ITU-T Fax Standards: What Do They Cover?

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.

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.