  Low-power wireless IoT networks have traditionally operated over a single
physical layer (PHY) -- many based on the IEEE 802.15.4 standard. However,
recent low-power wireless chipsets offer both the IEEE 802.15.4 and all four
PHYs of the Bluetooth 5 (BT 5) standard. This introduces the intriguing
possibility that IoT solutions might not necessarily be bound by the limits of
a single PHY, and could actively or proactively adapt their PHY depending on RF
or networking conditions (e.g., to offer a higher throughput or a longer radio
range). Several recent studies have explored such use-cases. However, these
studies lack comprehensive evaluation over various metrics (such as
reliability, latency, and energy) with regards to scalability and the Radio
Frequency (RF) environment. In this work we evaluate the performance of IEEE
802.15.4 and the four BT 5 2.4GHz PHY options for the recently completed IETF
6TiSCH low-power wireless standard. To the best of our knowledge, this is the
first work to directly compare these PHYs in identical settings. Specifically,
we use a recently released 6TiSCH simulator, TSCH-Sim, to compare these PHY
options in networks of up to 250 nodes over different RF environments (home,
industrial, and outdoor), and highlight from these results how different PHY
options might be better suited to particular application use-cases.
