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Author Topic: Attainable rate  (Read 1135 times)

Weaver

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Attainable rate
« on: May 28, 2018, 12:42:58 PM »

Kitz has already talked about Ďattainable rateí in the page on line stats. However I don't understand the point about DSLAM restrictions etc. Could anyone help me understand?
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ejs

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Re: Attainable rate
« Reply #1 on: May 28, 2018, 07:01:45 PM »

The max attainable rate ignores any rate caps, such a banded profile applied by the DLM, or the FTTC speed tier (which, at this level, would be exactly the same type of cap).

The max attainable rate will take into account the target SNRM.
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re0

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Re: Attainable rate
« Reply #2 on: May 29, 2018, 02:11:54 AM »

It may be worth noting that during periods where the background noise increases to a point where the noise margin for the communications (DSL) drops below the target margin then it is not unusual to see the modem showing a max attainable below that of the current sync. The max attainable usually gives a good indication of the best times to resync for the highest speeds at your current target margin.

Sorry if I am just repeating what you already have in knowledge.
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aesmith

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Re: Attainable rate
« Reply #3 on: May 30, 2018, 05:01:03 PM »

I think attainable rate also assumes some standard distribution of frequencies.  Or maybe idealised for combination of attenuation and noise margin.   So for example my router at the moment is connected at 4000K, target noise margin is 6dB, actual is 5.4dB.   Attainable rate reported as 4544K.
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ejs

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Re: Attainable rate
« Reply #4 on: May 30, 2018, 07:29:13 PM »

I'm pretty sure the modem will use the measured SNR per tone to calculate the attainable rate.

With FEC+interleaving, the attainable rate calculation does not need to meet the min INP constraint, but of course the actual rate will have to, so then the attainable can end up higher than the actual. You can gain more bandwidth from the FEC/interleaving coding than the bandwidth used to carry the FEC data.
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Weaver

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Re: Attainable rate
« Reply #5 on: January 19, 2019, 07:17:00 AM »

Thank you all. Even though a lot of uncertainty remains [?] these comments have been very helpful.

@re0 - that helps me understand. Iíve seen this iirc, a current rate going above the attainable, and your opinion sounds just right, helping me understand such a confusing and surprising observation.

I realise that I should have asked Ďwhen does the attainable rate figure change?í in your experience. I ought to check mine, I just assumed that what I see is set at sync time, dsl connection establishment time, and does not change after that until the next resync. I donít know why I assumed that and I donít know whether or not itís true. Is that right? Or does it change dynamically during the lifetime if the connection?

@aesmith - could be, thanks for that. Those figures are similar to mine relatively. For example I might see 3100 kbps sync rate downstream and an attainable of 3400 kbps, a figure which despite the name is completely unattainable.

Thatís why they call it Ďattainableí, because itís completely impossible. (Unless perhaps thatís at 0dB SNRM, but how long would that last, so whatís the point?)

@ejs - very helpful, the insight about min INP requirement being part of the key to understanding the definition of Ďattainableí rate. And thereís also the part about artificial limits, Ďcapsí, being ignited too. I donít think that would help me understand in my own particular case of course. Not that that challenges this opinion about the contributors to the definition of attainable.

In my case my real sync rates are not some magic round number, some low integer x times some integer multiple of a power of ten say.
« Last Edit: January 19, 2019, 07:34:33 AM by Weaver »
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aesmith

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Re: Attainable rate
« Reply #6 on: January 19, 2019, 08:42:50 AM »

I realise that I should have asked Ďwhen does the attainable rate figure change?í in your experience.
Attainable rate as reported by any of my routers varies from moment to moment.
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ejs

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Re: Attainable rate
« Reply #7 on: January 19, 2019, 09:41:30 AM »

Why do people seem to come to the conclusion that the attainable rate is not attainable? If the attainable rate is substantially higher than what you actually achieve, and you have FEC+interleaving+INP, then I think the idea is that the attainable rate is what you could get if the amount of FEC+interleaving were set for maximum bandwidth, while for the actual rate, the FEC+interleaving has to be set to provide the INP level specified. People don't seem to get the idea that it's possible for FEC+interleaving to give you a higher net date rate than without, because the way it's configured on FTTC (usually INP 3 delay 8ms) tends to result in a lower net date rate than without.

There's an improved attainable rate calculation method defined in the various ITU-T documents, which takes into account more parameters, but support for and using the improved calculation method is of course optional.
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aesmith

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Re: Attainable rate
« Reply #8 on: January 20, 2019, 09:14:49 AM »

Here's an example of "something" being applied to our line and it's affect on attainable rate.  Change from a 9dB target to 6dB target with I guess some sort of banding applied and certainly some interleave changes.   Steady line is the sync speed, wiggly one is Attainable Rate.   
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Weaver

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Re: Attainable rate
« Reply #9 on: January 20, 2019, 09:52:05 AM »

@ejs - Ah, itís a well-known defined term, defined in the specs. I didnít know that - thatís the key to it, so Iíll look that up.

Many thanks  ;D

@aesmith - thatís interesting, thanks!
« Last Edit: January 20, 2019, 09:56:38 AM by Weaver »
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