Understanding Line Loss and Measurements

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:

1. 512K Service. There is now no upper limit and BT will attempt than make it work on any line if possible.
   
   
2. 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.
   
   
3. 2 Meg Service. You need a line loss of less than 45 dB which means you will be 3.5 Kilometres or less from the exchange.
   

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:

1. Use good quality Splitter / Filters.
   
2. Use good quality extension cables.
   
3. 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.

Hopefully the above information will have been useful to those who wanted to know more about their ADSL connection.