By David Thompson · Published December 4, 2024 · Updated June 8, 2026 · 9 min read
Quick Answer: A fax machine works by scanning a document into a grid of black-and-white pixels, compressing the data, and converting it into audio tones that travel over a telephone line. The receiving fax machine decodes the tones back into pixels and prints an exact copy of the page. The whole process takes about 30 seconds to 2 minutes per page. Today you don't need the machine at all — an online fax service like mFax does the same thing from your phone or browser.
A fax machine is one of the oldest pieces of office technology still in daily use. Courts, hospitals, the IRS, and government agencies still rely on fax because it provides a legally recognized delivery record that email cannot match. Yet most people have no idea what's happening inside that beige box.
Understanding how a fax machine works takes about five minutes and explains why fax has outlasted every prediction of its death — and why modern online fax services can do the same job without any hardware at all.
What Is a Fax Machine?
A fax machine (short for facsimile machine) is a device that scans a printed document, converts it into a digital image, and transmits that image over a telephone line to a second fax machine, which prints an exact copy. Faxing is simply the act of sending a document this way — as an image of the page rather than editable text.
The key word is exact image. Unlike email, which sends editable text or attachments that can be modified, a fax transmits a bitmap — a pixel-for-pixel picture of your document. The receiver gets a reproduction, not a file. This immutability is precisely why legal, medical, and government systems trust it.
Fax machines are most commonly used to send contracts, medical records, legal filings, and tax forms — anywhere a verifiable, tamper-resistant paper trail matters more than speed or editability.
A Technology Older Than the Telephone
The fax machine predates the telephone by nearly two decades. Scottish inventor Alexander Bain patented the first facsimile device in 1843, using synchronized pendulums to transmit images over telegraph wires. The modern phone-line fax we recognize today became standard office equipment in the 1980s.
The Core Components of a Fax Machine
Every fax machine — from a 1985 standalone unit to a modern all-in-one printer — contains the same three fundamental systems working together:
Scanner
Reads the document line by line using a row of light-sensitive sensors (a CCD array). Converts what it sees into a stream of pixel values — black or white, at 200 DPI for standard quality.
Modem
The brain of the machine. Converts digital pixel data into audio tones that travel over the phone line — and decodes incoming tones back into pixel data when receiving. Operates in the 300–3,400 Hz frequency range.
Printer
Reproduces the received image on paper. Thermal fax machines use heat-sensitive paper; modern plain-paper machines use laser or inkjet technology. Many print line by line as data arrives.
Supporting these three core components:
- Document feeder — feeds pages through the scanner at a controlled rate
- Telephone line interface — physically connects the machine to the phone network (RJ-11 jack)
- Control panel — keypad for dialing, buttons for speed dial and settings, status display
- Memory buffer — stores pages when the machine is busy or between transmissions
- Paper tray — holds the blank paper for incoming faxes (on plain-paper models)
How Does Faxing Work?
Faxing works in five basic stages: scan, compress, connect, transmit, and print. The sending machine scans your page into pixels and compresses the image, dials the recipient and negotiates a connection (the "handshake"), converts the compressed data into audio tones, and sends them down the phone line. The receiving machine reverses the process — decoding the tones and printing an exact copy. Here is exactly what happens at each stage, from the moment you press Send to the moment the page prints on the other end:
Document Scanning
You place your document face-down in the feeder. A motorized roller pulls it through at a steady speed. As it passes over the CCD (Charge-Coupled Device) array, a light source illuminates each line of the page. The sensors measure the reflected light for each point — bright means white paper, dark means ink. The scanner typically captures 200 lines per inch (standard quality) or up to 400 DPI for fine mode. Each line becomes a row of 1,728 pixels across a standard letter-width page.
Image Compression
Raw bitmap data for a single page would be enormous over a voice-quality phone line. Before transmission, the machine compresses the image using the Modified Huffman (MH) or Modified Read (MR) algorithm — both defined in the ITU T.4 standard. A page of mostly white space (like a typed letter) compresses by 90% or more. A dense hand-drawn document compresses much less. This compression is what makes fax transmission practical over a narrow telephone channel.
