OFDMA SNR oid | docsis.org

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I have one challenge at the moment is that I'm interested to know the SNR in a OFDMA interface but I don't find it yet, I do found the oid for upstream but it doesn't include ofdma .{ifindex}

Is there any oid for OFDMA? also looking the same for corrected and uncorrected codewords

Appreciate any help

Some CMTS vendors include it

Some CMTS vendors include it in docsIf3CmtsCmUsStatusSignalNoise ( the Docsis 3 version of the OID you referenced.

However, the mechanism that OFDMA uses versus ATDMA makes even that metric pretty worthless. See this post for deeper explanation: https://www.docsis.org/comment/9450#comment-9450

ATDMA channels have a single modem transmitting at a time over the entire 1 (potentially 6.4 MHz wide) channel, for a very short duration.

OFDMA channels have up to 1,896 sub carriers (channels 50 KHz spacing) where multiple modems talk at the same time (up to 237) on 400 KHz segments, for much longer periods of time.

on ATDMA channels CMTSes have a mechanism to look at the single channel SNR and FEC and if it is low adjust the entire channel (modulation / channel width) to compensate (all modems suffer even if problem is local to one segment of the network)

on OFDMA Channels CMTSes assign each modem with a High IUC for data and a Low IUC for communication with the CMTS. They have a mechanism that when a particular modem is having trouble communicating with the CMTS on the high IUC with low MER / FEC, the CMTS adjusts just that single modem's high IUC to something more appropriate for RF conditions.

Because of the disparity in number of carriers in the two different channels, and the time duration individual modems transmit for (6.25 usecs for ATDMA, 20-40 uSecs for OFDMA per symbol, typically 16 symbols in a frame thats 320-640 uSecs) a per-channel SNR value on an OFDMA channel is not terribly useful.

for example on an atdma channel if the monitor time when you read docsIf3CmtsCmUsStatusSignalNoise is 100 uSecs (made up number for illustration purposes) this will calculate the SNR across the entire 6.4 MHz channel 16 times (100 / 6.25) potentially 16 different modems. The CMTS would average the SNR measurement and you would have the average SNR across all 6.4 MHz of spectrum.

The same duration of a test on OFDMA would only capture 1/3 of the time for a single transmission on as little as 400 KHz of the OFDMA channel (1 modem using 1 minislot 400KHz to transmit). It could also catch 1/3 of the time for a single transmission for up to 237 different modems using 96 MHz of spectrum. Averaging either of those won't tell you how many modems it measured, nor what portion of the spectrum (up to 96 MHz) it measured.

This is why you need per subcarrier RxMER per modem, so that you can get the measurement of whole spectrum of the channel.

See my previously linked post for the way to conclusively test OFDMA RxMER, but if you still want to approximate the value for a single channel you would need to:

For each cable modem on an OFDMA Channel read docsIf31CmtsCmUsOfdmaChannelMeanRxMer (
total those MERs
divide by the number of modems

This will give you the average of the MER on the channel.

If you also add in the Standard Deviation docsIf31CmtsCmUsOfdmaChannelStdDevRxMer ( you can get a range per modem too.

However, due to the differences between OFDMA / ATDMA the number derived this way could be very different from the real RxMER because all of the modems transmitting on that particular channel could be in a clean part of the spectrum and showing a good overall MER, but another part of the channel could have very poor MER and due to no modems transmitting there and the averaging you would not know.

Some CMTSes have the ability to look at per minislot (400 KHz) SNR/FEC on a channel level, but it is not part of the Docsis spec to look at the channel that way. And it is not directly readable via an OID (requires CLI)

Some CMTSes also have the ability to schedule a Probe, to which every modem responds and measures MER and power on each subcarrier and then summarizes the response across the whole channel. This is also only available via CLI and takes 2-20 secs depending on number of modems and how quiet (read empty) the channel is at the moment.

If you have Casa CMTS I can help you with commands for last two methods, other vendors your mileage will vary.


the CMTS measures the FEC per OFDMA channel and per IUC
docsIf31CmtsUsOfdmaDataIucStatsTotalCodewords (
docsIf31CmtsUsOfdmaDataIucStatsCorrectedCodewords (
docsIf31CmtsUsOfdmaDataIucStatsUnreliableCodewords (

It also measures it per modem, per OFDMA channel and per IUC
docsIf31CmtsCmUsOfdmaProfileTotalCodewords (
docsIf31CmtsCmUsOfdmaProfileCorrectedCodewords (
docsIf31CmtsCmUsOfdmaProfileUnreliableCodewords (

Thanks a lot for you

Thanks a lot for you explanation it is very useful!

have a great day

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