Distributed TSCH scheduling: A comparative analysis

Industrial applications demand for ease of deployment, reliability and low-power in wireless networks, leading to standards like IEEE802.15.4, which include time synchronized channel hopping mechanisms. Yet, scheduling of transmissions on timeslots and channels falls outside the scope of current standards. Given the application requirements above, distributed (rather than centralized) scheduling has been identified as a vehicle to keep reliability within boundaries, even when network topology changes, offering scalability and flexibility. However, assigning timeslots and channels is challenging in a distributed environment, as the scheduling device can only rely on limited knowledge about its surroundings. This paper evaluates existing slot assignation methods - additional messaging and arbitrary allocation-in distributed Time Synchronized Channel Hopping (TSCH) networks, providing an experimental comparative analysis of the different methods. We find that, through additional messaging, it is possible to reduce conflicts - and increase reliability-, compared to pseudo-random slot and channel access. However, these benefits come at extra latency in scheduling time, which is problematic in time-critical wireless sensor networks. On the other hand, arbitrary cell allocation does not impact scheduling time, but creates conflicts for small slotframe sizes (high data throughput), affecting reliability. This study pinpoints the criticalities in distributed TSCH networks, advocating arbitrary cell allocation for mobile or scheduling-time-critical wireless sensor networks.