The Handshake
The sending machine dials the recipient's fax number just like a phone call. When the receiving machine picks up, the two machines don't talk — they beep at each other. This 3–5 second negotiation is called the T.30 handshake (an ITU standard). During the handshake, the machines agree on: transmission speed, image resolution, compression method, and whether to enable Error Correction Mode (ECM). Only after this negotiation does document data begin to flow.
Signal Transmission
The sending machine's modem converts the compressed pixel data into audio tones — frequencies between 300 Hz and 3,400 Hz, the same range the telephone network carries for voice. These tones travel over copper wire through telephone exchanges, switching centers, and whatever path the phone network chooses, arriving at the receiving machine. A standard V.17 modem transmits at 14,400 bps, sending a letter-sized page in roughly 30 seconds. Faster V.34 modems reach 33,600 bps and can send a page in under 10 seconds.
Decoding and Printing
The receiving machine's modem listens to the incoming tones and decodes them back into compressed pixel data. It decompresses the data to reconstruct the full bitmap image. The printer then reproduces the image on paper — thermal machines by heating dots on heat-sensitive paper, laser machines by fusing toner, line by line. On many machines, printing begins before the full page is received, so you can watch the fax emerge in real time.
Confirmation
After the last page transmits, the machines exchange final signals. The sending machine prints (or displays) a transmission confirmation report showing: date and time, recipient number, number of pages, transmission time, and result (OK or error code). This report is what courts and the IRS accept as proof of delivery.
The Fax Handshake: What That Screeching Sound Actually Is
The distinctive noise when a fax machine connects is the T.30 protocol handshake — not an error, not a fault, but a structured conversation between two machines. Each tone carries specific information:
| Signal | Sent By | Purpose |
|---|---|---|
| CNG (Calling Tone) | Sending machine | Identifies itself as a fax device, not a voice caller |
| CED (Called Station ID) | Receiving machine | Confirms it is a fax machine and is ready |
| DIS (Digital Identification Signal) | Receiving machine | Announces its capabilities: max speed, resolution, ECM support |
| DCS (Digital Command Signal) | Sending machine | Confirms the settings it will use based on DIS |
| TCF (Training Check Frame) | Sending machine | Short test burst to verify line quality before committing to full speed |
If the line quality check fails, the machines drop to a lower speed and try again — automatically. This is why fax degrades gracefully on noisy lines rather than failing outright.
Types of Fax Machines
Not all fax machines work the same way. The core scanning and transmission process is identical, but the printing technology differs significantly — and it matters for document quality and longevity.
Thermal Fax Machines
The original desktop fax type. These machines use a thermal print head that heats dots on specially coated paper, causing those areas to darken.
Pros: Simple, inexpensive, no toner or ink required Cons: Requires special thermal paper (rolls, not plain sheets); fades in sunlight and heat within a few years; cannot be highlighted or photocopied reliably
Thermal fax machines are still common in small offices and retail environments where cost matters more than document permanence.
Plain-Paper Fax Machines
Laser-based or inkjet fax machines print received faxes on standard paper, producing copies that are indistinguishable from laser-printed documents.
Pros: Permanent, archivable output; uses regular paper; works with copy machines and highlighters Cons: More expensive; requires toner or ink cartridges
Most standalone plain-paper fax machines manufactured since the 2000s are laser-based and handle high-volume office faxing.
Multifunction Printers (MFP) with Fax
The most common type in modern offices. These devices combine a printer, scanner, copier, and fax machine into a single unit connected to your phone line and your network.
Pros: Space-efficient; uses regular paper; integrates with computer workflows; some support fax-to-email forwarding Cons: Higher upfront cost; if the machine breaks, all four functions are down
Most businesses that still use physical fax hardware use MFPs rather than standalone fax machines.
