The Limes Arabicus was a desert frontier of the Roman Empire, in the province of Arabia Petraea. It ran -at its biggest extension- for about 1,500 km, from Northern Syria to Southern Palestine and northern Arabia, forming part of the wider Roman limes system. It had several forts and watchtowers.
The reason of this defensive "Limes" was to protect the Roman "Province of Arabia" from attacks of the barbarian tribes of the Arabian desert.
Next to the limes arabicus Trajan built a major road, the Via Nova Traiana, from Bostra to Aila on the Red Sea, a distance of 267 miles. Built between 111 and 114 AD, its primary purpose may have been to provide efficient transportation for troop movements and government officials. It was completed under Hadrian.
During the Severan dynasty (AD 193-235), the Romans strengthened their defenses on the Arabian frontier. They constructed several forts at the northwest end of the Wadi Sirhan, and they repaired and improved roads.Via http://byzantinemilitary.blogspot.com/2012/08/the-byzantine-fortress-of-qasr-bashir.html