Accurate Network Time Synchronisation
All PC's and networking devices use clocks to maintain an internal system time. These clocks, called Real Time Clock chips (RTC) provide time and date information. The chips are battery backed so that even during power outages, they can maintain time. However, personal computers are not designed to be perfect clocks, their design has been optimized for mass production and low-cost rather than maintaining accurate time.
These internal clocks are prone to drift and although for many application this is can be quite adequate, often machines need to work together on a network and if the computers drift at different rates the computers will become out of sync with each other and problems can arise particularly with time sensitive transactions.
A time server uses the Network Time Protocol (NTP) which was developed over 20 years ago. It uses an algorithm (Marzullo's algorithm) to synchronise time on a network. NTP (version 4) can maintain time over the public Internet to within 10 milliseconds (1/100th of a second) and can perform even better over LANs with accuracies of 200 microseconds (1/5000th of a second) under ideal conditions.
NTP uses a single time reference and synchronises all machines on the network to that time. This time reference can be either relative (a computer's internal clock or the time on a wrist-watch perhaps) or absolute such as a UTC (Universal Coordinated Time) clock source like an atomic clock that is as accurate as is humanely possible.
Jam atom adalah peranti masa penyimpanan yang paling mutlak. Mereka bekerja pada prinsip bahawa atom, sesium-133, mempunyai nombor yang tepat kitaran radiasi setiap saat (9,192,631,770). Ini telah membuktikan begitu tepat Sistem Unit Antarabangsa (SI) kini telah mendefinisikan kedua sebagai tempoh kitaran 9,192,631,770 radiasi atom sesium-133.
Walau bagaimanapun, jam atom sangat mahal dan umumnya hanya terdapat dalam makmal fizik berskala besar. Walau bagaimanapun, NTP boleh menyegerakkan rangkaian ke jam atom menggunakan sama ada rangkaian Sistem Posisi Global (GPS) atau penghantaran radio pakar.
Yang paling banyak digunakan adalah sistem GPS yang terdiri daripada beberapa satelit menyediakan tepat kedudukan dan lokasi maklumat. Setiap satelit GPS hanya boleh melakukan ini dengan menggunakan jam atom yang seterusnya boleh boleh digunakan sebagai rujukan masa.
A biasa pelayan masa GPS boleh memberikan masa maklumat kepada satu nanosaat beberapa UTC selagi terdapat antena terletak dengan tujuan yang baik dari langit.
There are also a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National.
Physics Laboratory in Cumbria which serves as the United Kingdom's national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77). These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.
Semua versi Microsoft Windows sejak 2000 termasuk Perkhidmatan Masa Windows (w32time.exe) yang mempunyai keupayaan untuk menyegerakkan jam komputer ke pelayan NTP.
