By the beginning of Sasanian rule, a number of buffer states existed between the empires. These were absorbed by the central state over time, and by the 7th century the last buffer state, the Arab Lakhmids, was annexed to the Sasanian Empire. Frye notes that in the 3rd century AD such client states played an important role in Roman–Sasanian relations, but both empires gradually replaced them by an organized defense system run by the central government and based on a line of fortifications (the "limes") and the fortified frontier cities, such as Dara. Towards the end of the 1st century AD, Rome organized the protection of its eastern frontiers through the "limes" system, which lasted until the Muslim conquests of the 7th century after improvements by Diocletian. Like the Romans, the Sasanians constructed defensive walls opposite the territory of their opponents. According to R. N. Frye, it was under Shapur II that the Persian system was extended, probably in imitation of Diocletian's construction of the "limes" of the Syrian and Mesopotamian frontiers of the Roman Empire. The Roman and Persian border units were known as "limitanei" and "marzoban"s, respectively.