The attenuation figures suggest maybe a 100 metre difference between the lines.

But I find it interesting that @jjpearce's transmit powers seem to be a lot lower! I wonder why that is?

From my understanding, power levels are adjusted (if necessary) at each cabinet, lowering them if crosstalk onto ADSL connections, where signals are already weakened by travelling long distances from the exchange, would be either a real, or a perceived issue.

I would guesstimate that my cabinet is around 4.3km from the exchange & I have wondered if that could be one reason why my speeds have deteriorated. i.e. that power levels have been reduced since the first month's higher speeds.

Unfortunately I do not have any connection stats from that first month for comparison against my current stats.

Alternatively, a 20Mb sync speed cap (for whatever reason) would produce an IP profile of around 19.4Mb & throughput as shown by speedtest.net of around 18.7Mb (maximum) at a quiet time.

Edited by deleted (Mon 06-Feb-12 07:14:42)

In reply to a post by Bald_Eagle1:

From my understanding, power levels are adjusted (if necessary) at each cabinet, lowering them if crosstalk onto ADSL connections, where signals are already weakened by travelling long distances from the exchange, would be either a real, or a perceived issue.

Yes - but the reduction only happens in the first 1-2 MHz. ie exactly where the ADSL and ADSL2+ frequency bands are.

The power levels over the higher frequencies can carry on as normal - so shouldn't affect (say) the values seen in the D2 and D3 bands.

I would guesstimate that my cabinet is around 4.3km from the exchange & I have wondered if that could be one reason why my speeds have deteriorated. i.e. that power levels have been reduced since the first month's higher speeds.

It is certainly a possibility - but without the detailed graphs, we'll never know.

Those cabinets close to the exchange should show less of a reduction in power (especially at lower frequencies), but over a larger portion of the 2MHz. Those cabinets farther away will see the power restricted more, but over fewer frequencies - under the assumption that the higher ADSL/ADSL2+ frequencies are no longer capable of sustaining data that far out. The furthest cabs will see a restriction only as high as 600kHz.

Picture of the power restrictions is in here, on page 31. The masks could have changed for the 17a profile, but probably only to account for the shift in bandplan (ie the 3-3.75MHz bit changing from up to down).

Looking at those, it is hard to tell whether you'll lose or gain by the choice of mask.

Incidentally - you can see this effect of the masks on the graphs we've been getting from the modems:

My original line is closer to the exchange, and shows the reduction in SNR up to tone 500, but note that the peak around tone 100 is 10dB higher than the peak around tone 500.

Conversely, my new line is farther from the exchange, and shows the reduction in SNR only up to tone 350. However, this time the peak at tone 100 is now around 5dB lower than the peak at tone 400.

Downloaded the files and thinking of giving it a go. Does one have to disconnect the LAN1 lead first or not? It is not clear from the instructions if it should be/can be left connected.

TIA

DrT