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[snadge]

can anyone tell me if a lower sync rate but with the same SNR Margin equals a more stable & error free line?
e.g.

forced profiles:

16,000k @ 7db
vs
20,000k @ 7db

and if so....how?

just it seems that SKY's DLM has a nack of setting users line speeds a tad lower than they are actually capable - in some cases by as much as 20% (like in example above) - ive been told its for stability but I cant imagine that reducing a line speed by about 10%-15% even 20% with same SNR Margin can make it more 'stable' or 'error free' in that it has less chance of causing problems with On-Demand-Content (which ive been told is the reason SKY's DLM does this)


thanks

[roseway]

In general, a higher noise margin will equate with higher stability.  If the connection speed is capped below the level at which it would otherwise connect, then the noise margin will end up higher than the target. Alternatively, if the target noise margin is raised, then the connection speed will be reduced. It's not possible for the ISP to specify the connection speed and the noise margin simultaneously, because they are interdependent.

[snadge]

In general, a higher noise margin will equate with higher stability.  If the connection speed is capped below the level at which it would otherwise connect, then the noise margin will end up higher than the target. Alternatively, if the target noise margin is raised, then the connection speed will be reduced. It's not possible for the ISP to specify the connection speed and the noise margin simultaneously, because they are interdependent.

thats what I thought eric, but it happens on SKY LLu connections where the SNR Margin @ 7db is same on the lower sync rate (that DLM ends up setting your line on) as it is when you call up and get it uncapped...  ive shown the example above where it was 16,000k then 20,000k on my friends SKY connection and both had 7db SNR Margin, then on mine DLM settled on 15,400k @ 7db (25db line) - I called up to get it unlocked (although all he did was move up to next 18Mb profile) and now my line syncs at 18,013k @ 7db - ive seen lots of other SKY connections do exactly the same, the DLM does start off with a high SNR and low speed and as it goes up in speed the SNR comes down (as it should), and when it gets down to 7db it stops... but the sync rate is substantially lower than the line is capable of on all the new connections I see on SKY LLu on their help & support forums and on other forums..

when I was with o2 I sync'd at 17,000k (26db line) with 6.5db SNR - yet SKY sync'd as so: 15,400k / 25db / 7db - so I called up to ask if it can be uncapped so I can play with SNR margin but he put it on 18Mb profile (up from 16Mb profile) instead of uncapping it (or putting on 20Mb profile) so I can not go above 18,000k when i reduce SNR Margin - anyway the new sync was 18,013k / 25db / 7db and like I say Ive seen COUNTLESS new SKY connections do the same thing, my friends 20db line ended up as 16,000k / 20db / 7db - he called up and got 19,999k / 20db / 7db - theres loads on SKY's forums with same issue that DLM syncs their lines lower than they had previously with other ISP on same ADSL2+ connection (o2 / BE* in particular) and in such cases the SNR is the same default SNR on each rate (the DLM's "set" rate and the 'uncapped' higher rate)

so... I was under the impression if the SNR Margin is same 7db on each sync-rate then does this mean the line is no more stable? - because Iam being told that SKY deliberately throttle back on sync rates for stability ... but, the SNR Margins are same 7db in the lower sync rate set by DLM and the normal achievable sync rate when you get it uncapped.

I know what your saying with the relationship with the two but believe me this is what happens on SKY LLu connections , there are countless posts on SKY and other forums where new sky users are saying "not getting anywhere near what I had with other ISP" and when they show their stats the sync rate is 10-15% slower and SNR is 7db - those that call up get it uncapped and sync goes up but SNR stays same default.

the only thing I can think of is a BUG in the routers

so same SNR margin does not mean more protection....which I knew

[snadge]

so does anybody know if this is the case?

I want to know if a 16,000k / 7db / 20db line (capped at 16Mb profile) is more stable than a 19,999k / 7db / 20db (un-capped) connection on same line.
my line was 15,400k / 7db / 25db (capped at 16Mb profile) and then un-capped at 18,000k / 7db / 25db

seen loads more SKY connections do this.

as you can see, the Noise Margins remained the same at either sync rate, ive been told SKY do this for stability ... but if Noise Margin is the same it would not offer any extra stability would it?

