# No Free Lunch--Characterizing the Performance of 6 TiSCH When Using Different Physical Layers
## 3 Problem Statement and Contributions  
While active research proposed various enhancements in 6 TiSCH for achieving wire-like reliability and low power consumption, one common assumption is that it uses the IEEE802.15.4 O-QPSK at 2.4 GHz. While O-QPSK 2.4 GHz is appropriate for many applications, the increasing demands for long range (including for environmental monitoring and automated meter reading) has triggered the standardization of longer range PHYs [3]. A problem statement has been proposed in the Internet Engineering Task Force (IETF) detailing the theoretical challenges or side-effects of augmenting 6 TiSCH to include different PHYs [18]. Using a different PHY will cause the network to behave differently, leading to different overall performance. For example, a different energy consumption and link cost changes how links to neighbors are evaluated at the link-layer, hence which multi-hop paths are picked by the routing protocol. A different PHY also impacts the number of re-transmissions, the level of interference and the amount of contention is shared cells [26]. Changing the PHY layer triggers subtle changes in the behavior of the protocol stack, resulting in different performance. We believe measuring this difference is best done through a real-world system level evaluation [27]. We show that there are significant benefits of each of the radio options: FSK 868 MHz, OFDM 868 MHz, and O-QPSK 2.4 GHz, yet they all come at a certain cost.  
The contribution of this article is three-fold:  
* We augment the OpenWSN reference implementation of 6 TiSCH to support the three PHY layers (As an online addition to this paper, these extensions have been merged into the main codebase of OpenWSN at [[https://github.com/openwsn-berkeley](https://github.com/openwsn-berkeley)]([https://github.com/openwsn-berkeley](https://github.com/openwsn-berkeley)) and published under an open-source license.).
* We conduct an experimental performance evaluation campaign of the resulting code on the 42-note OpenTestbed. For each PHY, we measure the network formation time, the end-to-end reliability, the end-to-end latency, the estimated battery lifetime of each node, and the memory footprint of the implementation.
* We highlight the advantages and disadvantages of each PHY layer, specifically:Using FSK 868 MHz yields the fastest network formation, the highest end-to-end reliability, and the lowest end-to-end latency, at the cost of a 10\(\times\) decreased battery lifetime compared to O-QPSK 2.4 GHz.
* Using O-QPSK 2.4 GHz yields the longest battery lifetime, at the cost of the slowest network formation, the lowest end-to-end reliability, and the highest end-to-end latency.
* Using OFDM 868 MHz yields an intermediate network formation time, end-to-end reliability, and end-to-end latency at the cost of a 5\(\times\) decreased battery lifetime compared to O-QPSK 2.4 GHz. In line with Reference [3], a node using OFDM 868 MHz discovers a number of neighbors close to that of FSK 868 MHz despite having the highest data rate. This is because it is more robust against frequency selective interference than FSK 868 MHz [25] and less vulnerable to Wi-Fi interference than O-QPSK 2.4 GHz [1].  
This shows there is no PHY among our candidates that is best across all metrics.