Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[asbokid]

Can't claim to know anything about bit allocation algorithms but attached are two excerpts from books by those who do know!

Bingham, John (2000) ADSL, VDSL, and Multicarrier Modulation, ch.5: Fundamentals of Multicarrier Modulation
Jacobsen, Krista (2006) Fundamental of DSL Technology (ed Goulden, P, et. al) ch.6: Fundamentals of Multicarrier Modulation

Bingham references John Cioffi, Emeritus Professor of Engineering at Stanford.  Cioffi's (old) course website carries several bit allocation algorithms coded in MATLAB:

http://www.stanford.edu/class/ee379c/

cheers, a

[kitz]

Thanks asbokid.   It was late last night when I was in bed and it dawned on me that the algorithms actually were for bit allocation (ie working out how many bits the subchannels can use) rather than actually loading bits for transmission into the bins.

One of those links you provided though does give more info on the bit swapping process..  and it talks purely about the bit swapping process using power gain/decrease to adjust the SNR.  So the case (3) I mentioned above method is used regardless of what bins are loaded with however many bits.

However...  Yet another link  would seem to show a function for the router on how to actually fills the subchannels with bits...  which I believe is the key to the answer for kezzamans question
 
Having had a quick look at this code, it would appear the router does indeed use the 'waterfilling' process to decide which bits to load into the subchannels.   

This waterfilling process would seem to analyse all the channels, and determine the subchannels with the best SNR as those being the ones to fill first.

[kezzaman]

Do you guys ever check how many bitswaps occur in 1 hour on ur line?

[geep]

[kezzaman]

Does fibreoptic broadband use bitswap?

[snadge]

Does fibreoptic broadband use bitswap?

GOOD QUESTION!!

someone much more skilled than I will answer in full and 100% correct, but for now I will attempt some explanation... NO.... lol  at least I dont think it does, only FTTC does because it is still a form of xDSL, its fibre to the cab, but from there its VDSL2 to the premises, so i just like bringing the exchange closer to you - Virgin Media's network is a HFC Network (Hybrid Fibre/Coaxial) in that it also has fibre nodes (cabs) near to the home and from these nodes is coaxial copper cable, but these typically have amplifiers en route beyond the nodes towards the premises and inbetween premises which 'boost' the signal power to keep it strong en route to all customers, also the coaxial cable is a much better quality copper cable and has some shielding too, Virgin Media's network uses DOCSIS (as opposed to xDSL) and I THINK this protocol is designed to be used with such a Network (HFC) ad operates at 'Fixed' speeds only..and therefore no bit-swapping is required as its not designed to 'adapt' to the lines conditions (like in xDSL) because the line has to always be in good working order able to receive the max speed (e.g. 100Mbps)...or not... there is no "in-between" where speed can be 'shaped' around the condition of the line ...something like that anyway

there is some info
here -> http://en.wikipedia.org/wiki/Hybrid_fiber-coaxial
and here -> http://en.wikipedia.org/wiki/DOCSIS

And same applys for pure fibre networks where they exist...

A lot of people THINK (due to Virgin Media advertising lies) that they get superfast fibre right into their homes....when they dont.  Virgin used to criticise BT's copper network calling it "infrerior" and listing problems with it in full knowledge they too used copper as last mile...just a better type of copper that was less affected due to the way it was being used.

when are OFCOM gunna force Virgin to wholesale its network?  cant wait for that day!!! - even though HFC/DOCSIS outperforms xDSL slightly.. I still prefer DSL because its massive acheivement in technology...theyve been able to match Virgin speeds (almost) over 100 year old copper network...without the use of higher grade last mile wiring or amplifiers en route too boost power.

...oh and when Vectored VDSL2 comes into play you will see speeds faster than Virgin's current 100Mbps max (over 100Mbps for those 400m and less from the Cab) - Vectored VDSL2 makes use of Vectoring all the Pairs together to cancel out cross-talk at the Dslam by calculating the noise over the pairs and injecting "Anti-Phased" noise into the lines which cancels out the noise over all the lines...enabling even faster speeds  ...something like that anyway , for it too work at its best all the lines in the cable need to be vectored, a few strays that arent can affect performance, all you need to do this is upgrade software on Dslam and pack all the VDSL2 pairs in their own shielded cable away from other ADSL1/2/2+ pairs...so it would seem , see video below

[youtube]http://www.youtube.com/watch?v=zkVmej4urx4[/youtube]

[kitz]

>> Does fibre optic cable use bitswap.

No.

>> Does vdsl (FTTC) use bitswap

Yes.

------------------------------------



Bitswap is an essential part of xDSL which uses DMT technology.
The reasons why are all explained in more detail in these sections.

What is DMT.
What is DMT modulation.
What is bit Loading
What is bit swapping
Further info on those pages helps slot understanding into place.

