EPON Clock Frequency Drift Compensation via Dynamic Window Adjustment
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Solution Overview
Problem
Ethernet passive optical networks face challenges in accommodating different clock frequencies between the optical line terminal and optical network units, leading to potential errors and overlapping of upstream data bursts due to clock frequency drifts.
Innovation Solution
The system derives an OLT clock from received signals and maintains a local clock, adjusting the number of bits in upstream data bursts by inserting or removing unused bits to ensure the bursts fit within the assigned transmission window, thereby compensating for frequency differences between the local and OLT clocks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the ONU uses its local clock for upstream transmission, then the transmission can proceed independently without continuous clock synchronization, but the data burst may drift outside the assigned transmission window due to frequency differences
Solution Approach 1:
The system dynamically adjusts the transmission window size based on the detected clock frequency difference between OLT and ONU. The adjustment factor is calculated as the ratio of OLT clock frequency to ONU local clock frequency, and the transmission window is scaled accordingly to ensure the data burst fits within the adjusted window boundaries despite frequency drift
Solution Approach 2:
The patent changes the parameter of transmission window size to compensate for clock frequency differences. By multiplying the original transmission window by the frequency ratio (f_OLT / f ONU), the system adapts the window parameter to match the actual transmission rate, allowing independent ONU operation while maintaining timing accuracy
2Reliability
If the transmission window is adjusted to accommodate frequency drift, then data bursts can fit within the window, but the window size may become larger than necessary when clocks are synchronized
Solution Approach 1:
The transmission window size is made dynamic rather than static. The system calculates the frequency difference between OLT and ONU clocks, then adjusts the window size proportionally. When clocks are synchronized, the window remains at its original size; when frequency drift occurs, the window expands or contracts accordingly, optimizing resource usage while ensuring reliable transmission
Data Source
AI summary
One embodiment of the present invention provides a system that accommodates different clock frequencies in an EPON. The system receives a signal from the OLT at the ONU and derives an OLT clock. The system also maintains a local clock. The system further receives from the OLT an assignment for an upstream transmission window, during which the ONU can transmit an upstream data burst to the OLT based on the local clock. The system adjusts the number of bits of the data burst without affecting the payload data carried in the data burst, thereby allowing the data burst to fit properly within the upstream transmission window and compensating for frequency differences between the local clock and the OLT clock. The system transmits the data burst based on the local clock in the upstream transmission window.