Clock synchronization

Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of clock rate differences and several solutions, some being more appropriate than others in certain contexts.[1]

Terminology

In serial communication, clock synchronization can refer to clock recovery which achieves frequency synchronization, as opposed to full phase synchronization. Such clock synchronization is used in synchronization in telecommunications and automatic baud rate detection.[2]

Plesiochronous or isochronous operation refers to a system with frequency synchronization and loose constraints on phase synchronization. Synchronous operation implies a tighter synchronization based on time perhaps in addition to frequency.

Problems

As a result of the difficulties managing time at smaller scale, there are problems associated with clock skew that take on more complexity in distributed computing in which several computers will need to realize the same global time.

For instance, in Unix systems the make command is used to compile new or modified code and seeks to avoid recompiling unchanged code. The make command uses the clock of the machine it runs on to determine which source files need to be recompiled. If the sources reside on a separate file server and the two machines have unsynchronized clocks, the make program might not produce the correct results.[3]

Solutions

In a system with central server, the synchronization solution is trivial; the server will dictate the system time. Cristian's algorithm and the Berkeley algorithm are potential solutions to the clock synchronization problem in this environment.

In a distributed system the problem takes on more complexity because a global time is not easily known. The most used clock synchronization solution on the Internet is the Network Time Protocol (NTP) which is a layered client-server architecture based on UDP message passing. Lamport timestamps and vector clocks are concepts of the logical clock in distributed systems.

In a wireless network, the problem becomes even more challenging due to the possibility of collision of the synchronization packets on the wireless medium and the higher drift rate of clocks on the low-cost wireless devices.[4][5]

Berkeley algorithm

The Berkeley algorithm is suitable for systems where a radio clock is not present, this system has no way of making sure of the actual time other than by maintaining a global average time as the global time. A time server will periodically fetch the time from all the time clients, average the results, and then report back to the clients the adjustment that needs be made to their local clocks to achieve the average. This algorithm highlights the fact that internal clocks may vary not only in the time they contain but also in the clock rate.

Clock Sampling Mutual Network Synchronization

CS-MNS is suitable for distributed and mobile applications. It has been shown to be scalable over mesh networks that include indirectly linked non-adjacent nodes, and compatible to IEEE 802.11 and similar standards. It can be accurate to the order of few microseconds, but requires direct physical wireless connectivity with negligible link delay (less than 1 microsecond) on links between adjacent nodes, limiting the distance between neighboring nodes to a few hundred meters.[6]

Cristian's algorithm

Cristian's algorithm relies on the existence of a time server.[7] The time server maintains its clock by using a radio clock or other accurate time source, then all other computers in the system stay synchronized with it. A time client will maintain its clock by making a procedure call to the time server. Variations of this algorithm make more precise time calculations by factoring in network radio propagation time.

Global Positioning System

The Global Positioning System can also be used for clock synchronization. The accuracy of GPS time signals is ±10 ns[8] and is second only to the atomic clocks upon which they are based.

Inter-range instrumentation group time codes

IRIG timecode are standard formats for transferring timing information. Atomic frequency standards and GPS receivers designed for precision timing are often equipped with an IRIG output. The standards were created by the TeleCommunications Working Group of the U.S. military's Inter-Range Instrumentation Group (IRIG), the standards body of the Range Commanders Council. Work on these standards started in October 1956, and the original standards were accepted in 1960.[9]

Network Time Protocol

Network Time Protocol (NTP) is a highly robust protocol, widely deployed throughout the Internet. Well tested over the years, it is generally regarded as the state of the art in distributed time synchronization protocols for unreliable networks. It can reduce synchronization offsets to times of the order of a few milliseconds over the public Internet, and to sub-millisecond levels over local area networks.

A simplified version of the NTP protocol, SNTP, can also be used as a pure single-shot stateless master-slave synchronization protocol, but lacks the sophisticated features of NTP, and thus has much lower performance and reliability levels.

Precision Time Protocol

Precision Time Protocol (PTP) is a master/slave protocol for delivery of highly accurate time over local area networks

Reference broadcast synchronization

The Reference Broadcast Time Synchronization (RBS) algorithm is often used in wireless networks and sensor networks. In this scheme, an initiator broadcasts a reference message to urge the receivers to adjust their clocks.

Reference Broadcast Infrastructure Synchronization

The Reference Broadcast Infrastructure Synchronization (RBIS)[10] protocol is a master/slave synchronization protocol based on the receiver/receiver synchronization paradigm, as RBS. It is specifically tailored to be used in IEEE 802.11 Wi-Fi networks configured in infrastructure mode (i.e., coordinated by an access point). The protocol does not require any modification to the access point.

Synchronous Ethernet

Synchronous Ethernet uses Ethernet in a synchronous manner such that when combined with synchronization protocols such as Precision Time Protocol in the case of the White Rabbit Project, sub-nanosecond synchronization accuracy may be achieved.

Synchronization in Ad-hoc Wireless Networks

Synchronization is achieved in ad-hoc wireless networks through sending synchronization messages in a multi-hop manner and each node progressively synchronizing with the node that is the immediate sender of a synchronization message. A prominent example is the Flooding Time Synchronization Protocol (FTSP),[4] which achieves highly accurate synchronization in the order of a few microseconds. Another protocol, Harmonia,[5] is able to achieve synchronization even when the device firmware cannot be modified and is the fastest known synchronization protocol in ad-hoc wireless networks.

See also

References

  1. ↑ Tanenbaum, Andrew S.; van Steen, Maarten (2002), Distributed Systems : Principles and Paradigms, Prentice Hall, ISBN 0-13-088893-1
  2. ↑ Norman Matloff (September 3, 2001), Transmission on a Serial Line (PDF), retrieved 2018-04-17
  3. ↑ Marco Platania (2018-06-03). "Clock Synchronization" (PDF). p. 11.
  4. 1 2 Maróti, Miklós; Kusy, Branislav; Simon, Gyula; Lédeczi, Ákos (2004). "The Flooding Time Synchronization Protocol". Proceedings of the 2nd International Conference on Embedded Networked Sensor Systems. SenSys '04. New York, NY, USA: ACM: 39–49. doi:10.1145/1031495.1031501. ISBN 1581138792.
  5. 1 2 Koo, Jinkyu; Panta, Rajesh K.; Bagchi, Saurabh; Montestruque, Luis (2009). "A Tale of Two Synchronizing Clocks". Proceedings of the 7th ACM Conference on Embedded Networked Sensor Systems. SenSys '09. New York, NY, USA: ACM: 239–252. doi:10.1145/1644038.1644062. ISBN 9781605585192.
  6. ↑ Rentel, C.; Kunz, T. (March 2005), "A clock-sampling mutual network synchronization algorithm for wireless ad hoc networks", IEEE Wireless Communications and Networking Conference, IEEE Press, 1: 638–644, doi:10.1109/WCNC.2005.1424575, ISBN 0-7803-8966-2
  7. ↑ Cristian, F. (1989), "Probabilistic clock synchronization", Distributed Computing, Springer, 3 (3): 146–158, doi:10.1007/BF01784024
  8. ↑ "Common View GPS Time Transfer". nist.gov. Archived from the original on 2012-10-28. Retrieved 2011-07-23.
  9. ↑ http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/enet-wp030_-en-e.pdf
  10. ↑ Cena, G.; Scanzio, S.; Valenzano, A.; Zunino, C. (June 2015), "Implementation and Evaluation of the Reference Broadcast Infrastructure Synchronization Protocol", IEEE Transactions on Industrial Informatics, IEEE Press, 11 (3): 801–811, doi:10.1109/TII.2015.2396003
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