Multiplex communications – Pathfinding or routing – Switching a message which includes an address header
Reexamination Certificate
1998-09-10
2001-08-21
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Pathfinding or routing
Switching a message which includes an address header
C375S371000
Reexamination Certificate
active
06278710
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention pertains to the field of distributed systems. More particularly, this invention relates to enhancements to time synchronization in distributed systems.
2. Art Background
Distributed systems are commonly arranged as a collection of nodes which are interconnected via one or more network communication links. These network communication links may be packetized links such as Ethernet or one or more of a variety of other packetized links that are adapted to distributed control system applications.
Distributed control systems commonly benefit from precise control of the timing at the distributed nodes. U.S. Pat. No. 5,566,180 of Eidson et. al. teaches a method and apparatus for providing precise control of timing in distributed nodes by synchronizing the local clocks in the distributed nodes. The synchronization protocol of Eidson et. al. involves the exchange of timing data packets and follow up packets among the nodes so that the delay in the transfer of a timing data packet from a first node to a second node in combination with timing information in a follow up packet can be used to accurately adjust a local clock in the second node.
A variety of conditions that are commonly found in distributed systems may introduce variation or jitter in the delay in the transfer of a timing data packet. For example, communication circuitry at various points in the distributed system may introduce jitter. In addition, communication circuits such as gateways can introduce jitter that depends on the volume of traffic in the system. Unfortunately, such jitter may reduce the accuracy of time synchronization in a distributed system.
SUMMARY OF THE INVENTION
A variety of enhancements to a time synchronization protocol for a distributed system are disclosed. The enhancements may be embodied in a distributed system which includes a first node and a second node and one or more intervening communication links that may includes communication devices such as repeaters or gateways. The first node includes a local clock and circuitry that generates a timing data packet and a follow up packet. The timing data packet has a unique timing point and the follow up packet includes a time-stamp obtained from the local clock that indicates a time at which the timing data packet is generated. The second node includes circuitry that receives the timing data packet and the follow up packet via a communication link. The second node further includes a local clock and circuitry that obtains a local time value from the local clock when the unique timing point is detected. The difference between the time-stamp from the follow up packet and the local time value indicates a relative synchronization of the local clocks in the first and second nodes.
The enhancements disclosed herein include techniques for improving the accuracy in time synchronization by separating the unique timing point from a delimiter for the timing data packet. The enhancements include techniques that compensate for jitter associated with communication circuitry in the distributed system including jitter associated with physical interfaces and gateways in the distributed system. These techniques may involve specialized circuitry in the communication circuitry to compensate for jitter or special processing of received timing data packets or the introduction of follow up packets that inform receiving nodes of measured jitter or a combination of these techniques.
Other features and advantages of the present invention will be apparent from the detailed description that follows.
REFERENCES:
patent: 4890222 (1989-12-01), Kirk
patent: 5392421 (1995-02-01), Lennartsson
patent: 5537549 (1996-07-01), Gee et al.
patent: 5640388 (1997-06-01), Woodhead et al.
patent: 5896427 (1999-04-01), Muntz et al.
Agilent Technologie,s Inc.
Hsu Alpus H.
Qureshi Afsar M.
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