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The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems.〔(【引用サイトリンク】title=IEEE 1588 Systems )〕 PTP was originally defined in the IEEE 1588-2002 standard, officially entitled ''"Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems"'' and published in 2002. In 2008, IEEE 1588-2008 was released as a revised standard; also known as PTP Version 2, it improves accuracy, precision and robustness but is not backward compatible with the original 2002 version. "IEEE 1588 is designed to fill a niche not well served by either of the two dominant protocols, NTP and GPS. IEEE 1588 is designed for local systems requiring accuracies beyond those attainable using NTP. It is also designed for applications that cannot bear the cost of a GPS receiver at each node, or for which GPS signals are inaccessible." ==Architecture== The IEEE 1588 standards describe a hierarchical master-slave architecture for clock distribution. Under this architecture, a time distribution system consists of one or more communication media (network segments), and one or more clocks. An ''ordinary clock'' is a device with a single network connection and is either the source of (master) or destination for (slave) a synchronization reference. A ''boundary clock'' has multiple network connections and can accurately synchronize one network segment to another. A synchronization ''master'' is selected for each of the network segments in the system. The root timing reference is called the ''grandmaster''. The grandmaster transmits synchronization information to the clocks residing on its network segment. The boundary clocks with a presence on that segment then relay accurate time to the other segments to which they are also connected. A simplified PTP system frequently consists of ordinary clocks connected to a single network, and no boundary clocks are used. A grandmaster is elected and all other clocks synchronize directly to it. IEEE 1588-2008 introduces a clock associated with network equipment used to convey PTP messages. The ''transparent clock'' modifies PTP messages as they pass through the device. Timestamps in the messages are corrected for time spent traversing the network equipment. This scheme improves distribution accuracy by compensating for delivery variability across the network. PTP typically uses the same epoch as Unix time (Midnight, 1 January 1970). While the Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds, PTP is based on International Atomic Time (TAI). The PTP grandmaster communicates the current offset between UTC and TAI, so that UTC can be computed from the received PTP time. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Precision Time Protocol」の詳細全文を読む スポンサード リンク
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