I can absolutely confirm that I do get variable 'Attainable' readings when sat right on top of the DSLAM. How can this be if it is purely theoretical ??
The attenuation will be very similar and obviously a small integer such as 1.2dB for example. What else does the 'Attainable' theorise about It's doing my nut in.
Allow me to add some soothing lotion
As in the links that @J0hn posted for me, the determination of max attainable is derived from calculation involving the SNR on all tones. It is a real calculation, not a theory (although some of the allowances it makes in that calculation are based on theory).
The calculation entirely hinges on the SNR on each tone - by which I mean the real signal-to-noise ratio, not the SNRM. (Reminder: SNRM is the margin, or the amount of "unused" SNR after the modem has "used" some of the SNR by deciding how many bits can be carried on a particular tone).
The SNR for each tone can be seen in the SNR/tone graphs on MDWS, and varies from over 50dB down to 6dB.
Why can N vary?(With the expectation that you make your measurements in the PCP, not in the FTTC cab...)Crosstalk will already exist on the line, even at the PCP, from the signals being carried on the tie pairs. This noise will increase in proportion to the length of the tie pair, and the number of neighbouring lines carrying a VDSL2 signal.
I'd expect to see more noise on longer tie pairs. I'd expect to see more noise on a line when all 10 pairs in the internal bundle have been allocated, than in a line where the other 9 pairs haven't yet been allocated (
if tie-pairs are grouped in 10s, not 25s). I'd expect to see more noise on a line as all the other bundles fill up.
How much? I don't know ... if the JDSU has a QLN graph, it might give you an idea. It wouldn't surprise me if the noise could reach more than a couple of dB on the longer tie pairs. I'd guess that crosstalk from 100m of tie pair will be worse than from 100m of plain D-side, where the disturbers are statistically spread out more. And I certainly get more than 12Mbps-worth of crosstalk on
my 100m D-side, where the DSLAM sits side-by-side with the PCP.
Why can S vary?Two main reasons... One you have mentioned (attenuation), one not mentioned on here yet.
a) The length of the tie pair plays a significant role.
10m distance might put an attenuation of 0.5dB on the line, while 50m might put 2.5dB attenuation on the line, and 100m leaving 5dB (using attenuation as measured on Broadcom VDSL2 chipsets, anyway).
b) The transmission power of the signal varies from tone to tone, and from DSLAM to DSLAM, controlled by the PSD masks that protect the ADSL/2+ spectrum.
Those PSD masks are governed by the distance between DSLAM and exchange. The main controlling factor (as you'd expect) is really the electrical distance, so is measured in dB, and is known as CAL (Cabinet Assigned Loss)
CAL is the loss measured at 300kHz, so is the same as the old attenuation values seen on ADSL modems; I believe Mr Sheep has confirmed this to be 10dB per km of 0.5mm copper. It varies between 0 and 50, in steps of 2. I guess that means it increases for every 200m from the exchange, up to 5km, adjusted for different gauges.
I've attached the graph from the ANFP, which shows how the transmission power is attenuated for some selected CAL levels. The more the power is attenuated, the lower the SNR will be, even when measured at the PCP.
- The red line, CAL=0, doesn't reduce power at all. This is for cabinets within 200m of the exchange, so these should have the best speeds.
- The green line, CAL=20, reduces most power over most frequencies, so is probably about the worst case, where resulting SNR is lowest and speed reductions greatest. This is for cabinets around 2km from the exchange.
- The blue and purple lines, CAL=40 and 50, reduce power over a more limited set of frequencies, so should have lesser speed reductions.
SummaryIf you are seeing 45Mbps differences, then I could believe such a value - with the cause split across these 3 aspects.
Overall, your worst case ought to be a cabinet 2km from the exchange, where the DSLAM is furthest from the PCP, and take-up is high on that tie-pair cable.
The best case, conversely, will be with side-by-side PCP and DSLAM, located outside the exchange OR at least 5km from the exchange, measured on the first line being installed.