An Algorithm for Synchronizing a Clock When the Data Are Received Over a Network With an Unstable Delay

Abstract

A method is presented for synchronizing the time of a clock to a remote time standard when the channel connecting the two has significant delay variation that can be described only statistically. The method compares the Allan deviation of the channel fluctuations to the free-running stability of the local clock, and computes the optimum interval between requests based on one of three selectable requirements: (1) choosing the highest possible accuracy, (2) choosing the best tradeoff of cost vs. accuracy, or (3) minimizing the number of requests to realize a specific accuracy. Once the interval between requests is chosen, the final step is to steer the local clock based on the received data. A typical adjustment algorithm, which supports both the statistical considerations based on the Allan deviation comparison and the timely detection of errors is included as an example.

Accurate, Traceable, and Verifiable Time Synchronization for World Financial Markets

“…In practice networks are asymmetric, which means that the incoming and outgoing delays are not equal, and this asymmetry adds uncertainty to the time received by the client. The maximum possible time uncertainty that can be added by network asymmetry would equal 50 % of the round trip delay [52], a situation that could only occur if 100 % of the delay was in one direction. To reduce the maximum possible uncertainty that can be contributed by the network, the round trip delay should be made as small as possible.…”

Metrological and Legal Traceability of Time Signals

“…where TD is the time difference between the server and client clocks, T1 is the time when the client made the request, T2 is when the request was received by the server, T 3 is when the server transmitted its response, and T 4 is when the time packets transmitted by the server arrive at the client. Using these same four time stamps, the round trip delay between the client and server [15] is computed by the client as…”

“…This means that the incoming and outgoing delays are not equal and that the difference in delays will contribute uncertainty to the time received by the client. The worst-case time uncertainty that can be added by network asymmetry is 50% of the round trip delay [15,16], a situation that could, of course, only occur if 100% of the delay were in one direction.…”

Clock synchronization algorithms for networked systems

“…工业以太网是基于有线网通信, 而无线传感器网 络基于无线通信, 那么延时、丢包、信道衰落等现象在无线传感器网络中更加突出, 而在工业以太网 中, 量化、 拥塞导致的滞后等不确定因素会更突出, 这些因素会不同程度地影响节点之间的可靠信息传 输 [98] ; 同时, 随机移动传感器网络的节点具有移动性 [64,99] , 导致它们之间的通信也具有随机性, 这些 通信拓扑的随机性将直接给时钟同步带来困难和挑战, 例如协调拓扑切换时刻和通信时刻以保证时 钟信息的有效交换. 此外, 时间延迟将会直接影响时钟的同步精度和同步速度, 甚至造成同步算法发 散 [100] . 然而目前, 不单是 IEEE 1588 存在对称延时路径等理想假设, 一致性时钟同步算法的理论分 析也大都忽略了这些不确定性, 特别对随机拓扑下的时钟同步问题, 目前还缺少系统的研究.…”