Internet Fax Adapters
These are not fax machines themselves — they are adapters that connect a standard fax machine to a VoIP or internet connection instead of a traditional phone line. They translate fax signals into T.38 protocol packets for transport over IP networks.
VoIP and Fax Compatibility Problems
Standard VoIP lines compress audio in ways that destroy fax tones — causing transmission failures and garbled pages. If you're faxing over a VoIP phone system, you either need a T.38-capable adapter or (much simpler) an online fax service that bypasses the hardware problem entirely.
Fax Machine Transmission Speeds
Fax modem speed determines how long a page takes to transmit. The telephone network negotiates speed automatically — faster when the line is clean, slower when there's noise or interference.
| Modem Standard | Max Speed | Time Per Page (approx.) |
|---|---|---|
| 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 |
| T.38 (over IP) | Variable | ~10–20 seconds |
Most faxes today use V.17 or V.34. The actual time per page also depends on document complexity — a page dense with handwriting transmits slower than a typed letter on white paper because it compresses less.
How Online Fax Works (Without the Machine)
Online fax services like mFax replicate the entire fax machine process in software — you supply the document, the service handles everything else.
Here's the parallel:
| Physical Fax Machine | Online Fax Service |
|---|---|
| You feed paper into the scanner | You upload a PDF or photo |
| Machine scans to bitmap | Service converts to TIFF/F fax format |
| Modem dials recipient number | Service dials using real phone infrastructure |
| Transmits tones over phone line | Transmits via T.38 or dedicated fax lines |
| Prints confirmation report | Sends digital delivery receipt to your email |
| Recipient machine prints | Recipient gets PDF in inbox (or physical printout if using a machine) |
The recipient cannot tell whether the fax came from a physical machine or an online service. The signal arriving at their end is identical.
No Hardware Required
With mFax, you get the same legally recognized fax delivery — timestamped confirmation, recipient verification, and delivery proof — without a fax machine, phone line, paper, or ink. Over 5 million people use mFax to send faxes from their phone or browser.
Why Fax Machines Are Still Used in 2026
Fax has survived email, Slack, DocuSign, and cloud storage. The reason is specific, not sentimental:
Legal delivery proof. A fax transmission confirmation report shows the exact date, time, recipient number, and number of pages transmitted. Courts and the IRS have decades of legal precedent treating this as proof of delivery. Email read receipts are optional and easy to dispute.
HIPAA compliance. A dedicated fax line transmits to a specific number without routing through a server that stores the document. Healthcare providers have used fax for PHI (Protected Health Information) for decades under established HIPAA guidelines.
Interoperability. Every fax machine speaks the same T.30 protocol. There's no vendor lock-in, no account required, no app to download. A fax sent in 1995 can reach a machine set up yesterday.
Government and court requirements. Many courts still only accept filings by fax. The IRS lists specific fax numbers for dozens of tax forms. Government systems move slowly — the infrastructure is entrenched.
Fax by the Numbers
An estimated 17 billion fax pages are sent annually worldwide. Healthcare accounts for the majority, followed by legal services and government. The fax services market is projected to grow to USD 4.47 billion by 2030 — driven not by hardware sales but by online fax adoption.
How to Send a Fax Without a Fax Machine
You don't need any hardware to send a legally valid fax. With mFax:
- Open the mFax app on your phone or visit mFax.to in a browser
- Upload your document — PDF, photo, Word file, or scan
- Enter the recipient's fax number (with area code)
- Tap Send
- Receive a digital delivery confirmation automatically
The fax arrives at the recipient's machine or fax inbox identically to one sent by a physical machine.
For business and healthcare teams, mFax Business adds virtual fax numbers, HIPAA compliance with Business Associate Agreements, team accounts, and full audit logging — starting at about $9/mo (billed annually). Rather than forcing you into fixed tiers, it lets you build your own plan with a live calculator, picking the exact seats and pages you need and paying only for what you use.