
- Upload your document (PDF, photo, or scan)
- Enter the recipient's fax number
- Tap Send — the fax is delivered in under 2 minutes
- You receive an automatic confirmation receipt
Over 5 million people use mFax to send faxes from their phone. It uses the same underlying fax infrastructure as a traditional machine — just without any hardware on your end.
For business teams, mFax Business adds virtual fax numbers, team accounts, HIPAA compliance, and audit logging — starting at about $9/mo (billed annually). There are no rigid tiers: you build your own plan with a live calculator, choosing the exact seats and pages you need and paying only for what you use.