SNR margin after MAX upgrade

[#1]

I recently moved from 2M fixed to MAX and tracked the performance of the link during BT’s magic ten days and beyond. 
DSL synch speed has varied only slightly: 8128 before conversion and 7616 (now). The router’s ‘attainable rate’ shows 8384(Dn) / 1068 (Up).
The IP profile from both BT and PN started at 7150 and dropped to 6500 on Day eight.
The error rate shown by the router is always negligible (new line, ADSLNation faceplate, spitting distance from exchange etc) and the line appears good with attenuation of 2.5(Dn) and 3.0(Up).
Output power shown by the router has been consistently 11.9(Dn);  upstream went from 7.3 before conversion to 9.7 afterwards.
 
The SNR margin moved from:
30.3(Dn) / 26.0(Up) when on 2M fixed
to
7.4-8.3(Dn) and 8.0(Up) around day seven
But…since then it has crept back up to 12.1-13.7 (Dn) and 8.0-10.0 (Up).
Throughput shown by the BT tester and PN Beta tester has been up to 6000kb/s but has now tailed off to between 4800 (at 08:30) and 2000 (evening).
Finally, I have been switching the router off overnight to give BT’s rate-adaptive gubbins a chance to do its thing (rather than just power-cycling it during the day lest the exchange thinks it is a blip). 
Anyone ideas why:
a. the SNR margin would creep back up again (and hence presumably force my rate down)?
b. given that exchange congestion is unlikely to change much over the conversion period and the following week, why throughput would fall so much given that my IP profile is vastly higher than even my best throughput?   

[#2]

a) Sounds like your SNR target may have been adjusted upwards by BT to make the line more stable.
b) Difficult to say on the throughput , there are so many things can affect it. Have you actually checked your exhange on the capacity checker ?

[#3]

The sync rate whilst on 2M fixed should be 2272kbps so the figure of 8128kbps must have been immediately *after* conversion.  8128 is the maximum for "up to 8Mbps" and this is the only sync rate that results in an IP Profile of 7150.  The first few sync speed ranges with corresponding profiles are:
[tt]Sync speed    IP Profile
8128 only      7150
7968 - 8096    7000
7392 - 7936    6500
6816 - 7360    6000[/tt]
So for sync at 7616 you have the correct profile - 6500.
Variation of SNR margin through the day with reductions in the evening when electrical equipment, heating, street lights, etc, etc, come on is quite normal.  The problem is not *high* margins it is *low* ones.  Normally  the target SNR margin is set at 6db and the equipment at the exchange finds the *highest* sync speed that will respect that margin.
The quality of your line seems excellent, so why should your sync rate change from 8128 to 7616?  My guess is that interleaving was turned on when that happened.  This is used to improve the stability of noisy lines.  It is standard for new services to have this on.  Some routers seem to respond to interleaving by syncing at a maximum of 7616, perhaps yours is one of them.  Interleaving can be turned off (at the risk of increased disconnections), but I would suggest leaving things to settle down first.  You mentioned "10 days and beyond" - how long is "beyond"?
As far as actual throughput vs IP Profile rate - you will never achieve the latter, but if you are using *Windows XP* have a look at the RWIN/MTU help pages on the portal (Help & Support -> Broadband -> Troubleshooting -> Guide to RWIN and MTU).  If you have Vista increasing RWIN will not help and other issues are the most likely reason.

[#4]

Thanks.
It's less than a week after the training period so things may yet improve.
It just seemed odd that with an apparently good and error-free line, the RA-ADSL logic in the exchange had bumped up my SNR margin.
I have already invoked the good Dr Tweak, and his comments were that 'RWIN is within range'. MTU (etc) 'looking good', 'good data stream' and with 'looking good' stable pings.

[#5]

I'm not convinced the DLM *has* bumped up your SNR margin, it seems to me that you simply have an unprecedentedly excellent line - is it underground and newly installed with well made connections in the exchange? - and with an ADSLNation faceplate you've done your best in your own premises.
If your SNR margin falls significantly below 9db (say to around 8db) at night you could try restarting your router.  Since target margins go up in 3db steps that would cause re-sync at a lower speed to achieve 9db (or 12db) margin; my guess is that you would sync again at the same speed with a similar (8db) margin.  That should demonstrate what your target SNR is.  However, be warned - if it is 9 or 12db you'll get a lower sync speed and possibly lower profile.
Re RWIN/MTU: are you using Windows XP?

[#6]

Thanks - food for thought. I may well rise at some ungodly hour for a few nights, check my synch rate, and reboot my router if my downstream margin is under 8dB  - which might mean several nights lost sleep...
Yes - I am using Win XP SP2 (and avoid Vista Autotune)

[#7]

My minimum SNR margin generally occurs between 11PM-1AM (while it's dark).  At the moment my stats show:
[tt]              DS        US
sync rate    7520      448 kbps
SNR margin    10.3      23.0 db
Attenuation  32.0      16.5 db
IP Profile is 6500 kbps[/tt]
I've just got the following on mybroadbandspeed.

