Beacon fires
If a specific message might need to be passed with great urgency, beacon fires could be planned from point A to point B to relay a single, planned message. For example, when the Spanish Armada was expected, the English lit the beacon fires to warn that the ships had been sighted. No special technology needed, just a series of high points within sight. Of course, that can only send one specific, urgent message...but in time of need, very effective. (IIRC, the beacons of Rohan in Lord of the Ring were similar...)