# No Free Lunch--Characterizing the Performance of 6 TiSCH When Using Different Physical Layers
## 1 Introduction  
Applications for low-power wireless technology are numerous, including smart building [1; 2], environmental monitoring [3], precision agriculture [4], automated meter reading [5], indoor localization [6], smart grid [7] and predictive maintenance [8]. Several physical layers (PHYs) exist, each finding a different trade-off between energy consumption, datarate and range. One particularly interesting development in low-power wireless technology is the standardization in 2016 of the IEEE802.15.4g amendment [9], which defines additional PHY layers for the older IEEE802.15.4 standard [10]. IEEE802.15.4g defines PHY specifications initially for smart metering utility networks. IEEE802.15.4g defines three families of modulations (Frequency Shift Keying--FSK, Offset Quadrature Phase-Shift Keying--O-QPSK, Orthogonal Frequency Division Multiplexing--OFDM), data rate ranging from 25 kbps to 800 kbps, and two frequency bands (sub-GHz and 2.4 GHz). These options result in a large set of combinations to address a wide variety of applications, from small home installations [11] to kilometer-scale smart agriculture deployments [12]. In the remainder of this article, we call a \"PHY\" a radio configuration, consisting of a modulation, a data rate and a frequency band. A traditional approach to having this array of choices is to characterize each PHY, select the one that best corresponds to the application, and build a network solution using that PHY. Two elements allow for a more dynamic approach--technology-agile radio chips and Time Synchronized Channel Hopping (TSCH). As the processes used in integrated circuits advance, chip manufacturers add more features into their products for the same die size, maintaining low-power operation and competitive pricing. System-on-Chip (SoC), such as Texas Instruments' CC2538 [13], which combine a micro-controller and a radio in the same die, are now commonplace. Advanced SoC such as the Texas Instruments' CC2650 [14] contain a co-processor which, combined with a 2.4 GHz front-end, implements either the Bluetooth Low Energy (BLE) or the IEEE802.15.4 standards. Ehile the CC2650 requires that the co-processor be reprogrammed to switch from one to another, the newer nRF52840 [15] from Nordic Semiconductor allow switching between these standards on a frame-by-frame basis. The AT86 RF215 from Atmel arguably goes one step further in that it implements the entire set of modulations, data rates and frequency bands defined in the IEEE802.15.4 standard. Using such technology-agile radio chips opens up many possibilities to dynamically match the PHY used to the application. While early low-power wireless networks used contention-based approach, TSCH, initially developed for industrial applications, is now supported by many commercial products and open-source implementations [16]. In a TSCH network, time is cut into timeslots and a schedule orchestrates all communication; it indicates to every node what to do in each timeslot: transmit, listen or sleep. TSCH is the base principle of standards such as WirelessHART, ISA100.11a, IEEE802.15.4e, and, more recently, 6 TiSCH, a set of IETF standards which combines the industrial performance of TSCH with the ease of use of IPv6 [17]. In today's TSCH networks, for each cell, the schedule indicates the frequency to communicate on, resulting in \"channel hopping\", a technique known to efficiently combat external interference and multi-path fading. Our long-term vision is that low-power wireless networks will combine the technology-agility of modern radio chips which TSCH-like scheduling approaches so nodes dynamically change their PHY layer on a frame-by-frame basis. That is, if the next hop is far, use a long-range (but slower and more energy-hungry) PHY. If the next hop is close, use a fast PHY so the radio can be switched of as fast as possible. The goal of this article is to get one step closer to this vision by comparing the performance of a 6 TiSCH network for different PHY layers.