New wireless technologies have been developed in recent years which enable applications that require the transmission of small amounts of data over long distances in an energy efficient manner. One of these technologies, LoRaWAN, includes a server-initiated communication mode named Class B which provides a deterministic latency for downlink communications. In this paper, we model Class B of LoRaWAN in ns-3 to explore the limits of scale at which this form of bi-directional communication remains feasible in large networks. The simulation results show that the principle restriction on scalability is caused by the duty cycle limits that the gateway must adhere to. In addition, we identify a limitation in the protocol which in certain configurations allows a gateway node to block the future transmission of its own beacon frames. Our contributions are the development of the first implementation and simulation of LoRaWAN Class B in ns-3, and an evaluation of the scalability limits of Class B.