Fax Transmission: Everything You Need to Know

By David Thompson · Published October 15, 2025 · Updated June 8, 2026 · 12 min read

Fax transmission has been moving documents across phone lines since the 1960s — and it still delivers 17+ billion pages a year worldwide. Yet most people treat it as a black box: you feed paper in, paper comes out elsewhere. This guide removes the mystery. You will learn exactly how every byte of a fax travels from sender to receiver, what the five T.30 phases are, why VoIP breaks fax so often, and how modern online fax transmission eliminates the hardware entirely.

Try mFax to send a fax from your phone in under 2 minutes — no fax machine required.

What Is Fax Transmission?

Fax transmission is the telephonic transfer of a scanned document image from one location to another. The sending machine converts a printed page into a series of audio-frequency tones; the phone network carries those tones to the receiving machine, which decodes them back into an image and prints it.

The word fax comes from facsimile — Latin for "make similar." The goal is always exact reproduction: what went in one end must come out the other end, pixel for pixel.

A fax transmission has three core stages:

1. Scanning — the original document is captured as a bitmap image
2. Transmission — the bitmap is encoded, compressed, and sent as audio signals
3. Reconstruction — the receiving machine decodes the signals and prints or stores the image

All three stages are governed by international ITU-T standards, which is why a fax machine made in 1994 can still talk to a cloud fax service in 2026.

How Fax Transmission Works: Step by Step

Step 1 — Document Scanning

A CCD (charge-coupled device) inside the fax machine scans the document line by line, moving from top to bottom. Each row of the CCD contains 1,728 photosensors. As the sensor passes over the page:

• Light pixels (white) reflect more light → higher voltage reading
• Dark pixels (black text) reflect less light → lower voltage reading

One A4 page generates approximately 2 million voltage variations before any compression is applied.

Step 2 — Data Compression and Encoding

Raw 2-million-bit pages would take minutes to send. Fax machines compress the data dramatically before transmitting. The ITU-T T.4 and T.6 standards define four compression methods:

A typical text page compresses from 2 million bits to fewer than 400,000 bits — roughly a 5:1 reduction — before it ever touches the phone line.

Step 3 — The T.30 Handshake (Capability Negotiation)

Before any page data is sent, the two machines must agree on how to communicate. This negotiation is defined in ITU-T T.30, the core standard for all Group 3 fax calls. It happens in two of T.30's five phases:

Phase A — Call Establishment

The calling machine sends a CNG (Calling Tone) — a 1,100 Hz beep repeated every 3 seconds. This tells the answering device: "I am a fax machine." The answering machine responds with a CED (Called Station Identification) tone at 2,100 Hz, lasting 3 seconds. The characteristic fax screech you hear on a phone is this CED tone.

Phase B — Capability Negotiation

Both machines switch to slow 300 bps signaling (V.21 modulation) and exchange capability packets:

• DIS (Digital Identification Signal) — the receiving machine announces what it can do: maximum speed, supported resolutions, compression formats, and whether it supports Error Correction Mode (ECM)
• DCS (Digital Command Signal) — the sending machine replies with the settings it wants to use, limited to what both sides support

The handshake typically takes 3–15 seconds. Super G3 machines running V.34 can complete it in as little as 9 seconds.

Step 4 — Image Data Transfer

With parameters agreed, the machines switch to high-speed modem transmission for the actual page data. This is Phase C.

Page data is transmitted one scan line at a time. Each line ends with an EOL (End of Line) marker. The sending machine also enforces a minimum transmission time per line — a deliberate pause to prevent the receiving machine's print buffer from overflowing.

The modem speed used depends on the handshake negotiation. The rule is simple: the lower of the two machines' maximum speeds is always used.

