Cisco IOS Software Releases 12.2 Mainline

Signal-to-Noise Ratio (SNR) Reporting Updated in Later Cisco IOS Versions

August 8, 2003

Products Affected



Cisco IOS ® software versions: 12.2, 12.0SC, 12.1EC

These images will be affected by an inaccurate Signal-to-Noise Ratio (SNR) that will read approximately 4 dB higher than the real values.

Corrected IOS versions: 12.2(6.3)S, 12.1(9.5)EC01, 12.2(6.3)T, 12.2(6.4), 12.0(16)SC02, 12.2(6.8)DA, 12.2(3.6)BC, 12.2(6.4)B, 12.2(6.8)S, 12.2(6.08a)DA, 12.2(15)BW, 12.2(15)BX, 12.2(15)ZN

These and later IOS versions will read the SNR closer to the real value (about 4 dB lower that the older IOS versions)

Hardware: uBRMC11c, MC12c, MC14c, MC16c, MC16e, MC28c, uBR7111, uBR7114

This a list of hardware that is affected. Note: MC16s can be affected under earlier IOS versions when the linecard is running in compatibility mode (non- S mode).

Problem Description

The reported Signal-to-Noise Ratio (SNR) value is suddenly four dB lower with the loading of a later IOS image.


The formula that is used to calculate the signal-to-noise ratio(SNR) that is used by the controller chip has been updated by the chip manufacturer to better reflect the true value being measured. The DDTS mentioned below corrects the formula that is used for calculating the SNR values that are reported by the controller chip. The change implements the newest algorithm to calculate the SNR as recommended by the chip manufacturer. The new reported value is four dB lower than the old value in most cases. The code change is actually a constant log value changing from 10 in the earlier IOS versions to 16 in the later IOS versions. The four dB comes from 20*(log 16 - log10) which is 4.08. This is a lab qualified number using a pure noise source and can vary as high as six to eight dB in the presence of real world impairments that include impulse noise and Common Path Distortion (CPD).

TIP: The SNR estimate is relatively accurate for Added White Gaussian Noise, but often reads erroneously for typical Hybrid Fibre Coaxial (HFC) plant noise. Customers have reported good success comparing the errored packets to received packets, and creating an impulse noise counter index.

If the value is greater than one, there is some impulse noise, and values of 25-30 indicates a lot of noise which indicates it is time to fix the node.

$impulsenoise= sprintf "%.6f" , (.000001+$uncorrpack*10000)/(.000001+$inputpack$)

uncorrpack == uncorrectable packets from the show interface / show controllers outputs

input packets == received packets from the show interface / show controllers outputs.

Problem Symptoms

A four dB reduction in SNR on a linecard.


Workaround: Understand that the SNR value provided by the chip on non S type linecards is not the most accurate method of determining the SNR of a cable plant. It is not intended to provide the functionality of Cisco S type linecards. It should be used only as a rough indicator. SNR estimate is similar to modulation error ratio (MER), and is not intended to be an indication of cable network carrier-to-noise ratio (CNR).

Understand that the reduction in SNR reporting does not reflect any change in the cable plant. It is part of the chip manufacturer's recommended way of calculating the SNR and should not be relied upon to engineer the cable plant. It is used as an indication, along with other measurements, to target field activities.


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CSCdv78225 (registered customers only)

SNR formula is incorrect

For More Information

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