Kitz ADSL Broadband Information
adsl spacer  
Support this site
Home Broadband ISPs Tech Routers Wiki Forum
 
     
   Compare ISP   Rate your ISP
   Glossary   Glossary
 
Please login or register.

Login with username, password and session length
Advanced search  

News:

Pages: 1 [2]

Author Topic: Question for the clever ones ....  (Read 4027 times)

WWWombat

  • Kitizen
  • ****
  • Posts: 1674
Re: Question for the clever ones ....
« Reply #15 on: April 04, 2017, 12:42:16 PM »

The BCM chipset modems dont appear to do a very good job at projecting the attainable though if the line already has INP applied.   :(
I think I know why this is - its possibly something to do with broadcoms interpretation of the 'SNR gap'.  I believe they may be using the lines current SNR gap rather than recalculating one without INP.   

I agree that the Broadcom's seem to be getting something wrong in the area of SNRGAP.

One of the links in @J0hn's post shows SNRGAP to depend on a 10-7 bit error rate (BER) with no coding gain.

As @ejs mentioned recently, turning on FEC has two side effects: one (undesirable) impact is that it adds overhead, but the second (desired) impact is that it decreases the BER. Turning on FEC is a coding gain.

Once FEC is turned on, then adding interleaving can improve things - making a further coding gain.

So, when analysing how many bits can be carried on a tone, the calculation considers how many dB of SNR are needed per bit to achieve  that 10-7 BER. If FEC is turned on, then this ratio is affected: more bits can be carried per dB to achieve the same BER. Even more so if interleaving is added to FEC.

The miscalculation of maximum attainable seems to be something to do with this ratio when coding gain has been added.

(So, @Weaver, I'm not sure if your question was really related to how interleaving affects "attainable" rather than "overheads". If so, then this is the reason).
Logged

WWWombat

  • Kitizen
  • ****
  • Posts: 1674
Re: Question for the clever ones ....
« Reply #16 on: April 04, 2017, 02:45:30 PM »

@Wwwombat - You and I are on the same page, it seems. "Too lazy to mention the FEC side of things" is exactly how I assumed it must be. The irony is that more interleaving is the sign of a happier connection in the face of a line that is subject to noise spikes, so more interleave depth means more reliability and better throughput performance (though of course increased latency) because of fewer wasteful retransmissions. That's the way I see it anyway.

so more interleave depth means more reliability and better throughput performance (though of course increased latency) because of fewer wasteful retransmissions.

Exactly.

In old-style DLM interventions (heavier FEC/interleaving), then that retransmission was, of course, at TCP level. Layer 4.

In new-style G.INP interventions (very light FEC/interleaving), then FEC and interleaving combine to play the same role - to reduce the retransmissions. Except these ones are at the VDSL2 level - Layer 1.
Logged

kitz

  • Administrator
  • Senior Kitizen
  • *
  • Posts: 33881
  • Trinity: Most guys do.
    • http://www.kitz.co.uk
Re: Question for the clever ones ....
« Reply #17 on: April 04, 2017, 03:18:33 PM »

Quote
One of the links in @J0hn's post shows SNRGAP to depend on a 10-7 bit error rate (BER) with no coding gain.

So, when analysing how many bits can be carried on a tone, the calculation considers how many dB of SNR are needed per bit to achieve  that 10-7 BER. If FEC is turned on, then this ratio is affected: more bits can be carried per dB to achieve the same BER.

That's what I was trying to explain in my other post but obviously not doing a very good job of explaining the whole situation.    I was going to try and get some further evidence but didn't have time as my guest arrived right after I made that post, then with the funeral and having to sort out lots of other things totally forgot about it.

My post was here.

What I meant is that Im guessing that when the BCM devices calculate the SNRgap they are using the current SNRgap which includes the current INP figure rather than calculating a separate SNRGAP with INP=0. 
I thought I'd found a doc somewhere that appeared to state in amendment 6 clarification that INP=0 should be used.  The doc I found that day Im sure specifically said something like 'it had been observed that different vendors had been using different methods'.

Logged
Please do not PM me with queries for broadband help as I may not be able to respond.
-----
How to get your router line stats :: ADSL Exchange Checker

ejs

  • Kitizen
  • ****
  • Posts: 2078
Re: Question for the clever ones ....
« Reply #18 on: April 04, 2017, 04:37:12 PM »

I do not believe the difference between max attainable net data rate, and actual net data rate, with interleaving+FEC, should be considered a miscalculation, or that the modem is not properly accounting for something. It's the modem answering a hypothetical question of "If the interleaving+FEC parameters could be adjusted for maximum bandwidth, what would the bandwidth be?"

The difference is due to the basic ATTNDR method, which leaves various things unspecified.
ATTNDR then gets calculated based on adjusting the FEC ratio for the maximum coding gain with the minimum overhead, it has to stay within the maximum interleaving delay, but does not have to meet the minimum INP requirement.
The actual net data rate will have to have the FEC and interleaving parameters set to provide at least the minimum INP level specified in the line profile.

so more interleave depth means more reliability and better throughput performance (though of course increased latency)
It's not really the interleaving depth that is the important parameter, it's the delay, and the INP. Think of tickmike's recent post, with an interleaving depth of 416, much larger than normal for ADSL2, but with a fairly typical delay of 7.47ms (probably the max delay specified in the line profile was 8ms).

I suspect that the FEC and very small interleaving levels typically used with retransmission (G.INP) are for optimizing the bandwidth from the coding gain, and not really for providing additional noise protection.
Logged

Black Sheep

  • Helpful
  • Addicted Kitizen
  • *
  • Posts: 5717
Re: Question for the clever ones ....
« Reply #19 on: April 04, 2017, 07:41:53 PM »

Thanks one and all for the input thus far, with an extra  :congrats: to the Wombat for that in-depth break down. I won't pretend it's all sunk in ..... but most of it has !!!  :) :)
Logged
Pages: 1 [2]
 

anything