Comparison of Data-driven Link Estimation Methods in Low-power Wireless Networks HONGWEI ZHANG LIFENG SANG ANISH ARORA
From sensor networks to cyber-physical systems (CPS) Sensing, networking, and computing tightly coupled with the physical world Automotive Alternative energy grid Industrial monitoring and control Wireless networks as carriers of mission- critical sensing and control information Stringent requirements on predictable QoS such as reliability and latency
Dynamic wireless links Link estimation becomes a basic element of routing in wireless networks. 5.5 meters ( 2 secs) transitional region (unstable & unreliable)
Why not beacon-based link estimation?
Sampling error due to traffic- induced interference Unicast ETX in different traffic/interference scenarios
Sampling error due to temporal link correlation mean reliability of each unicast-physical- transmission minus that of broadcast Errors in estimating unicast ETX via broadcast reliability: estimated unicast ETX minus actual unicast ETX and then divided by actual unicast ETX
Two representative methods for estimating ETX L-NT uses aggregate unicast feedback {NT i } represents SPEED, LOF, CARP L-ETX uses derived information for individual unicast-physical-transmission represents four-bit-estimation, EAR, NADV, MintRoute EWMA {NT i }ETX PDR calculation {NT i }{PDR j } EWMA PDR 1/PDR ETX
Relative accuracy in L-NT and L-ETX where P 0 is the failure probability of a unicast-physical- transmission, and W is the window size for calculating PDR j ; COV[NT i ] > COV[PDR j ] if (which generally holds), thus DE k (L-NT) > DE k (PDR) L-ETX tends to be more accurate than L-NT in estimating link ETX. DE k (L-ETX)
Testbed based link-level experimentation We use Mica2 motes that are deployed in a 14 7 grid Focus on links of the middle row Interferers randomly distributed in the rest 6 rows, with 7 motes on each row on average; interfering traffic is controlled by the probability d of generating a packet at an arbitrary time
Variants of L-NT and L-ETX Variant/stabilized L-NT: L-WNT L-NADV (variant of L-ETX): estimate PER instead of PDR
L-NT vs. L-ETX: forwarders used MethodForwarderPercentage(%)Cost ratio L-NT L-ETX
Testbed based routing experiments Convergecast routing in a 7 7 grid A node at one corner as the sink Other 48 nodes as sources generating packets based on the event traffic trace from “A Line in the Sand” sink
L-NT vs. L-ETX: routing performance Event reliability Number of transmissions per packet received Seemingly similar methods may differ significantly in routing behaviors (e.g., stability, optimality, and energy efficiency)
Other experimental results Related data-driven protocols L-ETX-geo, L-ETX Periodic traffic, other event traffic load Sparser network Random network Network throughput
Concluding remarks Two seemingly methods L-ETX and L-NT differ significantly in routing performance Variability of parameters being estimated significantly affects the reliability, stability, latency, and energy efficiency of data-driven link estimation and routing Future work Other metrics (e.g., RT oriented) Opportunistic routing and biased-link-sampling