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Standard User pdes
(newbie) Wed 18-Feb-15 14:05:19
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Should sync speed vary with SNR changes


[link to this post]
 
I am with Sky Unlimited.

I have read in other forums that the sync speed varies dynamically with SNR. However, my sync speed is fixed from the point at which the connection is re-set until I reset it again. During this time the SNR can vary greatly. Could someone explain please as I have obviously misunderstood something.

Thanks
Administrator MrSaffron
(staff) Wed 18-Feb-15 14:43:15
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Re: Should sync speed vary with SNR changes


[re: pdes] [link to this post]
 
Sync speed will only vary if the connection drops or you force a reboot of the modem.

The SNR margin will vary to reflect the current situation.

The 30 seconds or so for the sync speed to change is usually easily spotted as you lose internet connection.

The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User TheEulerID
(member) Wed 18-Feb-15 15:13:09
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Re: Should sync speed vary with SNR changes


[re: pdes] [link to this post]
 
What you are seeing is not the actual SNR, but the SNR margin. (Unfortunately some modems report it- wrongly - as the SNR). Essentially what happens is when your modem synchronises, it establishes what the ratio is between the maximum signal strength it can receive and the background noise on the line. The higher the ratio, the faster it can send data. However, if the modem synchronised at that theoretical maximum rate, it would constantly drop out of sync as noise levels vary due to all sorts of things - other modems starting up, sources of electrical interference etc. In order to avoid this, a certain amount of "spare" SNR is kept in hand to absorb these fluctuations. That is called the "SNR margin", or maybe the "noise margin". As the noise level on your line varies over time, then that spare margin fluctuates, and that is what you are seeing.

Target SNR margins are set for each line. The lower the margin, then the faster data can go, but the more likely it can drop out. Conversely, the higher the margin, the slower the data rate, but it's less likely to drop out. These target SNR margins are usually defined by the way the ISP cares to set policies. Sometimes they are manually set, but more often they are adjusted automatically according to how well behaved your line is. Some modems allow users to "hack" these margins (at least on the downstream side), but that's not generally approved of by ISPs as it can confuse management systems.

Historically, 6dB has been considered a normal target SNR margin, but on good, stable lines 3dB might be set. However, on a poor line you might see as much as 12dB margins. That is generally not a good sign as, not only does it lose speed, but it probably points to a line subject to lots of interference from sources of noise (often sub-optimal extension wiring is at fault).

So, the sync speed will remain constant until a re-sync. You might expect to see SNR margin vary; maybe by the off couple of dB. In general, that has not impact on throughput.

If you have very high SNR margins (of the order of 12dB), then you have issues with variance in noise.


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Standard User XRaySpeX
(eat-sleep-adslguide) Wed 18-Feb-15 21:28:37
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Re: Should sync speed vary with SNR changes


[re: pdes] [link to this post]
 
Sync speed varies dynamically with Sync-time Target SNR Margin (not wrongly labelled 'SNR'). Run-time SNRM varies dynamically with noise whilst Sync speed remains unchanged.

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Standard User ukhardy07
(knowledge is power) Wed 18-Feb-15 21:59:22
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Re: Should sync speed vary with SNR changes


[re: pdes] [link to this post]
 
THIS IS MOST ISPs

Lets use BT as an example.
The ISP runs line training and determines that 3db, 6db, 9db, 12db or 15db is a stable TARGET noise margin.

Lets say I get a 6db noise margin target (one of the most common).

This means that when you turn the BT Hub on, it will sync at whatever speed a 6db noise margin gives.

IE I turn the home hub on at 10am. I sync at 10,000kbps with a 6db noise margin

I turn the hub on at 7pm I may now sync at 9500kbps with a 6db noise margin

I turn the hub on at midnight I sync at 9300kbps with a 6 db noise margin.

Notice in the example above, at the time of turning the router on, I get an arbitrary sync speed but the noise margin is always fixed, this is the target.

