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ADSL: Understanding Line Loss and measurements

Community Veteran
Posts: 6,983
Thanks: 8
Registered: 10-04-2007

ADSL: Understanding Line Loss and measurements

Tutorials and FAQs: ADSL: Understanding Line Loss and Measurements


This tutorial will hopefully give you an insight into what causes loss on your line, how it is measured, what the figures you see mean and the maximim loss acceptable for ADSL products.
Also steps you can take to try to minimise the loss on your line.


What causes line loss?

The telephone line from your local Telephone Exchange to your house is made up of a twisted pair of wires within a cable

Everything that carries electricity has what is known as "Resistance". This is measured in "Ohms" and resistance impedes the flow of current in a conductor.

Because your connection is two wires twisted together it also has some "Capacitance" between the wires. A capacitor is an electrical component and there is more current passed through it as the frequency gets higher.

Also your pair of wires has yet another characteristic called "Inductance" and inductance allows less current to flow through it as the frequency gets higher.

So as you can see from the above the electrical characteristics of your phone line are quite complex.

To sum up:

Resistance reduces the current so increases the loss.
Capacitance effectivly short circuits the line more as frequencies increase so increasing loss.
Inductance resists current flow more as frequencies get higher thus increasing the loss.

Obviously the longer your line then the greater the effect of the above characteristics and the greater the loss.


How is line loss measured?

Loss is measured by comparing the power level of the signal sent from one end of the line with that received at the other end. The difference between these levels is expressed in Decibels (dB).
The decibel scale is logarithmic and works as follows.

If the power received was 1/2 the power sent, then that would calculate as
10 x log base10 of 0.5 = -3.010

So a loss of 3 dB is equal to almost exactly half the power being received.
Similarly, if you were to look at log tables and calculate other figures you would find that losses of:
    10 dB = 1/10th of the power
    20 dB = 1/100th of the power
    30 dB = 1/1000th of the power.
If you were to carry on until you got to 60 dB loss you would find you are only receiving one millionth of the power that was originally sent out.


How do you find out what your line figures are?

Many ADSL Modems and Routers have a function in the set-up options that will actually measure the losses for you and give you an on-screen display.

If you have one that doesn't give you this feature then you could contact Customer Support and ask to be told the figures for a "Whoosh" test on your line .

One point to bear in mind with the Whoosh test is that the activation of your line actually adds about another 4 dB to the original loss. BT take this view when doing a test so if a loss figure of say 64 dB is produced from the test you will still be just within the limit of 60 dB (for 1meg service).


What do the figures mean?

Let's look at some typical figures, and here I will use the ones shown by my own equipment a D-link DSL-504 Router using 512K 50:1 ADSL.

These are as follows:
  • Attenuation Downstream: 28dB. This means I am receiving just under 1/500th of the signal sent from the exchange.

  • Attenuation Upstream: 31dB. This means the Exchange is receiving just less than 1/1000th of the signal sent from my modem.

  • SNR: 19dB. SNR stands for Signal to Noise Ratio and is basically the difference between the level of the signal being received compared with the natural noise level on the line.

    Here, the higher the figure the better. In my case 19dB means that the signal I am receiving is almost 100 times stronger than the noise level.

    Noise on your line is caused by many things. Some examples are other wires in the cable running alongside your wires, interference from power cables, radio signals, higher than normal resistance joints in the telephone wires and damp in the wires or cables.

Other figures you may see.

Upstream power 10 dBm
Downstream power 12 dBm

These relate to the output power from the transmitters at your modem and the exchange.

The dBm notation means decibels relative to one milliwatt (the "m" in the figure) so in the above case the powers are 10 milliwatts and approx 18 milliwatts (13 dBm would be 20 milliwatts).


What levels are acceptable for ADSL service?

Now you know what the figures above mean, you may be amazed that ADSL works at all!!

To get a reliable service your line needs to meet the following criteria:[list=1]
  • 512K Service. There is now no upper limit and BT will attempt than make it work on any line if possible.

  • 1 Meg Services. You need a line loss of less than 60 dB, and so, typically, will be no more than 6.0 kilometres from the exchange.

  • 2 Meg Service. You need a line loss of less than 43 dB which means you will be 3.5 Kilometres or less from the exchange.
    [/listShocked]The distances from the exchange are based on the average signal losses versus line lengths and you may find that even if you are under the distance you may still have too much loss for the chosen service.

