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[owenojo]

Hello,

Maybe someone here can help me try to make sense of a curious problem, I use a Billion Bipac 7700N router, until recently using BE  - now on plusnet as in a few months o2 are of course switching their network off. My line is fairly long with a 52db attenuation - can manage a sync of  5200 @6db. When on BE I used to manually set my router to sync on plain adsl2, as 2+ was less stable, had no benefit on my line and took ages to sync. Since moving to plusnet i can only sync on adsl/g.dmt which is ok if that how its been provisioned - the speeds are no worse, and its certainly a 21CN connection as I'm on an inner london exchange and the old adsl max service has been withdrawn.

My question : On my billion, bitswap is set to OFF by default, indeed its been off the entire time i was using BE. Since reading up on what bitswapping actually does I turned it on as understand it should be on at all times particularly when using g.dmt. But - turning it on seems to reduce the error count, but without fail causes the SNR to fall right down to 0.5 after around 12 hours uptime, sending the line errors through the roof , causing all trafiic to grind to a halt and resulting in a resync.

Has anyone seen anything like this before? it seems almost backwards to how bitswap should work?

cheers

[Loading]

It was already enabled on my router(td8960n), when i bought it, i plugged it in and wouldn't hold a connection for longer than 4 hours, as a last attempt before giving up with the router i disabled it, to my surprise the line is better, doesn't drop, although i get the same number of errors(millions). Also as i was aware it's meant to make the connection better, but it made mine completely worse 

[kitz]

The theory is that it should make the line more stable by swapping 'spare' SNR to tones as  needed.  It works very well for burst type noise - which it is mostly designed to protect against.

However a lot of routers may mark a frequency as unusable if the SNRm @ that tone has gone down to 0dB after a bit swap.  If the line has lots of bitswapping going on, then after a while it can seriously erode the overall SNRm value (which is what it appears to be doing in your case).  If your line is long and has a more constant noise source across all tones, then I could perhaps see in some rarer cases why its not working too well.

I also know that during sync, there is supposed somewhere to be a little margin to allow for bit-swap.  Ive no idea, but if bit-swap is turned off, would that margin be calculated and may give you a tiny bit more SRNm.   It wouldnt be much but it may make all the difference on a poor line. 

Normally I would not recommend turning it off, because it can and usually does make the line less stable if its not enabled and certainly should cause more re-syncs without it.   

[Chrysalis]

bitswapping should most of the time be beneficial and not only that, hugely beneficial.

In my adsl days, especially on speedtouch routers there was different firmware versions which had bitswapping on or off.

The 5.x firmwares had no bitswapping and it was a serious mess with stability, error rates skyrocketing disconnects every 30 mins or so, aweful.  6.x firmwares brought bitswapping but many of them it was buggy eg. it would work for 2-3 hours then just stop working.  The error rate when it stopped would multiply by a factor of 10.  The best firmware was the infamous 6.1.4.3 which was reasonably stable with bitswapping, those were the days eh.

I seen some later routers buggy as well, even on ukonline with billion 7402 routers some of the newer firmwares had the bitswapping bug where it would stop working after a certian amount of uptime.  In every instance I had no bitswapping my line performed a lot worse. I put bitswapping as similiar importance to SRA if not more importance.

On FTTC in 6 days uptime my modem has done over 100k bitswaps.

[Bald_Eagle1]

FWIW, my VDSL2 connection had been up for 10 days 5 hours 8 min 29 sec, with 648128 DS bitswaps & 12756 US bitswaps (11 day ongoing montage attached).





 

[owenojo]

Thanks for the replies everyone,

Kitz, I think that is pretty much what is happening in my case, an increase in noise  suddenly causes the router to swap away until there is nothing left to swap as it were - as supposed to turning it off and letting errors  build up higher. I have turned bitswap back on for now just to see if it was a "one off" type scenario.

In the last few months I was on BE - on adsl2, I did enable it and this problem did not occur whatsoever, only since moving to this BT based connection on plain g.dmt - is the bitswapping in some way different between the two ADSL modes? Have not read anything to suggest there is!

Cheers!

[kitz]

The theory behind bitswap in the 2 modes isnt any different.

BUT there are differences between adsl1 and adsl2+ that will affect you.   

adsl1 (g.dmt) needs a minimum of 6dB whilst adsl 2/2+ needs 3dB of SNR before any particular frequency is usable to load  bits into the bin.   The algorithm used by adsl2/2+ is much more efficient and uses less overheads than adsl1 - particularly when it comes to error correction.   

If youre not using any of the tones in the adsl2+ frequency range, then asking your ISP to put on on adsl2 mode..  or alternatively you may be able to force your router to use adsl2.   That way you get best of both worlds - the efficiency of adsl2/2+ encoding & less overheads...  but without opening your line up to any additional frequencies which may cause noise on the line.

A line at >55dB is usually much better & more stable on adsl1 than adsl2+ because of the unused frequencies.  At 52dB, youre certainly in the region which may benefit being on adsl2. 

The only reason I can think of why your line wont sync at adsl2 is that it needs to be enabled at the dslam.   The modes are backwards compatible.  So if your SP has it set at adsl2+ then you should be able to force adsl1/adsl2 via your router. However you cant do it the other way round.   It may be a 21CN exchange, but the modulation can still be set for adsl1/adsl2/2+

[owenojo]

Thank you to all who replied to my thread!

Well I figured out what the issue was.....and yep, it was the modem. My Billion 7700N was fine on ADSL2 modulation, but on ADSL1, when the noise margin falls more than about 2db (ie normal, on winter evenings when its dark) it goes nuts, wont bitswap properly whether that setting is on or off and resyncs.

I ordered the TG582N from Plusnet, and it works a charm, - would have used my old one from BE if it were not locked down (it went in the bin)

You live and learn!

Owen