Noise margin что это
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Noise margin что это

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Заметки эникейщика

Статьи и заметки об установке и настройке Windows, Linux, маршрутизаторов и т.д.

Параметры линии ADSL

Если вам не повезло стать пользователем интернета по технологии ADSL, то у вас нередко могут возникать проблемы с линией ADSL. Многие модемы могут осуществлять некоторую диагностику линии.
Выдача этой диагностики будет примерно следующая:

Downstream Upstream
SNR Margin: 15.5 db 26.0 db
Line Attenuation: 35.1 db 20.1 db
Data Rate: 8192 kbps 511 kbps
Max Rate: 16880 kbps 520 kbps
POWER: 19.7 dbm 12.2 dbm
CRC: 2 1

Что это обозначает и на что стоит обращать внимание в первую очередь?

SNR Margin:
Данный параметр отражает отношение сигнал/шум.
до 7 dB — плохая линия, присутствуют проблемы синхронизации
от 7 dB до 10 dB — возможны сбои
от 10 dB до 20 dB — хорошая линия, без проблем с синхронизацией
от 20 dB до 29 dB — очень хорошая линия
от 29 dB — отличная линия

Line Attenuation:
Данный параметр отражает затухание линии.
до 20 dB — отличная линия
от 20 dB до 40 dB — рабочая линия
от 40 dB до 50 dB — возможны сбои
от 50 dB до 60 dB — периодически пропадает синхронизация
от 60 dB и выше — оборудование работать не будет

Output Power:
Данный параметр отражает мощность принимаемого/передаваемого сигнала.
Реальные числа тут от +10 до +20dBm
Если числа меньше или отрицательные, то это проблема с оборудованием,
либо на DSLAM порт глючит, либо клиентский модем.

Названия параметров могут слегка отличаться.

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Noise Margin and How It Impacts Your Internet Speeds

Imagine this: you’re at your desk at work and your internet connection suddenly slows down. Maybe you’re at home and you just want to watch a funny video that someone sent to you but the video just won’t load – and neither will anything else. While you may be tempted to just pick up the phone and call your ISP or go spend a fortune upgrading your router and other equipment, the problem may actually be something that you can solve yourself, without having to contact your ISP or spend a fortune on new equipment. If you have ADSL internet, your problem may be the noise margin. Read on to find out how this could be impacting your internet speeds.

What is Noise Margin?

Noise margin – also known as SNR margin or noise margin – is a measure of the difference between actual signal-noise ratio (or SNR) and the minimal SNR required for syncing. So what does all of this mean? Let’s break it down in a way that’s more easily understood.

Signal-to-noise ratio, or SNR, is the ratio between the signal – or the meaningful information – and noise, which is background interference. This noise could be attributed to a variety of things, including but not limited to distortion and crosstalk.

If your noise is too high, you will experience interference with your ADSL. This could include problems with syncing and other issues that significantly delay your internet speeds, which isn’t ideal and can be quite frustrating. In order to avoid this issue, you have to make sure that your noise margin is right where it should be, with noise significantly reduced for best performance.

How Is Noise Measured and What’s Good (or Bad)

The SNR is measured in decibels. Anything at or below 6 dB is bad. You will experience lots of interference and difficulties with syncing. 7db to 10db is considered “fair.” It isn’t the best and will likely have interference. Any variations in conditions can lead to increased noise and cause connection issues. 11dB to 20dB is considered experiences very few sync problems, unless there is a large variation in conditions. 20dB to 28dB is considered excellent, while 29dB is outstanding.

This isn’t the only value you need to know, however. You’ll also need to calculate the SNR margin. Once you know your SNR, you’ll be able to calculate your SNR margin.

Calculating Your SNR Margin

Your SNR margin is the difference between your actual SNR and the lowest SNR needed to sync at a specific speed. This isn’t a difficult calculation at all once you’ve determined actual SNR.

Here’s an example:

Your actual SNR is 40dB.

The required SNR to sync at 8Mbit/s is 35dB.

Simply subtract the two values, and you’ll find that your margin is 5dB.

As a rule of thumb, higher margin numbers translate to cleaner signals. Anything below 6dB is considered bad, so in this example, there is a clear problem. The higher the number, the better, but in general, anything above 6dB should be fine.

Router Settings

Some routers provide information for you about your SNR. You may be able to access SNR upstream and downstream traffic statistics that you can check out and monitor to ensure performance. Not all routers have this option, and you’ll have to consult your owner’s manual or manufacturer’s website to find out if and how this information can be accessed.

How Do I Improve My Margin?

So you’ve done the calculations and you’ve found that your margin falls below 6dB. If you’ve noticed interference, you definitely know that this is a problem that you need to address. But how do you raise your margins? It may be easier than you think.

First, understand that there are many conditions that contribute to this line interference. It could be something expected, like peak times for your provider. This crowding can lower your number. But other things, including home appliances, devices installed on telephone lines, and even lighting can contribute to this interference. This doesn’t mean you have to change out your lights and stop using your small kitchen appliances. There are other ways to get around this problem.

