ITU-T Fax Standards: What Do They Cover?

The ITU-T recommendations that define how two fax devices connect, encode, and exchange a document.

Sarah Mitchell

Published on

September 9, 2026

Last updated on

September 24, 2026

T.30, T.37, T.4, and V.34 are ITU-T recommendations that each govern a different part of fax transmission, not interchangeable versions of the same standard.
T.30 defines the call-setup and handshake procedure between two fax devices.
T.4 defines how a Group 3 fax page itself is encoded, and V.34 defines the modem modulation used to move that data over a phone line.
T.37 defines store-and-forward fax delivery over email or the internet rather than a live phone call.

What Are the ITU-T Fax Standards?

The ITU-T fax standards are a group of ITU-T recommendations, T.30, T.37, T.4, and V.34, that each govern a distinct part of how a fax transmission actually happens. They aren't different versions of the same standard, each one defines a separate layer of the process, from how two devices agree to talk, to how the page itself gets encoded.

What Each Standard Actually Defines

T.30: defines the handshake and call-control procedure, how a sending and receiving fax device identify each other, negotiate speed, and confirm a page was received before hanging up.
T.4: defines how a Group 3 fax page is encoded into transmittable data, including how a fax page's resolution is encoded and the compression scheme applied to the scanned image.
V.34: defines the modem modulation used to move that encoded data over a standard phone line at a higher speed than older modulation schemes.
T.37: different from the other three, it defines store-and-forward fax delivery over email or an internet connection rather than a live, real-time phone call, closer in spirit to fax delivered over an internet connection than to a traditional call.

How These Standards Relate to T.38

T.38 is a separate, newer ITU-T recommendation built specifically for T.38's real-time fax-over-IP approach, carrying the same T.30 handshake logic over an IP network instead of an analog line. It's closely related to this group, but distinct enough to warrant its own entry rather than folding into this one.

Related Modulation Standards

V.34 isn't the only ITU-T modulation standard used in fax transmission. Older Group 3 fax devices relied on modulation standards like V.21, V.27ter, and V.29 before V.34 (and the related V.17) became common, each defining a different data rate and signal method for moving fax data over a standard phone line. These are part of the same modulation family V.34 belongs to, not separate fax standards in their own right.

Why This Still Matters

Every fax device, whether a physical machine, a fax-enabled printer, or a cloud service, has to speak the same T.30/T.4/V.34 language to interoperate with the rest of the world's fax infrastructure. This is also part of what determines fax group and modem class compatibility between two devices. The ITU, the standards body behind them, publishes and maintains all of these recommendations.

A Common Case: Two Different Fax Brands Connecting Successfully

A small business replaces an aging fax machine with a new model from a different manufacturer and worries the new unit won't be able to talk to the client fax numbers already on file. Because both machines follow the same ITU-T T.30 and T.4 recommendations regardless of brand, the handshake and page encoding work exactly the same way, and the switch causes no compatibility issues at all.

FAQs

Do T.30, T.37, T.4, and V.34 all do the same job?

No. Each one covers a different part of the process: T.30 handles the handshake between devices, T.4 defines how the page is encoded, V.34 defines the modem speed, and T.37 covers store-and-forward delivery over email instead of a live call.

Is T.38 the same standard as T.30, T.4, and V.34?

No. T.38 is a separate, newer ITU-T recommendation built for real-time fax over an IP network. It carries the same T.30 handshake logic, but over an IP connection instead of an analog line.

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