Quite some time ago, following a high number of connection stability and vastly reduced speed problems (regular DS sync speeds as low as 12Mb etc.), I started looking at trying to calculate typical VDSL2 line lengths from known attenuation values & typical attenuation values from known line lengths.
This was with a view to seeing how my connection then stacked up against a "typical theoretical" VDSL2 connection.
i.e. to determine whether or not my own connection now had "faults" that could & should be repaired, or indeed whether or not my expectations were simply too high.
As extensively documented elsewhere, my connection was able to maintain DS throughput speeds of 32Mb to 33Mb for the first month.
The sudden trigger for the vastly reduced performance appeared to be some "unknown" event that caused me to completely lose telephone & FTTC services for a 4 day period.
My previous higher speeds & apparently stable connection have never returned.
I forget where I obtained it now, but the attached document has some basic guidance:-
"For VDSL2 systems that transmit over normal polyethylene-insulated pairs, there is a good approximation of the attenuation A (in dB) at a certain frequency f:
A = k * l * sqrt(f), where k is a wire-dependant constant and l is the length of the wire.
The available spectrum increases distinctly the closer you are to the end user. Whereas the ADSL2 standard (ITU G.992.3) is designed for longer distances (up to approximately 6km, working up to 2MHz), the VDSL2 standard is designed for shorter distances (up to approximately 2km, working up to 30MHz)."Using my own connection's "engineer reported" details & assuming an overall frequency of 4.15 MHz for the VDSL2 8c profile & a constant cable attenuation of 13.81dB per km we could calculate these examples:-
Attenuation (A) - from a Reported Line length of 0.82km:A = 13.81 * 0.82 * 2.04
A = 23.1dB (approx)
At one time, an engineer's JDSU reported my attenuation as 30.2dB (a single value for the 8c profile).
Line length (l) - from a Reported attenuation of 30.2dB30.2 = 13.81 * l * 2.04
1/l = 13.81 * 2.04 / 30.2
l = 1.08km (approx)
Now, for VDSL2 connections, user modems do not report a single attenuation value, rather they report a value for the D1 band, another value for the D2 band, & since the introduction of the 17a profile, yet another value for the higher frequency D3 band.
My modem currently reports:-
D1 = 21.7dB
D2 = 62.9dB
D3 = N/A or 0.1dB
The latest engineer's JDSU recently reported a single attenuation value of approximately 22dB for my 17a profile connection.
It also reported a single attenuation value of approximately 22dB for my 8c profile connection.
At this stage we aren't sure how a JDSU calculates a single attenuation value from the various frequency band attenuation values, but it looks suspiciously like it may have only picked up the value from the lower band.
It surely cannot be reporting an average value across the 3 bands as the maths just don't add up.
If I use the JDSU's single attenuation value of 22 dB to estimate my line length from the formula above, I get a line length of 0.78km.
However, at the time the JDSU's single attenuation value was reported as 22 dB, the JDSU also reported my line length as 1.0km
Due to these discrepancies we could make a number of possible assumptions:-
a) JDSUs are inaccurate
or
b) The formula isn't adequate
or both statements could apply
All the above was based upon the 8c profile.
As the 17a profile uses even higher frequencies than the 8c profile, attenuation values will increase accordingly.
The question "Why does any of this actually matter?" could be posed.
I see it as an aid to troubleshooting either connections that exhibit poor speeds from the outset, and/or an aid to troubleshooting connections that initially exhibited good speeds that have deteriotated.
If we have reasonably accurate ball park figures to work with, we are part way toward understanding what speeds could be reasonably expected for various line lengths etc.
Conversely, we could identify issues such as poor speeds on shorter lines.
Unfortunately I do not have any details of my connection's attenuation values from when its speed performance was much "better".
The simple fact that speeds had deteriorated for a prolonged period suggests (to me) that my connection's current attenuation values must be somewhat higher than they were initially.
I believe my connection is incorrectly being calculated as being 1000m or so in length (possibly based upon attenuation values), thus higher speeds cannot be expected.
If my connection really is closer to 800m or so, maybe I should still expect to achieve the previous higher speeds.
If that is the case, what has caused attenuation values to increase, & perhaps more importantly, what (if anything) could/should be done about it?
From the evidence I have seen in various forum posts, it appears that almost ALL other users who were previously achieving around 33Mb or so DS throughput on the 8c profile are now seeing DS throughput speeds approaching 40Mb since the switch to the 17a profile.
I saw a marginal speed improvement when I was switched to the 17a profile, but I am currently only able to achieve between 26Mb & 27Mb throughput at best (some 13Mb or 14Mb less than the 40Mb cap).
Even when taking typical losses into account, I am still achieving 10Mb or so less than maximum throughput speeds from the 40Mb service.
Thank you
asbokid for posting links to various documents in another thread (
http://forum.kitz.co.uk/index.php/topic,10547.msg209077.html#msg209077 )
As soon as time permits, I shall study those documents to hopefully acquire a better understanding of these matters, but I would really appreciate any comments/suggestions from anyone who already has a good understanding, even if it is to completely discredit my various calculations & assumptions by providing more accurate & factual detail.
I do accept that a lot more needs to be taken into account than attenuation values (noise, interference, error counts etc.), but if attenuation values are already artificially too high for any given connection, do other issues perhaps have to take a back seat?
e.g. If attenuation values are higher than expected, could that in itself cause high error rates as the transmitted data simply cannot be received at the same rate that it is being transmitted?
We are all aware that high error counts (amongst other things) can cause DLM to take avoidance action by adding/increasing Interleaving depth, reducing sync speeds etc.
Paul.
Author
Topic: FTTC - Line Length and Attenuation Calculations (Read 46941 times)