I connect to my router using Ethernet; is yours wired or wireless?

[#8]

Wired.
Looking at your figures, I guess what I find galling is that our synch rates and IP profiles are almost identical and my line attenuation is far better than yours (the exchange is only three houses away), but your SNR margin is lower and throughput better.
I'm more than happy to have a higher SNR margin provided it doesn't adversely affect my synch rate or IP profile, but both of these seem to be far higher than the throughput I get, even with tests at 02:00 when presumably the DSLAMs are sitting twiddling their thumbs.

[#9]

Noise margin as such has no direct effect on throughput, unless it becomes too small for the router to maintain sync.  My stats are currently

Sync rate (kbps)      7520    448 
Noise margin (dB)      7.9    23.0 
Attenuation (dB)      32.0    16.5

I have RWIN=128480 and MTU=1500.  Pings from DSLreports are about 115ms.
I've done another mybroadbandspeed test:

essentially the same results as earlier.  A more robust test at Thinkbroadband gave 5909/367kbps (DS/US).
Are you familiar with the "more detailed" results that the DSLreports tweaktest gives?  If you'd like to post the results URL from a test I'll have a look at this information for you.

[#10]

decomplexity,
I only just noticed your attenuation figures in your first post.
With that low level ( 2.5 ) of D/s attenuation you should be getting a rock solid 8128 synch. For example below are the figures from my office line. As you can see it's got 19db D/S attenuation and it's been rock solid at 8128 for almost a year now.
ADSL Link Downstream Upstream
Connection Speed 8128 kbps 832 kbps
Line Attenuation 19 db 8.5 db
Noise Margin 10 db 15 db
My home line has 31.8db D/S attenuation and it synch's at 8128/8db albeit with a 2700HGV router.
Something is not quite right. I agree with spraxt it could possibly be interleaving has been turned on, but why ? you say the error rate is negligible. Your router should show whether it is on or not. Or maybe someone from the Comms Team might have a quick check ?
What router are you using ?

[#11]

decomplexity,
I've just been having a look at the reports for your line, on the 12th August the sync rate dropped to 7392, which meant your IP profile went down to 6500. 2 days later you synced at the same speed again but this time interleaving was applied to your line (14th). If you want interleaving turned off just let us know and we can get an order placed for you.

[#12]

Thanks Spraxyt. TweakTester url for a test done at 11:30 a.m. this morning (17th) is:
http://www.dslreports.com/tweakr/block:a313d?service=dsl&speed=6500&os=winXP&via=pppoA
Router is a (wired) USR 9105 with current firmware (V2.5) with MTU=1500 (the default).
Router stats are currently:
WAN
Received                             Transmitted
Bytes     Pkts   Errs  Drops     Bytes     Pkts     Errs  Drops
45934315 37599 0     0     4119931     38626 0       0

ADSL
Super Frames         1803927     1803925
Super Frame Errors:         0           0
RS Words:             122667091      15333362
RS Correctable Errors: 13           6
RS Uncorrectable Errors: 0       N/A
HEC Errors:                   0         0
OCD Errors:                   0            0
LCD Errors:                   0           0
ES Errors:                      0           0

ATM general
In octets: 46873968
Out octets:  6074304
Errors (of any type): none
ATM AAL5
Unicast packets in: 38583
Unicast packets out: 39583
Errors and discards: none
ATM AAL5 VCC
CRC errors 0
SAR timeouts 0
Oversized SDUs 0
Short packet errors 0
Which all look pretty good to me - which (unless the router is telling porkies about the error rate) keeps telling me RA-ADSL should be trying for a higher synch rate (given the substantial SNR margin which this morning is 12.9dB (Dn) / 10.0 (Up)

[#13]

For comparison I did a tweak test which is here.  In the "more details" section the plots seem to show bigger transients in your results, but I think that is a false impression - the variations are smaller in your case so the scales of the graphs show more details of the variations.
What I would suggest is trying RWIN=131400 (as I mentioned previously mine is 128480), I think you would benefit from that.
Are you on a business package with upstream sync rate 828kbps?  The reason I ask is your upstream SNR margin at 10.0 is lower than the downstream value, which I've seen before but is less usual.  I assume higher upstream speeds are associated with lower upstream margins as is the case with the downstream equivalents - that shouldn't be an issue (unless it does indicate a problem).

[#14]

You might be on to something there spraxyt. One tends to ignore upstream values cos they are usually insignificant. That upstream SNR is strangely low. Take a look at my earlier posting, my office connection is a business one with 832k upstream and still has 15db SNR upstream.

[#15]

Yes - we are on a business package (Teleworker Pro MAX) which is something of an anomaly since even Teleworker Pro fixed 2M  is not currently offered to customers and the MAX version did not officially exist when we took it out.
Given our negligible level of error and retransmits, I will try upping RWIN as you suggest (I see we use the same MTU).
I would guess (correct me if I am wrong) that this would be expected to improve throughput but could not affect synch rate or SNR margin since they are both linked (via the exchange's local loop checker) to the monitored line error rate which is independent of RWIN.