More expert advice as ever from the Tzarina of Kitz
If you take a look-see at the ITU-T specs G.992.3 [1]... described within are the parameters used to determine the output power for each subcarrier or tone.
The output power for a tone should never fall outside the bounds of the spectral shaping mask (which is in place to reduce crosstalk amongst the pairs in a cable bundle). That mask which restrains output power is defined as a mathematical function - the transmitter spectral shaping function (
tss).
The modem at each end of the transmission line can only control the output power levels used for the subcarriers in its own upstream bands. Neither the DSLAM nor the CPE modem has control over the TX power levels used by the Far End. So that means that our CPE modems can only control the upstream power output levels.
There is a fine tune gain control that is applied by the modem's transmitter for each subcarrier. The gain for an individual tone can be tweaked by +/- 1dBm (as a linear equivalent).
However, the output power can never, or should never, be tweaked to exceed the bounds of the power spectral density (PSD) mask for the bandplan.
In theory it's possible to retrieve the 'gains table' from a Broadcom chipset modem. That table contains those fine tune gain levels that are being applied to each subcarrier.
Here are all the diagnostic data tonemaps used by the Broadcom 6348/6358 chipset modems. The gains table is stored as an array of shorts (16-bit integers). The full source code for the Broadcom kernel driver is here [2]
/* ADSL diag data */
short snr[kAdslMibMaxToneNum];
short showtimeMargin[kAdslMibMaxToneNum];
uchar bitAlloc[kAdslMibMaxToneNum];
short gain[kAdslMibMaxToneNum];
ComplexShort chanCharLin[kAdslMibMaxToneNum];
short chanCharLog[kAdslMibMaxToneNum];
short quietLineNoise[kAdslMibMaxToneNum];
The xdsl diagnostic tool in the firmware of most Broadcom modems (known variously as the xdslcmd, adslctl or adslcfg tool) is not equipped to get the gains table data from the kernel driver. However, it is probably still possible to retrieve that data by hook or by crook.
One quirk of the Broadcom chipsets relates to output power. With the BCM6368 - the latest xDSL Broadcom chipset - the aggregate output power for the downstream bands often drops momentarily to zero for no obvious reason, before returning to a sane value of 19dBm or thereabouts.
Incidentally, you have very tasty looking limbs, kezzaman. Do you strictly need them all?!
cheers, a
[1]
http://www.analytic.ru/articles/lib26.pdf[2]
https://forum.openwrt.org/viewtopic.php?pid=120035