Hello, all the workers of the optical transmission network!
This time, I'll share the Q value in WDM system with you. I hope you like it!
What is the Q value?
The Q value and BER have a one-to-one mapping. The Q value can represent the system tolerance in dB, just like dBm substitutes for mW to represent optical power. A smaller pre-FEC BER indicates a larger Q value and better link performance.
Role of Q value
The Q value is a variable used to evaluate the transmission quality of a WDM system. The unit is DB. The Q value represents the transmission margin of the entire system and reflects the transmission performance of the entire physical transmission link. A larger Q value indicates a higher tolerance of the system link to bit errors. A smaller Q value indicates poorer system link transmission quality.
In terrestrial cable systems, wavelength performance is measured by OSNR.In a submarine cable system, the wavelength performance is measured by the Q value.
The Q value is closely related to the Pre-BER. Therefore, the transmission performance of wavelengths can be measured more accurately than the OSNR.
How to query the Q value of a board?
Query the Q value on NCE.
In the NE Explorer, select a board and choose Performance > Current Performance from the navigation tree.
In the screenshot, the value CUR of Q_VALUE_EER is the current Q value.
Note: The Q value is obtained by reading the pre-FEC BER of the OTU board through a series of mathematical calculations. The calculation process is completed by the board software. Not all OTU boards support the query of the Q value.
Additional notes on Q values
1. During the design of a terrestrial optical cable network, the OSNR-based performance commissioning is mainly performed.
2. Different vendors have the same requirements on the Q threshold. The Q threshold is calculated by the board based on the BER. As long as the customer has the same requirements on the pre-FEC BER, the Q threshold is the same.
3. The Q tolerance varies with optical modules. Because different optical modules have different transmit code formats and error correction capabilities, the worst BER thresholds that can be corrected are different and the BER tolerances are different.
Thank you!
