Optical Fiber Time Transfer Asymmetry Compensation
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current time and frequency alignment methods in optical fiber networks, such as PTP and NTP, face accuracy limitations due to transit delay asymmetry caused by varying path lengths and packet delay variations, which are not accounted for in existing protocols like Precision Time Protocol (PTP) and Network Time Protocol (NTP).
Innovation Solution
The method involves determining and compensating for transit delay asymmetry between network elements by using synchronized clocks and burst mode communications over optical fiber links, either over the same or different wavelength channels, to establish symmetric paths and correct time transfer protocols like PTP and NTP, ensuring accurate time synchronization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If packet-based methods (PTP/NTP) are used to transfer time references, then time synchronization can be distributed throughout the network, but accuracy is adversely affected by transit delay asymmetry and packet delay variation
Solution Approach 1:
The patent applies asymmetry by using different wavelengths for forward and reverse direction transmissions. Specifically, the master clock transmits timing packets to slave clocks on one wavelength while slave clocks transmit timing packets back to the master clock on a different wavelength. This wavelength differentiation allows the system to measure and compensate for asymmetric transit delays in each direction separately, thereby resolving the transit delay asymmetry problem that limits PTP/NTP accuracy.
Solution Approach 2:
The patent implements feedback by having slave clocks transmit timing packets back to the master clock on a different wavelength. The master clock measures the transit delay of these reverse-direction packets and uses this feedback information to calculate and compensate for asymmetric delays. This two-way timing packet exchange with feedback measurement enables the system to correct for transit delay asymmetry and improve time synchronization accuracy.
2Productivity
If separate fiber strands and wavelengths are used for carrying signals in each direction, then communication capacity is increased, but path length variation introduces tens of nanoseconds of asymmetry in transit delay
Solution Approach 1:
The patent uses feedback by measuring the actual transit delay of timing packets transmitted in each direction on their respective wavelengths. The master clock measures the delay of packets received from slave clocks on the reverse wavelength and uses this feedback to calculate compensation values. This allows the system to maintain separate fiber strands and wavelengths for high communication capacity while simultaneously compensating for the path length variation-induced asymmetry to achieve nanosecond-level time transfer accuracy.
Solution Approach 2:
The patent applies parameter changes by using different wavelengths for forward and reverse transmissions. This wavelength parameter differentiation allows the system to utilize separate fiber strands or wavelengths to increase communication capacity while enabling independent measurement and compensation of transit delays for each wavelength, thereby resolving the time transfer accuracy issue caused by path length variation.
3Measurement precision
If GPS signals are used at each network element for time alignment, then accurate time synchronization is achieved, but cost increases and GPS visibility requirements are imposed
Solution Approach 1:
The patent merges the time synchronization function into the existing packet-based communication infrastructure. Instead of requiring separate GPS receivers at each network element, the system combines time reference distribution with data packet transmissions by embedding timing information in protocol packets. This consolidation eliminates the need for additional GPS hardware at each node, reducing device complexity and cost while maintaining accurate time synchronization through the wavelength-based delay compensation mechanism.
Solution Approach 2:
The patent introduces an intermediary approach by using a master clock system that centralizes the time reference function. The master clock receives the primary time reference (which could be from GPS or other sources) and distributes synchronized time references to all slave clocks through the packet network with wavelength-based delay compensation. This intermediary master clock eliminates the need for each network element to independently process GPS signals, thereby reducing device complexity while maintaining synchronization accuracy.
Data Source
AI summary
A clock at a first network element that is connected to a second network element over an optical fiber link is aligned using bursts of timing information exchanged between the two network elements. According to one method, the bursts from the first network element to the second network element and the bursts from the second network element to the first network element are transmitted over the same wavelength channel of the optical fiber link, in which case zero asymmetry in the transit delays can be assumed during the alignment procedure. According to another method, the bursts from the first network element to the second network element and the bursts from the second network element to the first network element are transmitted over different wavelength channels of the optical fiber link, in which case the asymmetry in the transit delays can be quantified and applied during the alignment procedure.