Great stuff Bob - that really clarifies things

Just one thing about http://www2.farina1.com/ADSL/default.aspx or http://www.coolwebhome.co.uk/calc/calculator.php is that they don't differentiate between 21CN WBC and LLU. It would be the extent of the drop in performance due to the IP Profile on an adsl2+ line that would really be of interest to me in the context of this thread.

Of course it's very difficult to judge differences in throughput speed since the BT speedtester doesn't work on a LLU'ed line which for me, when previously on ADSL Max/IPStream (20CN), gave the most reliable results. However you say that 84% of downstream sync speed on a LLU line would be a good estimate of throughput and I can certainly verify that from my own partial LLU connection using the TBB speed tester.

Guess I need to find somebody with a downstream 37dB attenuation, 6dB SNRM and ~13107K sync on 21CN and run a few TBB speed tests in order to make a comparison...

Edited by 4M2 (Sat 14-Jul-12 12:26:20)

In reply to a post by jelv:

I'm on 20CN. speedtest.net results vary between 6.72 and 6.98 so I'm seeing less than 500 drop from my 7150 profile.

Yes, but that's speedtest.net, I saw ~500K less than the IP Profile with the BT speed tester

Regarding SNRM, which I find almost incomprehensible, could, for example, an upstream sync be improved by an ISP changing it?

When I was on adsl1 with Plusnet I had the upstream sync un-capped to 832K and at that time the SNRM changed to 9dB from 21dB - was it just by the ISP somehow changing the upstream SNRM that the upstream sync went from 448K to 832K?

We normally only bother about downstream SNRM (explained here). But yes, the change from 448 to 832 upstream is why the SNRM dropped. The same reason as high downstream SNRM being expected on 8128/7616 ADSL sync.

On non-LLU you/ISPs have a choice of up to 448kbps or up to 832kbps from BT Wholesale on ADSL, and apparently the ISP can also limit it to 448kbps on ADSL2+. (Plusnet do this by default). The norm on ADSL2+ is uncapped, so does whatever it can up to around 1.3Mbps - the maximum possible on the standard Annex A.

Still on non-LLU, Annex M is now available on ADSL2+, providing up to 2.5Mbps upstream on good enough lines. The rule of thumb I apply is that if the downstream sync is less than 16Mbps, then on a change to Annex M you lose 2.5Mbps downstream and are unlikely to get any increase in upstream. (For simple reasons technical reasons). From 16Mbps to 20Mbps downstream sync you get progressively increasing upstream and decreasing loss of downstream. At an original downstream sync around 20Mbps, or above, you get the original downstream and full 2.5Mbps upstream.

On LLU on O2/Be the upstream is uncapped. The 16Mbps rule applies wrt Annex M. I'm unsure how Sky and TT run things.

Edited by RobertoS (Sat 14-Jul-12 13:43:04)

In reply to a post by RobertoS:

On non-LLU you/ISPs have a choice of up to 448kbps or up to 832kbps from BT Wholesale on ADSL, and apparently the ISP can also limit it to 448kbps on ADSL2+. (Plusnet do this by default).

This is supposed to have changed although there are still a few reports of it being capped

This is really interesting: leaving aside Annex M, the only thing stopping the maximum possible sync, either upstream or downstream is the SNRM. Yet with adsl1 the upstream is generally capped at 448K, due to a high SNRM, even though the line could support 832K.

With LLU it's necessary to contact the ISP to get the SNRM changed (?) although with 21CN the exchange kit can negotiate a downstream SNRM of 3dB if the line proves stable enough to support it and hence a higher sync without the intervention of the end user - seems like 21CN WBC might well be a better option if one has a good, relatively noise free, line despite any possible lower throughput due to the IP Profile.

In reply to a post by RobertoS:

On LLU on O2/Be the upstream is uncapped. The 16Mbps rule applies wrt Annex M. I'm unsure how Sky and TT run things.

Upstream on Sky is capped, either by DLM or manually-applied line profiles.

On a decent line, and catching Tier 2 or DLM on a good day, 1,150 kbps should be achievable. Tier 1 typically cap it at 768 kbps.

In reply to a post by 4M2:

With LLU it's necessary to contact the ISP to get the SNRM changed (?) although with 21CN the exchange kit can negotiate a downstream SNRM of 3dB if the line proves stable enough to support it and hence a higher sync without the intervention of the end user - seems like 21CN WBC might well be a better option if one has a good, relatively noise free, line despite any possible lower throughput due to the IP Profile.

Surprisingly, Sky's DLM is now using a 3 dB noise margin on downstream if it thinks the line is good enough.

Another aspect with Sky's line profiles are that they can alter the power output. Traditionally, one would look at spare noise margin to see how much extra a line has in the tank. As Sky attenuate the power output, lower than optimum synch speeds can be reported with a low margin, so it may look like the line is at it's maximum even when it isn't. It makes troubleshooting that much harder, especially when you consider the Sky router does not report power output.

Edited by Oliver341 (Sat 14-Jul-12 14:44:27)

Oh, don't know anything about power output except that maybe it's normally at max with adsl2+???

My TG585v7 gives these numbers : Output Power (Up/Down) [dBm]: 12,0 / 19,5 with adsl2+ 21/37dB attenuations, currently 10 / 5.5dB SNRM's and 1.022 / 13.107dB sync's. Would that be considered max power?

In reply to a post by 4M2:

Oh, don't know anything about power output except that maybe it's normally at max with adsl2+???

I suppose that depends on the ISP.

My TG585v7 gives these numbers : Output Power (Up/Down) [dBm]: 12,0 / 19,5 with adsl2+ 21/37dB attenuations, currently 10 / 5.5dB SNRM's and 1.022 / 13.107dB sync's. Would that be considered max power?

Looks a pretty normal output power, yeah.