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Precision Time Protocol

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From Wikipedia, the free encyclopedia

Network time synchronization protocol

Precision Time ProtocolCommunication protocolAbbreviationPTPPurposeTimeDeveloper(s)IEEEIntroduction2002; 24 years ago (2002)Port(s)udp/319, udp/320<br>The Precision Time Protocol (PTP ) is a protocol for clock synchronization throughout a computer network with relatively high precision as compared to using the earlier developed Network Time Protocol (NTP) and therefore potentially higher accuracy depending on the configuration. In a local area network (LAN), accuracy can be sub-microsecond – making it suitable for measurement and control systems applications.[1] PTP can be used to synchronize financial transactions[2], mobile phone tower transmissions[3], sub-sea acoustic arrays[4], and networks that require precise timing as an alternative to using the timestamp of satellite navigation signals or where sub-nanosecond accuracy as provided by the White Rabbit Project is unnecessary.[5]

The first version of PTP, IEEE 1588-2002 , was published in 2002. IEEE 1588-2008 , also known as PTP Version 2, is not backward compatible with the 2002 version. IEEE 1588-2019 was published in November 2019 and includes backward-compatible improvements to the 2008 publication. IEEE 1588-2008 includes a profile concept defining PTP operating parameters and options. Several profiles have been defined for applications including telecommunications, electric power distribution and audiovisual uses. IEEE 802.1AS is an adaptation of PTP, called gPTP, for use with Audio Video Bridging (AVB) and Time-Sensitive Networking (TSN).

History<br>[edit]

According to John Eidson, who led the IEEE 1588-2002 standardization effort, "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."[6]

PTP was originally defined in the IEEE 1588-2002 standard, officially titled 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 (PTPv2), it improves accuracy, precision and robustness but is not backward compatible with the original 2002 version.[7] IEEE 1588-2019 was published in November 2019,[8] is informally known as PTPv2.1 and includes backwards-compatible improvements to the 2008 publication.[9]

Architecture<br>[edit]

The IEEE 1588 standards describe a hierarchical master–slave architecture for clock distribution consisting of one or more network segments and one or more clocks. An ordinary clock is a device with a single network connection that is either the source of or the destination for a synchronization reference. A source is called a master (alternately timeTransmitter[10]), and a destination is called a slave (alternately timeReceiver[10]). A boundary clock has multiple network connections and synchronizes one network segment to another. A single, synchronization leader is selected, a.k.a. elected, for each network segment. The root timing reference is called the grandmaster.[11]

A relatively simple PTP architecture consists of ordinary clocks on a single-segment network with no boundary clocks. A grandmaster is elected and all other clocks synchronize 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.[12] 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 (start of 1 January 1970).[a] 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.

Protocol details<br>[edit]

Synchronization and management of a PTP system is achieved through the exchange of messages across the communications medium. To this end, PTP uses the following message types.

Sync, Follow_Up, Delay_Req and Delay_Resp messages are used by ordinary and boundary clocks and communicate time-related information used to synchronize clocks across the network.

Pdelay_Req, Pdelay_Resp and Pdelay_Resp_Follow_Up are used by...