On the practical implementation of propagation delay and clock skew compensated high-precision time synchronization schemes with resource-constrained sensor nodes in multi-hop wireless sensor networks

Abstract

In wireless sensor networks (WSNs), implementing a high-precision time synchronization scheme on resource-constrained sensor nodes is a major challenge. Our investigation of the practical implementation on a real testbed of the state-of-the-art WSN time synchronization scheme based on the asynchronous source clock frequency recovery and the reverse two-way message exchange, which can compensate for both propagation delay and clock skew for higher precision, reveals that its performance on battery-powered, lowcomplexity sensor nodes is not up to that predicted from simulation experiments due to the limited precision floating-point arithmetic of sensor nodes. Noting the lower computational capability of typical sensor nodes and its impact on time synchronization, we propose an asymmetric high-precision time synchronization scheme that can provide high-precision time synchronization even with resource-constrained sensor nodes in multi-hop WSNs. In the proposed scheme, all synchronization-related computations are done at the head node equipped with abundant computing and power resources, while the sensor nodes are responsible for timestamping only. Experimental results with a testbed based on TelosB motes running TinyOS demonstrate that the proposed time synchronization scheme can avoid time synchronization errors resulting from the single-precision floating-point arithmetic of the resourceconstrained sensor nodes and achieve microsecond-level time synchronization

A Beaconless Asymmetric Energy-Efficient Time Synchronization Scheme for Resource-Constrained Multi-Hop Wireless Sensor Networks

“…Of particular note is that the second term in (2)-i.e., (T i (t) − T i (t k )) / 1 +ˆ i,k -for the logical clock update requires the division by the frequency ratio, instead of multiplication, unlike the time synchronization schemes based on conventional two-way message exchange. Considering the microsecond-level timestamps involved in the calculation, this division may cause significant numerical computation error due to the limited floating-point precision in the resourceconstrained sensor nodes embedded with low-cost MCU as discussed in [6] and [11].…”

“…Likewise, the actual performance of EE-ASCFR on resource-constrained sensor nodes turns out to be poorer than expected due to the precision loss resulting from the use of single-precision floating-point numbers. In this regard, we have proposed an improved version of the scheme along with its multi-hop extension based on the reverse asymmetric framework and demonstrated satisfactory time synchronization accuracy on a real WSN testbed [11].…”

“…The authors of [10] proposed a novel energy-efficient time synchronization scheme based on the reverse two-way message exchange and demonstrated through simulation experiments that sub-microsecond-level synchronization accuracy could be achieved. Still, the computational precision required by their scheme (i.e., the precise division of the floatingpoint numbers) is beyond the capability of most resourceconstrained sensor nodes equipped with a low-cost Micro-Controller Unit (MCU) providing only 32-bit floating-point as discovered in [11]. The idea of the reverse two-way message exchange together with message bundling for synchronization and measurement data, however, could be applied for designing more advanced energy-efficient time synchronization schemes targeting resource-constrained sensor nodes because it can greatly reduce the number of message transmissions by sensor nodes.…”

“…1 (a). In the proposed framework, all the synchronization procedures are moved from sensor nodes to the head 1 as in [11], and application messages (e.g., for reporting measurement data to the head) carry synchronization-related data as well. In addition, BATS does not rely on the "Beacon/Request" messages as shown in Fig.…”

“…Unlike the conventional schemes based on the one-way message dissemination including FTSP, however, BATS is optimized for energy efficiency by reversing the flow of synchronization messages from "head→sensor nodes" to "sensor nodes→head", which is the key idea of the reverse asymmetric time synchronization framework. The reverse asymmetric time synchronization framework, on the other hand, is a generalization of the schemes we have proposed in [10] and [11] and belongs to a broader category of reactive time synchronization protocols [15], where sensor nodes' local clocks, which are not synchronized to a common reference, are used to timestamp events; synchronization takes place after the event has been detected. However, BATS is also different from the existing reactive time synchronization protocols like Routing Integrated Time Synchronization protocol (RITS) [5] in its multi-hop extension, where, unlike RITS, there is no layer-by-layer time translation at gateway nodes along a multi-hop path; instead, the said time translation is completely moved to the head to relieve the burden of the gateway nodes.…”