cancel
Showing results for 
Search instead for 
Did you mean: 

some leftover maths

magnetism2772
Grafter
Posts: 983
Registered: 06-06-2010

some leftover maths

===========================part one==================================================
G.992.1 Full Rate ADSLor G.dmt  = 4000  times 15 (60Kb/s for each tone.)times 256
If all 256 tones are used in parallel, the total theoretical data rate can be as fast as 15.36Mb/s
and G.992.2, a lower data rate approach often called G.Lite
For G.Lite, only 8 bits are used per symbol with only half of the carrier tones used for a theoretical maximum data rate
of 4.096Mb/s.
4000  times 8 (32Kb/s for each tone.)times 128
If all 128 tones are used in parallel, the total theoretical data rate can be as fast as 4.096Mb/s
==============================================================================================

=================================part two===========================================================
The SNR Margin is the difference between the actual SNR and the SNR required to run at a specific speed. For example, if your line needs 35dB of SNR to run at 8Mbps, and the actual line SNR is 41dB, then the SNR Margin would be 6dB.
=====================================================================================================
the above is correct
but
the help i need is in two other parts
(part 1)
as each data bit represents a 3dB SNR (see part one  992.1 formula)
and  15 bits ::            a 45db sNR

then if my line SNR is 51 it can carry max number of bits, and has a safety SNR Margin of 6dB

one problem i see is that for every 3dB increase in the SNR Margin
it reduces  the number of bits that all my dsl buckets(or bins or  tones) will carry by 1 bit
each time
ie total bits per bin for G.dmt
15(bits) - (SNR)/3          dB
total bits per bin for G.lite(see part one G.992.2 formula)
8(bits) - (SNR) / 3
is (part 1) all correct

(part 2)
can i just add the line length dB (attenuation ) to the dB(SNR + SNR Margin) to calculate final bit allocation
to go on to formulate total dsl speeds  for both types?

what i'm doing is creating a max speed formula
9 REPLIES
magnetism2772
Grafter
Posts: 983
Registered: 06-06-2010

Re: some leftover maths

think it over during these cold nights and tia
------------------------------------part 3 ------------------------------------------
calculating power gains in dB  
example A
10 log (P1/P2)  where P1  = power 1 and P2 = power 2
if P1 is twice P2
then dB increase = 10 log (2/1)
dB = 10 times 0.3010299 = 3
so P1 times 2 = a 3dB increase
example B
now for the situation when P1 is four times P2
dB =   10 log(P1/P2)
dB = 10 log(4/1)
dB = 10 log 4
dB = 10 times 0.602059 = 6dB
example 3
now for the situation when P1 is 8 times P2
dB =   10 log(P1/P2)
dB = 10 log(8/1)
dB = 10 log 8
dB = 10 times 0.903089 = 9.0308dB
example 4
now for the situation when P1 is 16 times P2
dB = 10 log(P1/P2)
dB = 10 log(16/1)
dB = 10 log 16
dB = 10 times 1.204119 = 12.04119 dB

so 3db  +   3db  +   3db  +   3db = 12dB
=   2 times  2 times  2 times  2  = 16:1 increase
so knowing this
and without  involving any further maths
 3dB +    3dB  +   3dB   +  3dB  + 3dB = 15dB
= 2  times  2  times 2 times  2 times 2 = 32:1 (amplification)increase
checking using logs
10 log(P1/P2)
10 log(32/1)
10 times 1.505149 = 15dB to 2 decimal places

dB gains that are only fractional are preceeded by a minus sign
see the following
dB gain = 10 log (Pout/Pin) where Pout is Power Out and Pin is Power in
dB gain = 10 log (1/2)
dB gain = 10 log 0.5
dB gain = 10 times -0.301029
dB gain = -3.010dB = 3dB to 2 decimal places
and
-3dB +     -3dB   +    -3dB  +     -3dB  +   -3dB   = - 15dB
= 1/2 times 1/2 times  1/2 times 1/2 times 1/2    =   1/32   = 1:32

------------------------------------------part 3 is correct---------------------------------------------
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: some leftover maths

much easier to go to Kitz and but in a slot and get the answer out Roll eyes Roll eyes
magnetism2772
Grafter
Posts: 983
Registered: 06-06-2010

Re: some leftover maths

hi pierre  Smiley
i'm near but still quite far
198kHz
Seasoned Pro
Posts: 3,217
Thanks: 253
Fixes: 7
Registered: 30-07-2008

Re: some leftover maths

Can't argue with that  Wink
Not young enough to know everything
magnetism2772
Grafter
Posts: 983
Registered: 06-06-2010

Re: some leftover maths

i've just looked at my router stats
i've totaled   the line attenuation dB figure and the SNR margin dB figure by adding both together (attenuation (dB) + SNR Margin(dB)
then placed its  total as Downstream Attenuation in the Kitz max  speed calculator
noteSadwith a SNR margin it makes my line longer it places me a little further from the exchange )
and got ~my real download speed
coolio Smiley
magnetism2772
Grafter
Posts: 983
Registered: 06-06-2010

Re: some leftover maths

from part 1 and for G.992.1 full rate ADSL
a first attempt at an
estimated download speed  formula using  max bins and tones
and using graph to account for loss of  bits from signal attenuation
[4000  times 15 times 256] -[2000000 times signal attenuation /10] 
that needs testing
HuhHuhHuhHuh?Huh
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: some leftover maths

Einstein strikes again Cool Cool Huh Shocked Crazy
magnetism2772
Grafter
Posts: 983
Registered: 06-06-2010

Re: some leftover maths

Roll eyes
prichardson
Grafter
Posts: 1,503
Registered: 05-04-2007

Re: some leftover maths

Oh if only Me + Money = Rich
There are a number of factors to take into account. No line is perfect.
http://en.wikipedia.org/wiki/ITU_G.992.1
When POTS is present (as in the UK), we start at bin 6, bin 0 exists and 255 bins includes upstream as well (256 I presume includes bin 0, which cannot be used).
There are a number on bins between downstream and upstream which are typically not used anyway.
In the UK, AM radio accounts for a number of bins not being able to load up all the symbols in some bins.
One example is in the Ipswitch area. There are significantly more bins put to waste due to AM radio from the continent.
Oooh, don't forgot lone cross talk. Time to everybody else off in town, for the sake of your additional speed Wink