Step 5 — Confirmation and Disconnection (Phases D and E)

After the last line of a page is sent, the transmission enters Phase D:

• The sender signals EOP (End of Procedure) for a single-page fax, or MPS (Multi-Page Signal) if more pages follow
• The receiver responds with either:
  • MCF (Message Confirmation Frame) — success, all data received correctly
  • RTN (Retrain Negative) — failure, requesting the sender retrain to a lower speed

Once confirmed, Phase E begins: the sender transmits a DCN (Disconnect) frame and both machines hang up. The sender then prints or saves a transmission confirmation report showing the result.

Types of Fax Transmission

Analog Fax (PSTN)

The original form of fax transmission. Audio-frequency tones travel over POTS (Plain Old Telephone Service) copper-wire circuits. Groups 1 and 2 were purely analog. Group 3 faxes digitize the document internally, but the phone line still carries analog tones.

Best for: Organizations with existing PSTN infrastructure, very high security requirements (physical wire is hard to tap remotely).

Digital Group 4 Fax (ISDN)

Group 4 fax was designed for ISDN lines at 64 kbps — roughly 7× faster than the best Group 3 analog speeds. A single page transmits in under 10 seconds. The compression standard is ITU-T T.6. However, ISDN infrastructure has declined sharply since the 2000s, and Group 4 was never widely deployed commercially.

Fax over IP — T.38 (Real-Time FoIP)

ITU-T T.38 (published 1998) enables real-time fax transmission over IP networks. A T.38 gateway translates the analog T.30 fax signal into IP packets and sends them over the internet or VoIP infrastructure. At the destination, a second gateway converts the packets back into analog signals for the receiving fax machine.

Key features of T.38:

• Uses UDP (preferred) or TCP transport
• Sends redundant packets to compensate for packet loss
• Achieves roughly 10× bandwidth reduction compared to carrying the raw audio over a VoIP codec
• Sensitive to jitter, delay, and packet loss — VoIP networks must be configured carefully to support T.38

Store-and-Forward Internet Fax — T.37

ITU-T T.37 sends fax documents as email attachments in TIFF-F format via SMTP. Unlike T.38, this is not real-time — the document is routed through email servers before delivery. Acceptable for non-urgent documents where timing does not matter.

Online / Cloud Fax

Modern online fax services (including mFax) eliminate the fax machine entirely. The workflow:

1. You upload a PDF, photo, or document in the app or web portal
2. The service converts it to a fax-compatible format
3. A backend gateway dials the recipient's fax number using T.38 or PSTN infrastructure
4. The recipient's machine receives a standard G3 fax — it never knows the sender had no hardware

The transmission happens in the cloud. You get a delivery confirmation on your phone. The receiving end sees a perfectly normal fax.

mFax supports sending faxes from any device — iPhone, Android, or web browser — with a 98% delivery success rate.

Fax Transmission Standards and Speeds

Fax Groups Explained

The ITU standardized fax into four Groups, each representing a generation of technology:

Group 3 is the global standard in use today. When people say "fax machine," they mean a G3 device.

Modem Standards Used in G3 Fax

The V.21 300 bps channel is used exclusively for the T.30 handshake signals (DIS, DCS, etc.). All actual page image data travels at the negotiated higher speed.

How Long Does Fax Transmission Take?

Fax transmission time depends on the negotiated modem speed, the complexity of the document, and the overhead phases.

Add 15–30 seconds for the T.30 handshake on every call, regardless of page count. A 10-page fax at 9,600 bps takes approximately 10–12 minutes total (handshake + transmission).

Factors that slow transmission:

• Dense graphics or photographs (dark pixels require more encoding)
• Noisy phone line (machines negotiate down to a lower speed)
• Error Correction Mode retransmissions on poor lines
• VoIP jitter causing retraining

Error Correction Mode (ECM)

Error Correction Mode is an optional feature of Group 3 fax (defined in T.30 Annex A) that delivers bit-perfect pages despite line noise.