    Conversely you may also find that even if you are outside the distance you may still get the product.

    At the end of the day, it is the important line tests that are done after you apply for ADSL that matter.


    Effects of too high a line loss.

    Line losses can alter over a period due to factors such as temperature, rainfall, corrosion in cable joints, etc.

    If you have had a good ADSL service but find you start getting frequent disconnections, you need to check your line is still within the limits above to meet the type of service you have.

    So, if you have the original figures, keep a note of them so you can make a comparison later if you start getting problems.

    Remember from the above dB notation, an increase of 3dB in the loss figure means that you are only getting 1/2 of the signal you had when it was working fine.

    Also very important is the Signal to Noise Ratio. Remember that higher figures are better here.

    I have not as yet found any article that determines the minimum Signal to Noise Ratio as acceptable for your ADSL to work correctly. However it is generally accepted that anything less than 10 -12 dB will cause problems and I would worry if my line had a figure of, say, 10dB and normally I would expect 15 dB or better.


    Improving your loss figures.

    Unfortunately there is little you can do about the actual line itself other than get it maintained by BT. But you can take some steps to ensure that you are not adding more than the minimum loss yourself:[list=1]
  • Use good quality Splitter / Filters.
  • Use good quality extension cables.
  • Ensure that where you plug into cable sockets that the pins are clean and bright.
    It has been known for the connections here to corrode with time and you could try pulling out and re-inserting the plug into the BT socket several times to polish the connections.[/listShocked]Hopefully the above information will have been useful to those who wanted to know more about their ADSL connection.

    Edited to reflect the limit changes introduced by BT
  • 61 REPLIES
    richter
    Newbie
    Posts: 7
    Registered: 07-08-2007

    ADSL: Understanding Line Loss and measurements

    I bought some cheap filters from www.e-buyer.com for £1.29. Cheesy Can I buy extra ones from PlusNet because these ones don't do a wonderful job :?: :idea: Guess I can't expect a lot for £1.20 but it was worth a try!
    N/A

    ADSL: Understanding Line Loss and measurements

    I recomend the Z-blockers availabel from the likes of DSL Warehouse.
    N/A

    ADSL: Understanding Line Loss and measurements

    my SNR is 8.3.

    According to the BT engi I had round - that's acceptable. It came up Green on his router test.

    On the old line (before he swapped them) I was getting 4.1 which came up as amber.
    Community Veteran
    Posts: 6,983
    Thanks: 8
    Registered: 10-04-2007

    ADSL: Understanding Line Loss and measurements

    Are those figures actualy Db's though ?
    There is no way ASL would work with a SNR level of 4.1dB that implies that the signal is almost in the noise.

    They must be some other figure like an actual ratio of your signal being 4.1 times higher than the noise level.
    N/A

    ADSL: Understanding Line Loss and measurements

    His router came up with that figure.
    The reason he was there because my adsl kept dropping.
    Community Veteran
    Posts: 6,983
    Thanks: 8
    Registered: 10-04-2007

    ADSL: Understanding Line Loss and measurements

    Well yes as I said above at 4.1dB it should not work at all and it was obviously giving you problems.

    I am still very doubtfull though that a SNR of 8dB is good enough to work without any problems. Believe me I spent my working life on Transmission Networks and a figure of 8dB means that your signal is only about 5 times higher than the background noise. This would be very noticable with an audio circuit as you would actually hear the noise quite clearly.
    ADSL is less forgiving than the human ear and I would be less than happy with a figure below 10dB.
    N/A

    ADSL: Understanding Line Loss and measurements

    My connection has dropped a couple of times. But no major outages. - It came up again within a few minutes.

    I'm almost at the limit for line loss. 59.3dB. I really do live in the middle of no where.

    My cordless is a bit hissy - but I just put it down to BT being unable to make anything that works properly :shock: :lol:

    As I did say - The BT engi's router came up with that figure and it was green. - Perhaps a plus techy could shed some light on acceptable SNR figures?
    Community Veteran
    Posts: 6,983
    Thanks: 8
    Registered: 10-04-2007

    ADSL: Understanding Line Loss and measurements

    Quote
    Signal to noise ratio should really be above 10-12 dB to work properly, ADSL just doesn't work with SNR of 6dB or less, similarly with a line loss of 70dB or just will not work at all. So don't hold your breath for another extended reach trial soon.