But to simplify things to the most basic of levels :-
~ DMT technology is used between the dslam and CPE (your router)
~ Without DMT, telephone cable to the home is limited to 56kbps,(dial up) 
~ DMT modulation is splitting of frequencies so each subchannel can carry 'up to' 56kbps (if the bin is fully bit loaded)...  and merging to provide higher bandwidth.
  The downside being the higher frequencies are more subject to signal deterioration.
~ DMT modulation is why DSL needs a modem (adsl modem/router) rather than a 'standard' router used for cable broadband.
~ Without bitswapping the line would lose sync if the SNR in any of the subchannels dropped too low and became unstable.
~ Bitswapping can be performed between any of the available subchannels


Cable broadband doesnt use DMT at all.

Optical Fibre uses Frequency Division Multiplexing* or Wave Division Multiplexing.
(See Wave Division Multiplexing (WDM)).
Although fibre can be subject to attenuation, it is minimal when compared with copper.
Co-ax is used for the entry into the home, which as snadge mentioned is better quality copper than the twisted pair... and use of amplifiers if needed to ensure against signal deterioration.  - Think how ethernet doesnt need/use bitswap either. This length of co-ax only has a relatively short run.


*DSL also uses FDM to separate the upstream and downstream channels. DMT splits it into subchannels.

IIRC Bitswapping is a standard in g.DMT not just the rate adaptive products - its interleaving/FEC that doesnt occur on the traditional fixed rate products. 
I think adsl2/2+ may use a different/better bitswap algorithm as it handles SNR slightly differently.. and bit loading of min 1 bit per bin (3dB SNRM) when compared to min 2 bits (6dB SNRM) of adsl1.


-----


edited to add
Excellent and informative post by snadge - thank you

[kezzaman]

Hi Kitz,

The other day i disabled bitswap and it solved my gaming lag issue, i just can not believe it after all this time   
Now im interested to know does bitswap just have a bad affect on my line or is it everybodys line... (when it comes to gaming).

If you know any gamers on adsl or vdsl get them to disable bitswap and see if it improves for them.

When i did it my connection was alittle jumpy for about an hour and speedtest.net results were all over the place but then it settled down and now... im just so happy 

[Blackeagle]

Hmmm, four years ago Talktalk rolled out an update to their MSAN's which turned off bitswapping on 999 exchanges.  This led to customers experiencing intermittent sync, dropping connection, and a degradation of throughput speeds.  Further updates were rolled out over two evenings to put this right.

If bitswapping is not enabled then, as line conditions change, SNR will drop, BER will increase and eventually the line will drop.

[kezzaman]

 
Whats BER?

If disabling bitswap causes issue wouldnt raising snr profile fix it?

[burakkucat]

Whats BER?

BER is the Bit Error Rate.

[kitz]

hmmmm also from me too.  Disabling bitswap isnt something I'd ever recommend.
 
Its there for a purpose and without it the line in theory should become more unstable - although saying that it wont if the SNR (not SNRM) across ALL subchannels remains  fairly constant, but I would imagine over the course of the day just about every line will experience some fluctuations, bearing in mind we are talking a heck of a lot of subchannels on adsl2+.
Granted at sync time the Bit Allocation Table (BAT) will include a small safety margin for BER, but eventually over time that will deteriorate too.


Even if you do compensate by increased SNRM, the BER is at some point going to cause lag in itself, and eventual deterioration of the line

I really dont understand how disabling bitswap can have improved your line, the router will constantly be loading the datastream into the different subchannels as defined by the BAT anyhow - regardless if Bitswapping is on or not. 
Bitswapping isnt the shuffling around of data, its actually telling the modem where to load the bits and its a calculation that happens so fast it shouldnt have any impact on the data throughput.


-----------
>>get them to disable bitswap and see if it improves for them

No ty   

My own SNRM as reported by my router is rock steady - its practically flatlined for the past  4 yrs and never moves by more than .5 dB
To be perfectly frank, I dont think you will find many lines any more stable than mine* (see graph below). 
My Error rate over the past 13 days 16hrs is
208 CRC
0 ErrSec
0 FEC (obviously as im not interleaved)

Yet if I switch DMT tool on bitswapping is happening ALL the time... and I can see that the real SNR fluctuates on a couple of the subchannels very briefly for more than what my SNRM is. 
Without bitswap my BER would be silly stupid figures... and without a doubt its bitswapping that keeps my line stable at such a low SNRM.  I reckon I'd probably last about an hour (if that) before loosing sync.


*Admittedly I have an problem with physical line degradation of about 1Mbps of sync per annum that BTw wont do anything about yet because the line is still classed as way above average, but its unrelated to this topic of conversation

[GigabitEthernet]

Hi Kitz, just a quick question (sorry for the hijack), what router do you use and how do you graph your SNRM over time?

[kitz]

Hi Kitz, just a quick question (sorry for the hijack), what router do you use and how do you graph your SNRM over time?

Currently using a ST585v6 - which unfortunately is going to have to be retired pretty soon as the network switch doesnt work properly and access is via wireless
Although I have a new router, ive not yet installed it, cause I will be really sad to see my faithful ST go..   one of the things I will lose is the MRTG graphing and monitoring which you see above

[kezzaman]

But Kitz how many bitswaps to u get in a day or an hour or minute?