Clock Offset Estimation for Systems With Asymmetric Packet Delays

Abstract

This paper proposes a new clock offset estimation that mitigates unwanted link asymmetry for precise clock synchronization. The main contribution is to address the primary and traditional design issue of the IEEE 1588 standard precision time protocol (PTP), which estimates clock offset under the assumption that the delays of exchanged packets are symmetric. To mitigate the issue, we focus on the fact that PTP measures asymmetry variation through the derivatives of its timestamps with respect to the time step. By exploiting the measurement of the variation, the proposed approach defines the asymmetry in the form of a linear differential equation (LDE) and leverages the LDE to define and exclude asymmetry-induced errors. Additionally, we clearly derive the state transition of the asymmetry. Subsequently, we derive a novel state-space model from our approach. The model describes PTP clock offset estimation perfectly, allowing optimal clock offset estimation. We verify the theoretical validity of the proposed method with real data. Our approach improves PTP accuracy by more than thousand times and achieves an accuracy at the level of tens to hundreds of nanoseconds on an asymmetric communication link. Our approach realizes an accuracy comparable to that of PTPv2, without the cost of specialized hardware.

A Fuzzy-PI Clock Servo with Window Filter for Compensating Queue-Induced Delay Asymmetry in IEEE 1588 Networks

“…Nevertheless, the computational cost of solving the LP problem is high. Ha et al [43] directly modeled delay asymmetry through the linear differential equation and state space model and provided an optimal time offset estimation, and the accuracy can reach 1 µs. However, the estimation algorithm requires real-time measurement of the clock frequency offset using an oscilloscope, making it not easy to implement in practical applications.…”

“…This paper uses the minimum. Similarly, the frequency offset estimation in another direction using t 43 [k] can be denoted as ŷ43 [k]. If the minimum delay packets can be found in both the front and the back windows, ŷ21 [k] and ŷ43 [k] will have the same absolute value and opposite signs in theory.…”

“…Sensors 2024, 24, x FOR PEER REVIEW 10 of 27 and opposite signs in theory. On the contrary, if the packets cannot be found, y 21 [k] or y 43 [k] may have significant variation. To reduce the estimation error, the final frequency offset estimation takes the smaller absolute value of the two, and the sign is consistent with y 21 [k], which can be expressed as Compared with Ref.…”

CRT-Based Clock Synchronization for Millimeter-Wave Communication with Asymmetric Propagation Delays

“…However, in most of practical scenarios, this assumption does not hold due to the asymmetry of transmission links. In real distributed networks, packet transmission delay comprises nodal processing, queuing, transmission, and propagation delays, which are unpredictable and easily affected by link congestion or varying distances [16]. This discrepancy between the assumption and reality leads to errors in clock offset estimation in PTP time synchronization, limiting its effectiveness.…”