I was arguing that the maxattainable uses the actual target SNRM. MrSaffron and Robertos seemed to think modems use a fixed 6dB in all cases.
Never saw that - its been a while since read anything on TBB otherwise I could have said something earlier to point you in the direction of ITU standards to back up your claim.
I already knew that it is uses the target SNRm from when I attempted to do a bit of digging a few years ago to see if there was any clue why max attainable went so skewy when Interleave (or rather INP) was applied to the line.
I'm still none the wiser why INP causes the discrepancy. I assume that most of the modem stats we see on here use the basic method (in fact I can't even recall there being a choice of methods at the time I looked). I'm also assuming that because most of the stats we do see tend to come from BCM chipsets then it could be down to the way that BCM interpreted the original recommendation.
I note the latest version of ITU G.993.2 says
The conditions of the basic method in this version of the Recommendation are unchanged compared to the conditions for ATTNDR calculation in previous versions of this Recommendation. This set of conditions did not specify a number of conditions to calculate ATTNDR, which lead to vendor discretionary behaviour in the reported ATTNDR values. Looking at an archived version, which was the pre-published version of these changes relating to the max attainable rate was dated 05/2015 (amendment 6). Most of us here are used to seeing stats from the HG612/Zyxels/Billions all of which were around before that date.
I will say that shortcuts can be taken during development, and things that ought to be variable can get hard-coded.
Im talking about the BCM based modems that most of us use, but with this stat being part of the standard BCM commands then I would think its unlikely as we see enough movement of SNRm and target SNRm for it to be fairly accurate on lines which dont have INP. These modems are also used on adsl lines and with other SPs which have had variable target SNR (both more than and less than 6dB) long before Openreach decided to apply it. It only appears to go pete tong when INP is applied.
I dont know here and perhaps ejs may have some thoughts, but the other important figure used is SNRGAP. ITU states
The SNRGAP value is defined for a 10−7 bit error ratio on 4-QAM (no coding gain, INP_min0 = 0).Is it something to do with the use of INP=0 in the calculation when say INP 3 is applied to the line which is throwing the figures out. Whatever way the figures that Ive noticed always seem to over compensate by about double what they should be.
The modem has to calculate a [true] SNRGAP to get the actual sync speed which will be calculated using any existing DLM params. Therefore is it possible that BCM is using the actual SNRGAP figure rather than one with INP=0 in the ATTNDR calculation that is throwing the figures out for interleaved lines.