Факсы второго класса (Group 2, анал-логовые) описываются T.30 и T.3 рекомендациями ITU-T
Факсы третьего класса (Group 3, цифровые) описываются T.30 и T.4 рекомендациями ITU-T
А ещё модемы котороые тоже, туда же, факс-модемы!
* Class 1 fax devices do fax data transfer where the T.4/T.6 data compression and T.30 session management are performed by software on a controlling computer. This is described in ITU-T recommendation T.31.
* Class 2 fax devices perform T.30 session management themselves, but the T.4/T.6 data compression is performed by software on a controlling computer. The relevant ITU-T recommendation is T.32.
* Class 2.0 is different from Class 2.
* Class 2.1 is an improvement of Class 2.0. Class 2.1 fax devices are referred to as "super G3"; they seem to be a little faster than Class 1/2/2.0.
* Class 3 fax devices are responsible for virtually the entire fax session, given little more than a phone number and the text to send (including rendering ASCII text as a raster image). These devices are not common.
