Just the base unit. It is anything that presents a direct load to the ring cadence sent down the line and depending on the value of that load a REN figure will be given. A DSL modem will have a REN of 0.
In practice, you can normally exceed 4 but performance is not guaranteed.
Standard UK ring cadence is 400ms ring, 200ms silence, 400ms ring, 2000ms silence and then repeated. The old ring generators used to have three outputs, off set by 0, 1000, 2000 ms.
Somewhere I still have the old TIG documents (superseded by SINs) which defined Ring Generation and network tones.
M H C
taurus excreta cerebrum vincit