How ECM works:

1. Page data is divided into blocks called partial pages, each containing HDLC frames
2. Each frame includes a Frame Check Sequence (FCS) — a checksum
3. The receiving machine validates every frame's checksum
4. If a frame is corrupted, the receiver sends a PPR (Partial Page Reject) response
5. The sender retransmits only the bad frames — not the entire page

The result is perfect image quality. The tradeoff: slightly longer transmission time on noisy lines.

Fax Transmission Errors: Causes and Solutions

Most fax failures are transient — retrying once fixes the majority of them.

Common fax error codes:

For a full reference, see our complete guide to fax error codes.

Fax Transmission Confirmation Page

After every fax send attempt, most fax machines automatically print a transmission verification report (also called a fax confirmation page or activity report).

A standard confirmation page includes:

• Sender's fax number
• Recipient's fax number or name
• Date and time of transmission
• Total transmission duration
• Number of pages sent
• Result: OK (success) or NG / Error (failure)
• A thumbnail of the first page (on most modern machines)

Activity journals log the last 200+ incoming and outgoing transmissions and can be programmed to print automatically at set intervals (every 50 faxes, every 6/12/24 hours, etc.).

Online fax services like mFax send the confirmation as a push notification and email — no paper report needed.

Is Fax Transmission Secure?

Security depends entirely on the transmission method:

Analog PSTN Fax

Fax travels as unencrypted audio tones over copper telephone lines. Intercepting an analog fax requires physical access to the telephone line — wiretapping. This makes remote interception effectively impossible compared to email, which passes through dozens of internet servers.

Vulnerabilities that do exist:

• Misdialed numbers — the most common cause of fax going to the wrong recipient
• Unattended output trays — documents left visible on shared machines
• Physical line access — a motivated attacker on the same premises could tap the line

VoIP / T.38 Fax

Fax carried over an unencrypted IP network can be captured using standard packet analysis tools if an attacker has access to the network. The T.38 protocol has no built-in encryption or authentication.

Online Fax

Reputable services encrypt data in transit using TLS/SSL. The fax content is protected from your device to the service's gateway. For healthcare and legal use cases:

Fax Transmission Today: Who Still Uses It and Why

Fax transmission is far from obsolete. The numbers tell a clear story:

Three industries dominate fax usage:

Healthcare — hospitals, clinics, and pharmacies use fax to transmit prescriptions, referrals, lab results, and patient records. Many EHR systems lack real-time interoperability; fax remains the universal bridge. Fax also provides a court-admissible timestamp that email does not.

Legal — courts and law firms accept fax as a legally recognized form of document submission in most jurisdictions. A fax transmission record can be entered as evidence of timely filing.

Government / IRS — the IRS accepts faxed forms including Form SS-4, Form 2848, and Form 1040. Many federal agencies still require physical or faxed signatures for legal instruments.

The shift is not away from fax transmission — it is away from fax hardware. Cloud fax usage is growing at 5–11% annually as organizations replace physical machines with online services.

Online Fax Transmission: The Modern Alternative

Online fax services preserve the legal and operational benefits of fax transmission while eliminating the machine, the dedicated phone line, and the physical paper trail.

How a cloud fax transmission works:

The receiving fax machine sees a perfectly ordinary Group 3 fax. It has no idea the sender used a smartphone. The legal delivery record is identical.

For personal faxing, mFax handles the full transmission from your phone — upload, send, confirm. Over 5 million users, 4.8-star App Store rating, 98% delivery success rate.

For business teams, mFax Business adds virtual fax numbers, team accounts, HIPAA compliance, and a shared web dashboard — starting at about $9/mo (billed annually). Instead of rigid fixed tiers, you build your own plan with a live calculator, choosing the exact seats (1–35) and pages (200–5,000) you need and paying only for what you use.

Summary: Fax Transmission at a Glance

For more background on fax technology, see How Does a Fax Work? and our guide to Fax Over IP (VoIP Fax).

Ready to skip the hardware? mFax handles the full fax transmission from your phone — scan, send, and confirm delivery in under 2 minutes.