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[bensley]

Can anyone explain to me how Reed-Solomon encoding works within the context of ADSL?

I posted this question on serverfault:
http://serverfault.com/questions/403703/adsl-throughput-loss-from-reed-solomon-encoding

Probably the wrong placeto post it, so can anyone here explain it?

Thank you.

[kitz]

I'm not on the pc ATM so can't google or point to stuff easy, nor look at too many links... So ill try make it as simplistic as poss..... But it's something to do with the over heads required to carry the redundant data that limits the available bandwidth in adsl1 to 8mbps.   The redundant data is carried in channels that are marked as out of band.... Therefore you don't see them in the normal  254 available subchannels available for transmission of data.

On adsl1 if your line is able to sync at the full 8mbps, you probably have no idea of the the overhead being there. But if you line can't then you may well see some additional loss of sync when err corr is applied due to the OOB chans needing some of the normal frequencies.   

Adsl2 improved the FEC algorithm S= 1/2 and less overhead needed for error correction  opening up the downstream to 12 Mbps.    adsl 2+ doubled it due to the opening up of the additional frequencies. 

My pages on the main site try to discuss it at a basic understandable level..  Unfortunate can't direct link.. But try looking on the 3 pages with interleaving..  The one with error correction FEC.  and also the page that explains DMT modulation.

[bensley]

...But it's something to do with the over heads required to carry the redundant data that limits the available bandwidth in adsl1 to 8mbps.   The redundant data is carried in channels that are marked as out of band.... Therefore you don't see them in the normal  254 available subchannels available for transmission of data.

So if all 254 sub-channels are used for transmission and those overheads are in out-of-band carriers on different frequencies, then that is a moot point and no explanation of why the bandwidth is 8Mbps; as the full range of sub-carriers should be free for use Or have I miss-interpreted what you said?

Many thanks.

[bensley]

Sorry, I forgot to say; I understand that Reed-Solomon is splitting data into 255 byte codewords because an 8-bit symbol sized has been imposed to give a 32 byte parity block; but I don't see how that should lower the speed like this?

Many thanks.

[kitz]

Maybe I explained it wrong.   but whatever way there is very little information about this unless you want to go into it at high level and start using the complicated formulas. 

When I wrote what I did .. umm about 2006/2007? - there was zilch information out there.. and I had to get practically everything from text books and a basic understanding of algorithms from when I did a disseration in data compression (some similarish theories) and scraps of information here and there. 

The whole aim of the site was originally to put information out that at a level that could be understand without a degree in the subject...  (there wasnt even a wiki back in those early days).

It took quite a while to find out what information I could and even now theres some parts that arent explained and you will quite often see a few of the techies on this forum theorising about how it slots together if a new bit of information comes to light.

I really dont know how to explain it any further.. so Im not going to bull you, because I dont understand it all myself  but its not separate frequencies*..  the overheads are carried separate from 'normal data'.  The interleaver is a seperate module which happens before DMT modulation and its to do with the symbol size and bytes in the DMT frame that causes the reduction.  The bitloading in each channel remains the same (max 15 bits).

I dont know how or why its such a big overhead, but if you want to dig any deeper then perhaps look into Out of Band signalling, because you will find that adsl uses OOB for FEC overhead and its partially because adsl uses OOB that maximum sync speed is reduced. Also look into bytes in the DMT frame.
Its a payoff between useful data throughput and reach - ie which frequencies can be used for transmission of useful data. 

Unfortunately Im not in a position to be doing any research atm due to mostly using a mobile device.. and it could be towards the end of next week before I even get chance to start digging.

If you do find anything interesting that is capable of explaining it in a dumbed down way, then please do let me know.

[guest]

I thought the throughput reduction was more to do with the amount (duration?) of INP applied myself on ADSL2+ lines.

RS encoding isn't that hard to understand surely - here's a hint for the people struggling, its not linear and is designed for impulse noise. Constant bit-rate noise is easy peasy, impulse isn't.

OOB overheads are carried within the same carrier frequency range anyway, from memory the OOB overheads were only OOB on lower rate DSL.

So if you're at a sync of 18Mbps on ADSL2+ on "interleaved" then if you drop all "OOB correction/interleaving" then you're quite likely to end up with 20Mbps sync rate.

There's plenty of OOB signalling available on ADSL (8Mbps).

Of course we're also looking at the default level of INP applied by ISPs. A fastpath ADSL2+ 3dB connection from Be is likely to sync considerably faster than an interleaved ADSL2+ 3dB from Sky (yes Sky DLM does 3dB now).

I don't think I understand the question really. Nothing new there

[snadge]

The way I understand it is; that its data added to outgoing traffic that can be used to correct errors, the data is there all the time (whether there are errors or not) and that's why speed is reduced because of the added overhead, the higher the setting (more overhead) - the less speed, there is info (below) that says newer Broadcom MSAN's can now reduce/change the number of DMT Symbols per RS Codeword which allows a higher sync with the higher INP settings

I dont know if this is any help... but there is a lot info here:

1) http://www.draytek.co.uk/support/kb_vigor_inp.html
2) http://www.see.ed.ac.uk/~dgmc/PhDthesis/Nedev.pdf
3) http://en.wikipedia.org/wiki/Reed-Solomon_error_correction

offtopic: @ Rizla - are sky doing 3db upon request?

[guest]

offtopic: @ Rizla - are sky doing 3db upon request?

No. It has to be set by the DLM software, there's no manual option for tier1/2 to set for 3dB.