By means of signalling. A few basic facts about Roman signalling survive, including the use of fire, smoke, and wooden beams. The first two are fairly obvious but the last, which is only mentioned in passing by Vegetius, is rather obscure and not understood. Beacons are shown being prepared and used on Trajan’s Column, presumably for the purposes of signalling. Modern studies have examined lines of sight between signal stations, forts, milecastles and turrets in the region of the Wall. Some signal towers allowed sites that could not see each other to communicate, as Barcombe Hill signal station did for Housesteads and Vindolanda.
Further reading: Woolliscroft 2001