# Formula used to calculate the OSNR

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OSNR = 10lg (S/N)

###### Other related questions:
BER calculation formula
BER calculation formula: B1 BER = Number of B1 errored blocks/(N x 270 columns x 9 rows x 8 bits x 1s x 8000 frame/second). 　　 Note: N in the preceding formula is the number in STM-N. B2 BER = Number of B2 errored blocks/{[(261columns x 9 rows) + (9 columns x 5 rows)] x 8 bits x 1s x 8000 frame/second}. B3 BER = Number of B3 errored blocks/(261 columns x 9 rows x 8 bits x 1s x 8000 frame/second).

Base of the OSNR = 10 × log (Ps/Pa) formula
The base of the OSNR = 10 x log (Ps/Pa) formula is 10. Strictly speaking, lg should be used instead of log. However, the default base is recognized in the industry and related standards. Therefore, "log" is still used in the formula.

Method used to calculate a linked clone VM's storage resources
The storage resources required by a linked clone VM are calculated as follows: Formula: ROUNDUP (N/128, 0) x Size of the base volume + N x Average size of delta volumes. The average size of delta volumes needs to be calculated based on the site situation. Generally, the estimated average size is 5 GB.

Detection of the OSNR of a 40G or 100G system using the traditional OSNR detection method
The optical spectrum of 40G or 100G signals is wider than that of 10G signals. Therefore, the signal spectra of adjacent channels overlap each other. The channel noise includes not only the ASE noise but also the power crosstalk of some signals. Therefore, the OSNR tested using the traditional OSA method is less than the actual OSNR. The traditional OSA method is implemented based on the interpolation of inter-channel noise that is equivalent to in-band noise. The traditional OSNR detection methods, including the scanning method and the integral method, have the following disadvantages: Scanning method: applicable only to a system with a 100G channel spacing and without traversing an OEQ node. The detection results have a low precision. Integral method: When there are a few wavelengths, turning on or off the lasers will cause noise power fluctuation and therefore affect the detection accuracy. In addition, the noise power cannot be measured by turning on or off the laser when there is only one wavelength.

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