Results From Proof-of-Concept Time Based Communications Testing

Tom Celano, S. R. Stein, Al Gifford, Eric Swanson, Roe Hemingway, John Carney

Abstract:	Time-based communications is a concept whereby the synchronous layer of a communications channel is used as a vehicle for two-way time transfer. The technology originated in 1995 with prototype hardware designs for a self-calibrating optical two-way time transfer system. Point-to-point systems were built in 1996 and demonstrated synchronization error in a laboratory between a master clock and a recovered slave clock at the sub-nanosecond level over 10 km distribution distances using SONET. Since then, the communications payload capability has been added to the optical two-way system providing a true time-based communications channel with two-way time transfer functionality embedded in a OC-3 (155 Mb/s) data channel. In February 1997, two weeks of testing were performed at Lincoln Laboratory (MIT/LL) to collect an extensive data set on the timing and communications performance of the system. The test objectives included: A) Measure system performance with long-range (>10 km) cable runs in a laboratory environment. B) Establish timing and communications performance in a laboratory environment with typical communications hardware in the fiber link. C) Characterize system performances using "real-world" (outdoor) links exhibiting temperature dynamics that change the length of he fiber. D) Determine the suitability for application in future wavelength division multiplexed (WDM) optical communications networks. This paper presents the results of the time-based communications testing performed at Lincoln Labs (MIT/LL) using an Optical Two-Way Time Transfer Communications System. A Short explanation of system design and two-way time transfer implementation in a communications channel will be followed by a presentation of data sets from the Lincoln Laboratory testing. Time transfer measurement data will be presented from tests conducted from within the laboratory, as well as tests conducted over long-distance, outdoor links. The laboratory measurements to be presented include two-way time transfer results over fiber in a controlled (constant temperature) environment with typical communications hardware inserted in the link between clocks. The hardware used for laboratory measurements includes electro-optic repeaters and erbium-doped fiber-optic amplifiers. The outdoor fiber links (intended to represent real-world links) include a 75-km fiber run with OC-3 repeaters inserted at the midpoint, and 100-km with wave-division multiplexing equipment in the link. Synchronization error measurement data are presented for each test configuration. Two-way delay data will be presented for the real work links to show the changes in fiber length induced by temperature variations in the outdoor environment.
Published in:	Proceedings of the 29th Annual Precise Time and Time Interval Systems and Applications Meeting December 2 - 4, 1997 Sheraton Long Beach Hotel Long Beach, California
Pages:	423 - 434
Cite this article:	Celano, Tom, Stein, S. R., Gifford, Al, Swanson, Eric, Hemingway, Roe, Carney, John, "Results From Proof-of-Concept Time Based Communications Testing," Proceedings of the 29th Annual Precise Time and Time Interval Systems and Applications Meeting, Long Beach, California, December 1997, pp. 423-434.
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