Duty-Cycle Distribution in Low-Power Collection
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We characterize the interplay between collection and lowpower operation by studying the joint impact of the offered load and the Low Power Listening sleep time setting on the duty-cycle distribution in a large-scale sensor network testbed. Collection services must be able to work on top of a low-power MAC. The Collection Tree Protocol (CTP) [1] has been shown to perform very well on top of Box-MAC [2], the mainstream TinyOS MAC layer that provides Low Power Listening (LPL) [3] support: CTP achieves duty-cycles of 3% with an InterPacket Interval (IPI) of 5 minutes and an LPL sleep interval of 1s. Another tree-based collection protocol for low-end sensor networks, Arbutus, has been recently shown to compare favorably with CTP in a very extensive analysis [4] that is, however, limited to an always-on CSMA MAC layer. Our ongoing work has a twofold goal: (1) to validate Arbutus in the presence of MAC-layer dutycycling; (2) to explore the impact of the IPI and the sleep time settings on the duty cycle statistics. The motivation for (1) is clear: without duty-cycling, the radio is a toy. The motivation for (2) lies in the lack of a systematic study about the effect of different sleep time settings as a function of the offered load. It is well-accepted that nodes should sleep, but how often should they wake up given the extent of their projected load? This work presents our progress toward these two goals. Sleep Time [ms] IPI [min] Duty Cyle [%] (μ± σ) Average Delivery Rate 250 1 10.31± 3.39 0.97 1000 1 11.63± 9.04 0.93 250 5 4.81± 2.59 0.98 1000 5 3.16± 1.04 0.98 Table 1. Overall results: duty-cycle and delivery rate at four different operating points. We adapted the TinyOS implementation of Arbutus used in [4] for LPL support, and we tested it on top of BoX-MAC on the Motelab testbed [5]. We fixed the topology by focusing on a specific sink assignment (node 103) that the results in [4] showed to be particularly favorable. Table 1 shows the results with about 100 Motelab nodes, averaged over five 30min runs for four different IPI-Sleep operating points. The results at IPI=5min compare favorably with the 0 5 10 15 20 25 0 10 20 30 40 50 60 70 80 90 100
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