Optical Transport Network Clock Synchronization Dithering
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Solution Overview
Problem
In optical transport networks, synchronizing OTN transport or HO-ODU clocks can lead to issues with short-term wander and low-frequency jitter due to frequencies of synchronous services being near integer fractions of transport rates, causing systematic justification patterns and infrequent phase adjustments, especially when mapping client configurations using asynchronous or generic mapping procedures.
Innovation Solution
Dithering of justification events during client signal mapping by alternately adding and subtracting bits, creating regular phase shifts that increase the probability of justification events being reflected in the client or LO-ODU data, which can be filtered by the clock recovery process to average the phase and reduce low-frequency jitter and short-term wander.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If synchronous OTN transport is implemented to transport reference clock signals, then clock synchronization capability is improved, but short-term wander and low-frequency jitter increase due to systematic justification patterns
Solution Approach 1:
The patent applies periodic dithering action to the justification process by alternately adding and subtracting bits at regular intervals. This periodic modification of the justification pattern prevents the system from settling into stable systematic patterns that cause low-frequency jitter and short-term wander, while maintaining the overall synchronization function.
Solution Approach 2:
The patent changes the parameters of the justification process by dynamically adjusting the number of bits added or subtracted during mapping. By varying these parameters periodically through dithering, the system maintains clock synchronization capability while preventing the formation of harmful systematic patterns that degrade signal quality.
2Productivity
If frequencies of synchronous services are set near integer fractions of transport rates, then bandwidth utilization is improved, but justification events become systematic and infrequent phase adjustments occur
Solution Approach 1:
The patent introduces periodic dithering action that alternately adds and subtracts bits during the mapping process. This periodic intervention ensures that justification events occur regularly rather than systematically, increasing the frequency of phase adjustments and preventing the degradation of signal quality that would otherwise result from infrequent adjustments.
3Manufacturing precision
If dithering of justification events is implemented by alternately adding and subtracting bits, then phase oscillations are generated that can be filtered, but buffer sizes must be increased to accommodate the variation
Solution Approach 1:
The patent implements beforehand cushioning by increasing the buffer size to accommodate the periodic bit additions and subtractions performed during dithering. This pre-allocation of additional buffer capacity prevents overflow or underflow conditions that would otherwise occur during the dithering process, enabling the generation of filterable phase oscillations without risking data loss.
Data Source
AI summary
A technique is provided for transmitting client data included in a client signal via an optical transmission path of an optical transport network. The optical transport network uses transport frames include a transport frame period for transmitting client data. The method includes receiving multiple client entities comprising multiple client data bits; determining the number of client data entities received during a transport frame period to establish a mean number of client data entities to be included in a transport frame, the mean number of client data entities corresponding to a mean number of client data bits; mapping multiple client data entities into the transport frame wherein mapping comprises alternately adding and subtracting an amount of client data bits to/from the mean number of client data bits for at least two consecutive transport frames; and transmitting the transport frames comprising the client data via the optical transport network.