The first thing you can consider is purchasing a new router. The router that you purchase must be able to handle lower SNR margins, so make sure that you do your research. Look for online reviews, check manufacturers’ websites, or even ask a tech-savvy friend for recommendations.

You can also purchase filters that can be used on your ASDL lines. Again, make sure to research your options before you go out and spend a lot of money on something that may not perform as expected.

The faceplate to your cable line may be old, so you can check that too. That’s one inexpensive fix.

The lines in your home or building may be old, so these may need to be replaced. Contact your provider to find out what, if anything, they can do. If you rent, you’ll also want to speak to your landlord about options.

Finally, if worse comes to worse, you may have to consider switching providers. If nothing else is working or someone in your building or neighborhood have success with their service, this may be a possibility for you. While it may seem like a hassle, you don’t want to continue to pay for service that underperforms, do you?

Final Thoughts

Interference and bad connections can be a pain, but you shouldn’t just accept this as your fate. By considering the SNR and SNR margin, you can find and troubleshoot your connection to get rid of unwanted interference. Do you have a question about noise margin or SNR, or perhaps a helpful tip for those struggling with excess noise? Leave a comment below!

Noise margin что это

DSL line stats from your modem or router can be a useful tool to aid troubleshooting adsl problems and line faults. Some users may just wish to know what the figures mean, or if they can use this information to give a rough guide as to whether or not your line can cope with higher speeds.

What are adsl line stats?

When using your linestats as a diagnostic tool you have to look at all the figures and not just rely on say just the sync speed or SNR Margin.

How do I get my line stats?

Most adsl modems or routers will provide you with your dsl line stats.
I maintain a list of instructions for the most commonly used adsl modems and routers, which will help you find the figures you need.
— See How to get adsl line stats from your router.
Get your line stats from your router
Your ISP will also be able to get your line stats by performing something called a WOOSH test which will advise them your loop loss (attenuation) and SNR Margin. However, due to the time element involved, most ISPs are very reluctant to perform this test unless it is specifically in relation to an adsl fault. woosh test
A BT Openreach adsl faults engineer will also be able to access your line stats by hooking up his equipment to your line.

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Which figures do I need to look for?

  • Connection or Sync Speed or Rate
  • Attenuation or Atten or Loop Loss
  • SNR Margin or Noise Margin or Noise or Margin
  • Details of any CRC/HEC/RS/FEC errors and Errored Seconds may also be helpful.

It is important to remember that the sync speed at which you connect is not your "real speed" that you will be able to download at, as it has an inbuilt allowance to allow for such things as TCP/IP overheads.
For example a line syncing at 8128 kbps could expect to see a maximum throughput speed of around 7.1Mb (approx 13% for overheads).

What is Attenuation?

Attenuation is logarithmic and each 3dB of attenuation halves the strength of the signal power received, therefore a line with 30dB of attenuation only receives 1/1,000th of the power, whilst a 60dB line would only receive 1/1,000,000th.

Attenuation also depends on the quality and gauge of cable on your line, but a rough guide is 13.5dB — 14dB of loss per km.

True line attenuation — or Insertion Loss — can be measured at the DSLAM at the exchange via diagnostic tests and this figure should remain fairly static. Our routers can give us an indication of how much the signal is attenuated as an average against all the frequencies that it uses.

Because the router measures against the frequencies available, some users may notice a very slight increase in attenuation if say moving from a fixed rate 512 kb connection up to 2Mb.
Higher frequencies such as those used to transmit faster speeds are more likely to be attenuated (higher frequencies = higher attenuation).
ADSL 2+ has an increased frequency spectrum, therefore an increased attenuation of around 3-4dB is not unusual. As a very rough guide a speed increase of 4Mb is said to increase the attenuation by 1dB.

It is also important to note that different routers may load the frequency bins in slightly different ways, and on top of that some routers report the average across the frequencies actually in use, whilst some may report the average across the frequencies available — which is why sometimes using a different router may report a slightly different attenuation figures.

To confuse matters further, there are a few routers which record the insertion loss measured at 300 kHz as being the downstream attenuation figure. Obviously those routers are unlikely to show attenuation changes as the speed range increases. The reason why, is that insertion loss or attenuation measured at 300kHz can be used as a very good guide to calculate the loop length and line capacity.

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What is the difference between Line Attenuation and Signal Attenuation?

Routers that display two sets of attenuation figures are usually reporting the Signal Attenuation and Line Attenuation. In these cases it can be taken to mean:-

  • Line (or Loop) Attenuation is an average of all the tones available for use in the relevant dsl spectrum — regardless of if the tones are in use or not.
    Loop attenuation is calculated during the transceiver training phase of the initialisation process and is an estimation of the line attenuation averaged through all the upstream or downstream subcarriers.
  • Signal Attenuation is monitored and reported as an ongoing process and can sometimes be affected by ongoing changes in line conditions. Calculation is performed as the difference between the power transmitted at the far end and the power received at the near end.
    Signal attenuation is based an average of the frequency bins in use during showtime and therefore it is not considered unusual if Signal attenuation is slightly lower than Line attenuation.

