A Clock Skew Measuring Method for Networks with Large Delay Jitters

• Main Authors: Tsung-Han Chen, 陳宗翰
• Other Authors: Wei-Chung Teng
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/91691573722062515595

碩士 === 國立臺灣科技大學 === 資訊工程系 === 102 === The difference between two devices’ ticking frequencies is called clock skew, which is considered useful as a physical characteristic for device identification. In this study, it is observed that timestamps collected under high delay jitter communication channels, when mapped to a two-dimensional graph, would have some outliers below the line-shaped mass of samples. This kind of data is different from classic ones, and the clock skews when measured by traditional methods, e.g. quick piecewise minimum or linear programming, tend to be unstable due to the interference from these lower outliers. In this research, we developed a method based on Hough Transform method to precisely identify the slope of the line-shaped mass of samples. The proposed method includes a data filtering mechanism with parameter settings suggestions and a three-step line-fitting process to filter out all outliers. The remaining samples are then used to measure the clock skew by linear programming. In this way, we may prevent lower outliers from interfering the precision of measurement. We compared our method with three classical ones on high delay jitter samples collected from 7 devices. The experimental results show that the maximal differences between clock skews derived from 300 timestamps for 8 times of all devices by our method is at least 2ppm smaller than other methods. Also the maximal differences between short-term measurements and long-term measurement by our method are 2ppm smaller. We can thus conclude that the proposed method contributes more precise and stable measurement comparing to other approaches.