[Black Sheep]

Just guesswork, but are they doing it to lower your bandwidth as a 'Traffic shaping' excercise All ISP's only have so much backhaul services they pay rental for, if this is near saturation I can well imagine TS being carried out.

Just a guess with no evidence to back this up, cos I'm fed up waiting for my missus to get home from Uni. 

[snadge]

Just guesswork, but are they doing it to lower your bandwidth as a 'Traffic shaping' excercise All ISP's only have so much backhaul services they pay rental for, if this is near saturation I can well imagine TS being carried out.

Just a guess with no evidence to back this up, cos I'm fed up waiting for my missus to get home from Uni. 

hi BS,

nope, no traffic shaping - this is on SKY LLu which is totally unlimited with no Traffic Shaping of any kind.

this is what happens:- (bare with me lol)

SKY DLM trains your line for upto 10 days... 
sync rate starts low and Noise Margin is high, all's running correctly.
over space of a few days it notches up the speed and Noise Margin decreases, all normal there,
then eventually it gets to that "happy medium" where it syncs at what DLM says is best for your line,

SKY's DLM uses 'profiles' such as 10,12,14,16,18,20 Mbps profiles, the DLM sets the profile for your line based on the sync rate it has settled on, so as in my situation it was 15,400k (7db NM on a 25db line) which was a 16Mb profile, this means it will not go above 16,000k even if it could do so because of the profile cap.
for everyone ive seen migrate to SKY their connection sync is way lower (10-20%) than what they should have (and did have with previous ISP on same ADSL2+ connections) , ive seen loads on SKY's forums and a few of my friends and a neighbour.
when the speed is set the Noise Margin is SKY's default of 7db
knowing that SKY use "profiles" a lot of users ring up to ask if they can have it "uncapped" so they can sync higher by tweaking SNR (as the profile cap prevents this) but in most cases it goes up quite a bit anyway as ive already explained SKY's DLM seems to "aim low" and the uncapping lets it go up anyway, one can push it further by lowering SNR if one has a compatible router.

when you call up to ask if your line can be "un-capped" as to sync more out of it; the Noise Margin is still 7db but sync-rates go up considerably in some cases, like my friends, who was 16,000k (locked to a 16Mb profile) with 7db NM - he rang up and got it uncapped and now gets 19,999k on same 7db NM
my sync was 15,400k on 7db NM - I rang up to ask for it too be un-capped and is now 18,000k on same 7db (actually mine was screwed up and instead of being un-capped he locked it on an 18Mb profile instead of un-capping it, but Iam happy where it is so just leaving it and not bothering to ring back)

Like I say ive seen loads of other SKY connections do the same, people get a slightly lower sync than expected, for some reason DLM aims low with sync rate speeds and ive been told this is for "stability" - but this is where the whole Noise Margin issue sets in, cos the "stability" thing would be true if the Noise Margin was raised higher than the default of 7db to reflect the lower speeds...but it is not...so does this mean there is no extra protection at either the capped or un-capped speed because the Noise Margin is still 7db on each one ... if so then it would mean that DLM does not aim low for extra stability...

I personally think the DLM isnt very good at getting the correct line rate for the line, and thats why its low

when I called SKY and asked if I could have it uncapped cos I wanted to play with the SNR MARGIN etc - even he said "you should be able to get 19Mb from that line" yet SKY's DLM sync'd it at 15,400k ...poor  but this isnt the argument here, what Iam trying to find out is if the line is more stable and error free even if it has the same Noise Margin on the capped rate as it does on the un-capped rate.

he-he-he

I know LOGIC says that the Noise Margin should not and could not stay the same 7db on such a difference of sync rates (as Eric explained) but SKY's DLM manages to do so...or reports the Noise Margin as 7db when really it is not (I have seen same result in a Netgear, D-link and Sagemcom router, so cant be the routers at fault)