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SNR - signal to noise ratio

Each line needs a specific amount of SNR to run at a given speed. Very roughly its often said that 1dB of SNR equates to 450kbps of speed, but other factors such as attenuation and output power need be bourne in mind and some lines may only see around 1Mb per 3dB.

What is SNR Margin?

Whilst SNR Margin and SNR are related, they are not the same thing.
SNR Margin could be thought of as a "buffer zone" which is there to protect your connection against normal SNR fluctuations that can and do occur on a daily basis.

Consumer routers usually show the SNR Margin and not the SNR value regardless of the display name.
Depending on the manufacturer this may be "Receive Margin", "SNR", "Noise" or any variation of these.
It may also be abbreviated to SNRM or Noise Margin.

The SNR Margin is the difference between the actual SNR and the SNR required to run at a specific speed. For example, if your line needs 35dB of SNR to run at 8Mbps, and the actual line SNR is 41dB, then the SNR Margin would be 6dB.

Its perfectly normal for your SNR Margin to fluctuate during the course of the day, and its most likely to be at its lowest during the evenings. If your SNR Margin is too low this may lead to slower throughput or frequent disconnections.

Looking at your SNR Margin figure can often be a useful diagnostic for adsl faults. Attenuation and SNR Margin are independent figures and its possible to have a good attenuation, but poor SNRM — or vice versa.

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What is Target SNR Margin (TNMR)?

If you are on a rate adaptive "up to 8Mb" connection (such as MaxDSL or LLU) then Target Noise Margin plays an important role during the synchronisation process between your router and the DSLAM at the exchange.

Target SNRM affects the speed at which we sync at and sets the initial SNR Margin "buffer zone". You should be able to get a rough idea of your Target Noise Margin by looking at your linestats immediately after a resync.

During the sync process the DSLAM and router negotiate a sync speed based on the various conditions of your line, allowing "spare SNR" for normal fluctuations.

With the rate adaptive products, profiles that can be typically set are 6dB, 9dB, 12dB or 15dB -with 6dB being the default. Longer lines are more susceptible to noise and therefore often require a larger margin than shorter lines.

Its important to note that on very good lines which would be able to sync in excess of 8Mb and which are currently syncing at 8128 or 7616 will show the SNR Margin as being higher due to the line not having reached its full potential. Such lines may well have the output power reduced in order to minimise crosstalk for other connections, therefore the SNRM figure is not an indication of the true target in these particular cases.

Fixed rate lines don’t have a target SNRM as such. The line is set to connect at the fixed speed without any negotiation of line rate, therefore they will either connect at the fixed speed or wont connect at all if the SNR is too low.

Whilst Target Noise Margin is the correct term, it is often referred to as Target SNRM or the Target SNR.

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How fast should my connection go? (DSL Max & LLU).

What are BT’s Fixed Rate limitations?

Ideally your line length should be below 3.5km from the exchange to get a fixed rate 2Mb connection and 6km for a 1Mb connection.*

Some older adsl connections may still be on a fixed rate line, or there some instances where long lines which have large SNR fluctuations don’t perform too well on the "up to 8Mb" type accounts and can be moved back to a Fixed Rate Line.

Attenuation Fixed Rate Line Sync
Max — Capped Rate
> 60dB 512 kbps 576 64-288 Profile 500
43 — 60 dB 1 Mbps 1152 288 Profile 1000
<= 42 dB 2 Mbps 2272 288 Profile 2000

If you already have adsl enabled you may be able to add an extra couple of dB on those figures e.g. 44 dB would be the limit for a fixed 2Mb line. However, since Aug 2005 BT have adhered to the rules more strictly due to a large number of failed 2Mb upgrades that had to be regraded back down to 1Mb.

There is a supposed "soft cap" for adsl of about 70dB, but BTw will try connect anything up to 75dB attenuation.

*The new reach limits came into effect on the 6th of September 2004. Prior to this date the limits were 6km for a rate adaptive product (512k) and 3km for a fixed product (1 & 2Mb).

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Improving your Line Stats.

Many people have asked if there’s anything that can be done to improve your line stats.
There isn’t much you can do about attenuation, because that is largely dependant upon line length, and the amount of copper in/or joints on your line.

However there are a few things that may help improve your SNR Margin. Rate adaptive products such as the "up to 8Mb" maxdsl and LLU rely much more heavily on SNR Margin to attain the best possible speed so its well worth checking out to see if you can improve your connection speed.

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63.5 dB Attenuation?

The reason some routers/modems will report 63.5 dB as the maximum attenuation isn’t actually a bug. In fact its from the G.992.1 standard which quotes the recommended test parameter to be "The attenuation ranges from 0 to 63.5 dB with 0.5 dB steps."

As such many firmware versions have taken this literally and only go up to 63.5dB when it is possible to have a higher attenuation figure than this. If your router is displaying 63.5dB, then there’s a chance that your attenuation may in reality be higher.

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What do the other figures mean?

Different routers may also display various other data about your connection. Information and explanations about some of the other figures are explained on the following pages:-

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