Characteristics of Time Synchronization Response of NTP Clients on MS Windows OS and Linux OS

Abstract : The clock offset between a GPS-based NTP time server and NTP time client software, installed in the MS Windows Operating System (OS) and the Linux OS on PCs, are measured and evaluated. The clock offset on MS Windows 98 OS shows a trend with a range of about 55 ms. The NTP time client software on MS Windows OS adjusts the internal clock with the Application Program Interface (API) timer function. The resolution of the API timer function depends on the hardware interrupt of the PC system timer, which is 54.9 ms of IRQ0 for MS Windows 98 OS. Thus, this range of the trend in the clock offset is considered to be caused by the resolution of API timer function. The clock offset on MS Windows XP OS shows time resolutions of 1 ms for the 1 ms API timer mode and 10 ms for the 10 ms API timer mode. The resolution of 1 ms is dependent on a hardware interrupt of IRQ8 (976 microsecs) that is generated by a PC real-time clock. The resolution of the system timer on MS-windows XP depends on the hardware platform and shows 10 ms or 15ms. The clock offset on Linux OS also has a trend as well as a periodic divergence. The trend interval of the drift is estimated to be about 35 minutes. The origin of this trend in interval is presumed to be the loop constant of the kernel Phase Lock Loop (PLL). The Standard Deviation of this clock offset for 24 hours is 0.95 ms. Evidently, both MS Windows OS and Linux OS adopt different algorithms for keeping the internal clock. The accuracy of time synchronization by NTP is restricted by the algorithms. The limitation of the time synchronization accuracy on MS Windows OS is related to the resolution of API that depends on a hardware interrupt generated in the PC hardware system timer and real-time clock. On the other hand, the loop constant of the kernel clock algorithm restricts the time synchronization accuracy by NTP on Linux OS.