    When I was writing this tutorial I was in discussions with PlusNet and above is a quote from them during one on of the conversations.
    N/A

    ADSL: Understanding Line Loss and measurements

    The following figures were taken from my Solwise 110 router a few minutes ago. This was supplied by Plusnet and has recently been upgraded to the latest firmware. This made no change to the figures below:

    Local Tx. Power(dB) : 11.55
    Remote Tx.Power(dB) : 15.8
    Local Line Atten(dB) : 58.0
    Remote Line Atten(dB) : 31.5
    Local SNR Margin(dB) : 28.5
    Remote SNR Margin(dB) : 21.0

    The only figure that concerns me is the Local Line Attenuation which is getting close to the 60dB limit. When I first had my ADSL connect February 2003 this was around 43dB. A fault developed and the attenuation was varying rapidly between 38 and 55dB (typically over a period of 15 minutes). The actual fault was a 100% loss of service between midnight and 6am!

    Several tickets were raised at the time. After the second or third visit from BT something was changed at the exchange and my service was restored but the local line attentuation went to 53 dB. This was around June/July 2003. Since then it has been gradually getting larger and is now 58dB. I totally failed in my attempts to find out what was changed. A lift and shift was done (according to the information given to me by BT about 2 weeks earlier).

    Should I be getting worried? Should I ask for the results of a woosh test? Before I had ADSL the line checker reported that I was 3.1 km from the exchange.
    Community Veteran
    Posts: 6,983
    Thanks: 8
    Registered: 10-04-2007

    ADSL: Understanding Line Loss and measurements

    Yes it is getting close to the 60db limit,but a couple of questions.
    1. Is your normal phone working Ok.
    2. Is this 58dB loss consistant or is it still varying.
    3. Is ADSL working with no apparent problems.

    If it stays steady and you have no problems then I would leave it for the moment and hope that it stays below the 60dB level.
    If it's still varying then you still have some sort of line fault and further action is needed.
    A whoosh test may give you some more accurate figures but do bear in mind that this test will show approx 4dB more loss than there really is. (This is apparently caused by the ADSL equipement at the exchange) so 64dB is the limit with a whoosh test.
    N/A

    ADSL: Understanding Line Loss and measurements

    The phones are all working fine. There is no apparent ine noise. The line attenuation has been slowly getting higher. There was a short time when it did vary rapidly (53-5Cool but one of BT's street cabinets was knocked over and I suspect that his had something to do with it. I lost my ADSL connection for a number of short periods around the same time.

    I was getting frequent lockups, in common with most SAR110 users, but the firmware upgrade has stopped that. I am just concerned that as it has changed by 5 dB over the past 6 months it might rise to the level where my ADSL stops working. That is quite a large signal change (I am an electronic design engineer and do understand dB).

    The SNR looks quite good despite the high attenuation.
    Community Veteran
    Posts: 6,983
    Thanks: 8
    Registered: 10-04-2007

    ADSL: Understanding Line Loss and measurements

    Sorry for the delayed reply to this thread but I have been away sailing for a week.
    Yes I suspect that the distribution cabinet being knocked over has some baring on your figures.
    If the phones are working fine and ADSL is still working ok then I think all you can do is to monitor the situation. I'm not sure just how accurate the figures you get from the modem are in reality, and I suspect there is a fair margin of error in these readings.
    Just to offer some reassurance though I had a fault on my phone last year where the line was apparently dead with no dial tone and utter silence on the phones.
    My ADSL though was still working. When the BT engineer came out he found that the cable between my home and the cabinet had corroded and one of the wires in my "pair" was totaly disconnected.
    So it seems that ADSL does still work on wet string Cool
    N/A

    ADSL: Understanding Line Loss and measurements

    The line loss had deteriorated before the cabinet was knocked over. That caused a temporary blip and it has settled to 57.5 or 58 dB.

    I do check several times a week however.
    N/A

    ADSL: Understanding Line Loss and measurements

    An increasing attenuation figure normally occurs in damp conditions (Winter) as corrosion on contacts causes greater contact resistance and hence higher attenuation.

    Unplug every connection and replug, just to clean the contacts. Check that all plug contacts are good fit in sockets and clean. Check your own house is in order first.

    Connections on telegraph poles and in street corner boxes (nodes) can deteriorate also. Underground distribution nodes can fill with water.