The FEC overhead calculation is R parity bytes used in a block of size N;
Thank you - that explanation makes it simple. As soon as I see formulas such as this, then my brain just switches off
That calculation would suggest you could regain over 16Mbps (21% of 80) when DLM is removed. I don't think you do ... and the real answer would be around 9Mbps.
Just a thought - could s=1/2 mode have anything to do with this? As implied, s=1/2 mode reduces RS overheads by up to 50% by mapping 2 RS codewords to a single FEC output data frame.
RefThe limitation of maximum allowable Reed-Solomon codeword size can be overcome if the interleaved
data path is used (with S=1/2, where S is the number of data frames per RS codeword). Using the interleaved data
path, which will be discussed in detail later, two Reed-Solomon codewords can be mapped to a single FEC output
data frame.
DSL has two bearer channels (paths) that it can use: FAST and INTERLEAVED. Traditionally with adsl1 everything was sent down the FAST path. Interleaved path can still simulate fast path by using depth=1... but if s=1/2 mode is used on this channel then overheads can be reduced by half.
I'm the one who told Chrys this, as I've experienced it directly back when the 17a profile was introduced, but before the 80/20 packages were introduced.
Its been clearly in evidence since adsl2+ ... and in fact its also been seen as far back as on adsl1 on some lines
if you were attached to one of BTs Marconi or Fujitsu MSANs (adsl2+ capable) rather than the older DSLAMs
and if you had an adsl2+ router capable of s=1/2 mode.
I think there have been cases where the FEC overhead comes fully off the sync speed, even when capacity is available. I wonder if it a configuration issue, or a difference between DSLAM or CPE.
Several years ago I searched and searched to find an explanation for how adsl FEC overhead is transported because I wanted to know. I know from my various text books on compression & error correction (
these are from my college days which I dont have to hand now and TBH I was far more interested in compression for my dissertation than I was in adsl back in those days) that there are 2 methods available for overhead transportation ie out of bands or reduction of payload. When later researching specifically for adsl, the only thing I could come up with was a Cisco paper that said adsl & DMT uses the out of bounds method for FEC redundancy.
However like you I have also observed lines where it appears that the full overhead has been taken from the sync speed and why I was a bit fuzzy in my 3rd paragraph using the words 'believe' and 'cant seem to find anything that says otherwise'. On reflection you make an interesting point when you say difference in DSLAM - I got my info from a Cisco paper so it could be possible that other DSLAM makes
may be configured differently? As mentioned it was an area I searched extensively for, but drew a blank