# On-Demand Scheduling of Command and Responses for Low-Power Multihop Wireless Networks +
## 2 Related Work  
As the IoT technology pervades our everyday life, vast amount of research have been devoted to improve the performance of low-power multihop wireless networks for IoT applications. A number of studies [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22] proposed fast flooding schemes to disseminate messages into the whole network using various ideas such as exponential timer, time synchronization, capture effects, etc. There are also numerous prior work on efficient collection of data in multihop low-power networks [23; 24; 25; 26; 27; 28; 29; 30; 31]. Although they are all great work that improves performance for their respective scenarios, they only targeted one side of the system;: either the command dissemination phase or the response collection phase. However, many, if not most, real-world IoT applications usually require some mechanism to both send commands and collect responses [1; 2; 3; 4; 5; 6; 7; 8]. In other words, it is very common that command and response packets coexist in the network concurrently in real-world IoT applications.  
There are some studies which tried to schedule commands and responses jointly. [PERSON] et al. [33] proposed DeAMON, a decentralized packet scheduling for 6 TiSCH [34] in multihop wireless networks. DeAMON's resource pool is partitioned into signaling, data, and over-provisioned slotframes. The root disseminates a Build command via flooding in a scheduled manner, upon which a receiving node will start scheduling itself. DeAMON's scheduling guarantees parallel transmissions between far-away nodes by using Request-for-Slots and Assign messages. However, DeAMON uses one dedicated channel in 6 TiSCH just for disseminating Build commands (control channel), and signaling, data, and overprovision are all operated on separated channels. Thus, it is not suitable for single-channel scenarios. In addition, DeAMON requires extra control packet overheads for scheduling. DeTAS ([PERSON] et al. [35]) and OST ([PERSON] et al. [36]) are also decentralized packet scheduling schemes for 6 TiSCH/TSCH networks that address collision and congestion problems, but they focus only on either the collection phase or MAC layer and does not consider command dissemination phase jointly.  
[PERSON] et al. [1] proposed CoReDac which schedules both command and response packets in a multihop network. CoReDac's time slotting mechanism operates in the MAC layer, and scheduling is done via MAC layer packet exchanges between parent and child by piggybacking slot offset and sleep interval information into messages, whereas SCoRe's scheduling is done at a higher layer and at a larger scale for the whole multihop network recursively. Furthermore, although CoReDac schedules command and response phase together similar to our approach, their evaluation focuses on energy-efficiency not reliability nor latency. For most IoT applications, especially industrial applications, reliability and latency are both crucial performance metrics that must be satisfied jointly.  
[PERSON] et al. [37] proposed a latency-optimal convergecast scheme for WirelessHART network. WirelessHART standard supports channel hopping-based TDMA scheme, and the authors improve this in terms of latency and the number of channels used. However, WirelessHART employs proactive scheduling which requires time synchronization in contrast to our on-demand relative scheduling. Moreover, the root manages timeslots of each node one by one which incurs high overhead, while SCoRe uses recursive scheduling of subtree aggregate slots. In addition, the proposed scheme was evaluated via simulation only whereas we conduct experiments to demonstrate the effectiveness under real wireless environment.