system of flag signalisation in ancient fleets

[eugene]

Is there any survived information in sources about the system of flag signalisation in Roman, Greek or Phoenician fleets?

[Praefectusclassis]

Signalling in ancient fleets seems to have been very primitive, with only a colored flag or shield catching the sun used to signify the start of battle. That's it. The naval flag signalling system is, iirc, an 18th century invention.

[eugene]

I wonder why this system of signalisation was not invented: it is not very hard think
and does not requires much technical developement.

[Praefectusclassis]

That goes for many things. You just happen to have to think of them. And need them, of course.

[eugene]

I think that ancient fleets need such signalisation no less than in the 18-th century and later.

[M. Demetrius]

Somewhere, I think on RAT, I read about a system of flag signals that could relay messages between watchtowers, even with a changing-key code system. If on land, why not at sea?