Router Stats (sync, noise, attenuation) explained

[Saturday]

The following has been written specifically for O2’s Home ADSL2+ LLU products. However, the principles also apply to O2 Access.


Getting to your router stats

Several useful pieces of information about your connection can be found within your O2 Wireless Box's admin pages. To find these you need to first log in to the router:

Open up a web browser (IE, Firefox etc) and type http://192.168.1.254. into the address bar and click go or enter.

To quote O2, "when you first receive your O2 Wireless Box it is set to a default username and password to allow you quick and easy access without any need for additional usernames and passwords". This means access to the Box is wide open so you are strongly recommended to set a password to restrict access.

To do this go to "User Management" and next to Administrator choose "Change my password". You'll now be presented with three blank fields. In the first "Old Password" leave it blank, in the other two type your new password. After confirming you'll then be asked to enter your username and password. These will now be Administrator (note the capital A) and the password you've previously entered. You now have access to a number of functions within the router including the simple connection stats.

Due to restrictions that O2 have placed on the Wireless Box, the Administrator login doesn't provide access to the detailed router stats. To get to these you must log in using a "secret" higher level username. Log in again to the router but this time with username SuperUser (note the capital S and U) and password O2Br0ad64nd (thats digits 2064 and the first O and B being capital letters).

You'll now have access to pretty much all the functionality that you'd get with generic Thomson firmware including access to the router stats. For routers other than the O2 Home Wireless Box (Thomson 585v7), please refer to the user manual for instructions on accessing the required admin pages.

Assuming you've done all that, you now have sight of your router’s current connection details which should look something like this (depending on router model):

Bandwidth (Up/Down) [kbps/kbps]: 1,265 / 13,479
Line Attenuation (Up/Down) [dB]: 18.6 / 33.5
SN Margin (Up/Down) [dB]: 6.6 / 7.0



Connection Sync

Within the list of stats, first check your router sync speed. This will usually be labeled Bandwidth, Rate or Sync and be measured in thousands of bits per second (kbps).You’ll see two columns:

• Downstream/Down refers to the connection coming from the exchange to you
• Upstream/Up refers to the connection going from you to the exchange.

Most people are interested in their downstream speed. There are a couple of things to note:

The figure you see is the actual speed that your router negotiated with exchange when the router was connected and last powered up. If you were to run a speedtest at a test site then you will not see this speed because of transmission overheads – expect up to 80-90% of the router reported connection speed for a very fast site. Slow sites will give a much lower result. A more detailed explanation can be found here.

The figure reported may not be at all close to the speed you’ve “bought”. This is for a number of reasons which should soon become apparent if you read on.



Line Attenuation

For the purposes of this guide, we’re more interested in the Downstream Line Attenuation.

Attenuation is a measure of the loss in power of electromagnetic signals between transmission and reception points. Many factors affect attenuation but by far the largest factor is line length. In simple terms, the further you are away from the exchange the higher your attenuation figure will be as the signal loss increases.

Attenuation is the single most important factor in determining the speed your router can connect at. You may be signed up for 16Mb O2 Premium but you will only get close to this speed if your line is fairly short.

As a point of interest whilst you can identify the distance between your home and the exchange “as the crow flies” with a site like Samknows Broadband - Comprehensive Broadband Information, you can get a reasonable approximation of the actual line length by dividing your (ADSL2+) downstream attenuation by 13.8 to get the distance in kilometres. What many customers are disappointed to discover is that BT lines rarely take a straight line between the exchange and the home. It is not at all unusual to find that a line's actual length is 2 or even 3 times the straight line distance estimated by samknows.

The following table shows the relationship between downstream attenuation and potential connection speed on a relatively low noise line:

Attenuation.............Approximate Line Length............Potential Connection Speed
20db...................................1.4km...... ...............................20000kbps
26db...................................1.9km...... ...............................18000kbps
32db...................................2.3km...... ...............................16000kbps
35db...................................2.5km...... ...............................14500kbps
40db...................................2.9km...... ...............................11800kbps
45db...................................3.2km...... .................................8500kbps
50db...................................3.6km...... .................................6500kbps
55db...................................4.0km...... .................................4500kbps
60db...................................4.3km...... .................................3200kbps
65db...................................4.7km...... .................................2000kbps



Noise Margin

Before we go into noise margin we need to cover noise.

The connection between you and the exchange is made up of two copper wires. Copper is a great transmission medium but it also is susceptible to various electromagnetic interference, generally known as noise. Any length of copper wire will naturally carry noise but the longer it is the more likely it is to be noisy. The challenge for your router is to distinguish between background noise and the ADSL signal. You may have heard the term Signal to Noise Ratio (SNR). This is simply the ratio between the strength of the signal and the background noise on the line expressed in decibels.

The noise margin reported by your router is not a measure of noise on your line. It is a measure of the margin it has given itself when it negotiated with the exchange. This is in effect the difference between the actual SNR and the SNR the router requires to run at a given speed.



Why is any of this important?

Well to understand this we need to consider what is happening between your router and the exchange equipment.

Again, in simple terms, when your router talks to the exchange when powered up it measures and negotiates a number of factors. First it tests the transmission qualities of your line (in effect attenuation) then it measures noise level and quality. With this information it identifies which part of the frequency spectrum is available and usable and assigns upstream and downstream “channels”. In effect it blocks out part of the frequency bandwidth where noise is above a certain level and doesn’t use this part. A noisy line means less bandwidth is available. Less bandwidth means lower speeds. So going back to the above table, you will only get those potential connection speeds on a relatively quiet line.

So in summary, if you have a long line then this is the first thing that sets your connection speed. If it is noisy too then the connection speed is further reduced.

You can’t materially reduce your attenuation as it is largely line length related. However, poor line quality will increase the attenuation slightly. Improving the line condition may reduce your attenuation by 1 or 2db - that's all.



What is the right Noise Margin?

When the router negotiates with the exchange it is trying to sync at the highest speed possible with an O2 default noise margin of 6db or higher. Note: when you first moved to O2 your line will have run under DLM, see here, and this process may have determined your line is such that a higher than normal noise margin is needed and you may find that your default noise margin is set to higher than 6db.

Noise varies through the day depending on a variety of environmental factors plus cross-talk from other local O2 ADSL users (the more active other users are the more noise generated). If noise rises then the router noise margin drops. If noise drops then the router noise margin rises.

It will have been 6db (possibly higher with a short line) when it first connected but the current noise levels may be higher or lower than they were then so the margin will/may have changed. So, don't expect your router noise margin to be 6db at all times.



Note: This tutorial is in first draft and I will add to it and improve the clarity over the coming days. If any reader has suggestions for additions or corrections to this guide please let me know.