IEEE 802.11ah task group has developed a new wireless local area network (WLAN) technology, sub 1GHz (S1G) WLAN, for future Internet of Things (IoT) environments. Because of the superior propagation feature of S1G frequency band, S1G WLAN can provide very long transmission range, wild coverage, and support more than 8000 devices. In order to alleviate the high collision probability raised in the dense network environment, S1G WLAN adopts an restricted access window (RAW) mechanism. However, as the time slots of RAW are allocated without any consideration of traffic load of devices, the medium resource might be inefficiently used. In addition, devices belonging to a same group have to contend for channel, and thus severe collisions may occur in a large size group. In this paper, we propose a sequential transmission scheme, which provides an efficient way of source management in the S1G WLANs by considering the traffic loads and allocating channel access times of the devices. Through conducting mathematical analysis and performing simulations, we prove that our proposed protocol can enhance the performance of the S1G WLANs for the IoT environments.