Checking an ADSL2/2+ connection to ensure you’re getting the best possible performance can sometimes feel like more of an art than a science, with most suggestions being “try this…” or “try that …”.  So what’s the best way to check?

The place to start is the Online Status page on your Vigor Router.

  1. Look at Loop Attenuation and SNR.  Your objective is to decrease Loop Attenuation and/or increase SNR.
  2. Note the Modulation, Up Speed, and Down Speed. Periodically your modem will train with the DSLAM to re-evaluate which settings and speed is best.  Sometimes this will result in your synchronisation speed (shown as Down Speed and Up Speed on the Online Status page) going down over time.  Note that the actual data throughput could be significantly lower than the Sync speed.
    Many customers (particularly those further from the exchange) find that G.DMT modulation (the ADSL1 standard) provides fewer dropouts and higher actual throughput, even though the ADSL2 or ADSL2+ modulations might show a higher Sync. Speed.
  3. Also on the Online Status page, look at the Connection time on the WAN Status section, Corrected Blocks and Uncorrected blocks.  You can’t be sitting in front of your modem constantly to see every time it drops out but the Connection time indicates how long ago the last drop-out was.  So if your Modem’s Up Time is 200 hours but the Connection time never gets higher than 10 hours, the ADSL must be dropping out.  However if the Modem’s Up time never gets higher than 10 hours then the entire Vigor is rebooting periodically.
  4. Finally on the Online Status page, look at the Corrected Blocks and Uncorrected blocks.  Occasionally there will be a bit of noise on the phone line which corrupts the data received.  If it’s a simple 1-bit error the circuitry in the modem can work out which bit was wrong and correct it.  However more complex errors are uncorrectable and require the block to be re-transmitted,  which is going to take longer for your computer to receive the data.   Again there’s no hard limit to what is acceptable, but you will want the ratio of Uncorrected Blocks to RX Blocks to be as low as possible.

After trying several firmware/filters/etc you will have several sets of results – and probably no clear winner. Unfortunately there’s no simple answer; and you may have to trade off the speed of ADSL2/2+ against stability of ADSL1.

If you have a Vigor 2800 or Vigor 2600 model, you can look at the Disgnostics > ADSL Spectrum Analysis graphs.  The BINs along the x axis indicate the different frequencies which can carry data, and the height of the bar indicates how many bits of data can be trannmitted at that frequency. The red bars at the left are used for Upstream.  ADSL1 uses only the first 255 BINs.