ADSL+2 sync speeds now lower

Hi All,

I have just been upgraded to ADSL+2 and there has been a noticable difference in my upward sync speed (double) 956 Kbps from 450 . However the downward sync speed has dropped since changing by .5Mb now 5503Kbps from 6016Kbps with no change in SNR.
Tech support at Entanet tell me this is because of the training period and it will change in the course of a few days.
Has anyone else had similar issues to this drop, as this does not make any sense to me I would have expected a significant rise in either sync or a change in SNR before any stabilisation period.
Further I no longer appear to be able to access btspeedtest to check my BRA's profile.

Regards

GavinO

ADSL2 can sometimes result in lower syncs compared to ADSL on very long lines. I wouldn't expect the effect to kick in at the kind of syncs you are seeing.

How are you getting ADSL2? Very few lines have access to it through BT. If you're using an LLU operating like O2/Sky/BeThere etc. then the BT speedtester will not work and in most cases you won't have a BRAS.

He has been moved to Entanet WBC. Please be gentle with the poor unfortunate!

Ah, I wondered if that might be the case. It's not a good start for WBC if the sync drops :-/

ADSL2+ generally adds about 3dB in attenuation. This is because it uses higher frequencies as well as the ADSL ones, and these higher ones attenuate more.

A rise in attenuation has the same result as a rise in target SNRM and you are in just the wrong range of sync, the same as me.

ADSL2+ adds to the speed available because it can put stuff in the higher-frequency bins. But an increase in attenuation at our sort of speeds can reduce the sync by about the same amount. On top of that there is the question of how your router measures attenuation.

Many routers measure the attenuation averaged across all the possible bins, thus raising it as explained. But you could very well find that those higher bins are actually too unstable for you to use so your router discards them, after using them for the attenuation measurement . I can clearly see this if I examine the bin loadings with DMT. The result is you don't get the extra capacity from the higher bins but lose sync speed due to the higher attenuation.

You could very well find, as I have done by following the example of others, that if you force your router to ADSL2 rather than ADSL2+ that your speed increases to more than you had on ADSL. ADSL2 doesn't use those higher bins so the router doesn't calculate the attenuation higher, but you still get the speed improvement of ADSL2.

BUT!!! Leave well alone till after whatever training period Entanet and your ISP have said you will have, otherwise you could totally screw things up.

Edited by RobertoS (Wed 05-Nov-08 13:27:15)

WBC definately has issues*. I wouldn't even consider it without a good proactive ISP to help you through it (AAISP have been great sorting out the issues and hitting BT with a cluestick).

BT keep repeatedly forcing me onto a 25db SNR and I have to ask my ISP to reset it to 3db (which gives a nice and stable 14mbps).

Apparently their algorithm for deciding what your SNR should be is fubar - it only knows 0db (what I started on) and insane levels like 25.. it doesn't ramp up slowly. Of course I'm now on the 2mb BRAS and have to wait for that to reset.

I've been through a number of modems to get a good line:
Cisco 877 - sync OK but can only hold sync for ~5 minutes.
Billion 7402R2 - can't even sync to the exchange in adsl2 mode. Appears to be incompatible with WBC.
Billion 7200 - Same as Cisco, but syncs faster.
DG834G v4 - Works perfectly (ironic as I used to have one of these years ago but threw it away).

My own recommendation would be definately get a modem that supports SRA out of the box (only the DG834 does of those above and IIRC you have to enable it with DMT Tool). ADSL2+ is very susceptible to noise bursts from nearby electronics due to the frequencies it uses, and SRA capable modems will work around this without requiring a resync. eg. the DG834 will sometimes drop its speed dramatically, then recover, but it never needs to resync... so my wife doesn't need to shout at me for being disconnected during a raid

Note that you don't *have* to use ADSL2 with WBC. You can request that your line be put back onto ADSL1 or simply switch your modem to only accept that. I'm told a number of people have done that after gaining no benefit from ADSL2.

* WBMC is sometimes referred to as 'Who Broke My Connection' by those that are on it.

Edited by TonyHoyle (Wed 05-Nov-08 15:39:15)

The rise in attenuation that some (not all) modems give on ADSL2 is just a rise in the measured figure by the modem - it has no bearing on which speed you actually get.

As attenuation increases SNR will tend to drop (because the weaker signal is more affected by noise), but it's not a hard and fast rule because it's dependent on the quality of the wiring between you and the exchange.

Speed is determined by the availability of usable bins. ADSL2+ will nearly always have more available than normal ADSL2/1, no matter what the SNR figure actually is.

You can say that an average line with an attenuation of X will get speed Y, and that's true for the majority, but it's not a hard and fast rule.. different modems measure attenuation differently.

eg. according to the charts I could never have got 8mb on ADSL1 because my attenuation is too high.. but I had 8mb for years with plenty of SNR to spare, and get decent ADSL2+ speed now - because the connections are relatively noise free.

force adsl2 (without the +) and see if it improves.

Unless they using annexM or increased the snrm I would expect a adsl2 synch to be at least on par with adsl1, if its still lower and the noise margin isnt too high then you may need a different router.

Edited by Chrysalis (Wed 05-Nov-08 16:13:23)

Well it's not an issue for me yet. I'm with BeThere and see no reason to move just yet (occasional DNS failures not withstanding)

Thanks to All for your valuable imput.
I will give Entanet and ADSL24 the benifit of the doubt for a couple of days, before contacting them again.
I have a Belkin router F5D 7632 for my sins which I am unable to check the attenuation on as it is never listed in the appropriate place for some reason. However my previous router's attenuationwas always 37dB so it should be the same.
My SNR never drops below 12dB for some reason.
One other change that I have noticed from my stats is that since the change to ADSL+2 I am not registering any transmission errors, packets, CRC's, Near or Far which also seems a bit odd.

Regards

GavinO