Bias Estimation in
Asymmetric Packet-based Networks
In the context of the IEEE 1588 Precision Time Protocol
(PTP), estimating the delay's bias is a problem that appears in both one-way
(using transparent devices) or two-way message exchange mechanisms. For
estimating the offset via the two-way message exchange mechanism, it is usually
being assumed that the expected value of delays in forward and reverse
directions are equal with each other. However this is not a realistic
assumption for packet-based wide area networks, where delays in down-link and
up-link directions may have a significant difference. In this work we propose a
solution to estimate the random delay's bias and improve the synchronization
accuracy of IEEE 1588. Our method is easy to implement and is compatible with
the current version of the protocol. We compared our results with no bias
correction and the Boot-strap method. In addition to the improvement in
synchronization accuracy, our method allows us to update the slave clock
recursively. The proposed method works well even in the presence of large
frequency offsets and can also be implemented by using different filters.