Now lets say I sync at 10am. I sync at 10,000kbps with a 6db noise margin.
I do not reboot the router or disconnect it from the telephone line.
At 6pm, I will still have a 10,000kbps sync speed although the noise margin may now be 4db.
This effectively means the line has more external noise than it did at 10am, hence the noise margin has dropped down.
At midnight I will still have a 10,000kbps sync speed although the noise margin may be 3db. This shows that there is even more external noise.

The sync speed will not change unless the line drops. If the sync speed changes it means the line has dropped out, imagine you are on a Skype call and it cuts out or uploading a file to an email and it cuts out. Not ideal.

When may the sync speed change.
1. When the line drops out
E.g.. I connect at 10am with a sync speed of 10,000kbps and a noise margin of 6db.
I do not reboot the router or remove it from the phone line.

At 6pm the sync speed is 10,000kbps and the noise margin is 0.5db.
With a noise margin of 0.5db, there will be significant errors building up on the line. The router will try to do everything it can to hold the connected but eventually it is likely to drop out.

The router may drop the connection and now sync at 9000kbps, with a noise margin of 6db.
Notice when the line drops out, the router resyncs at the target noise margin, the sync speed is reduced to compensate.

The line only drops out when the connections too unstable to hold, it then trains in with a lower speed.

Sky line management is a little more complicated.
They have both noise margin target and a sync speed cap.
Sky DLM runs for 10 days. In this 10 days Sky says, you can sync at a maximum speed. Say 15Mbps.
A target noise margin is also set, say 3db.

You reboot the Sky Hub.

HOW SKY WORKS

At midnight it will sync at say 14,800kbps (below the sync speed cap) with a noise margin of 3db (the noise margin target)

At 10am the following day, you may sync at 15,000kbps (hit the sync speed cap) with a noise margin of 5db (above the noise margin target)

Effectively, the noise margin target is applied if you are below the sync speed cap set to your line.

If your line tries to connect above the sync speed cap, it won't be allowed to and will just sync at that speed.

HOW OTHER ISPS WORK
At midnight it will sync at say 14,800kbps with a noise margin of 3db

At 10am the following day, you may sync at 15,500kbps with a noise margin of 3db

OTHER ISPs you are guaranteed to sync at whatever speeds yield the noise margin target.
Sky, you sync at the noise margin target, unless the sync speed cap is hit. Then you sync at that speed and get whatever noise margin comes.
Standard User ironman12345
(newbie) Wed 18-Feb-15 23:37:43
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Re: Should sync speed vary with SNR changes


[re: ukhardy07] [link to this post]
 
Good post ukhardy07, explains the way it all works.
So the only way to keep the sync rate and hopefully the db rating is to use a separate router as if an all in one modem/router is used resetting one will reset the whole setup to the internet. ?
Standard User RobertoS
(elder) Wed 18-Feb-15 23:52:21
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Re: Should sync speed vary with SNR changes


[re: pdes] [link to this post]
 
See this page.

My broadband basic info/help site - www.robertos.me.uk. Domains,site and mail hosting - Tsohost.
Connection - Plusnet UnLim Fibre (FTTC). Sync ~ 56.5/15.1Mbps @ 600m. - IPv4BQM IPv6BQM
Standard User ukhardy07
(knowledge is power) Thu 19-Feb-15 00:51:56
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Re: Should sync speed vary with SNR changes


[re: ironman12345] [link to this post]
 
The noise margin will always flap around.

The sync speed should remain the same.

If that's what you are seeing it's fine.
Standard User pdes
(newbie) Thu 19-Feb-15 09:07:41
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Re: Should sync speed vary with SNR changes


[re: RobertoS] [link to this post]
 
In reply to a post by RobertoS:
See this page.


A good read. Thanks
Standard User pdes
(newbie) Thu 19-Feb-15 09:25:37
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Re: Should sync speed vary with SNR changes


[re: RobertoS] [link to this post]
 
In reply to a post by RobertoS:
See this page.


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