As I understand it, line attenuation is a characteristic of the line, measured under specified conditions, but signal attenuation is the actual attenuation measured by the router, under the conditions existing at the time of measurement. So the former should be fixed, but the latter can vary as signal conditions change.
Eric (Roseway) is 100% correct. The individual buckets/bins at each 4.3Khz intervals, will have varying degrees of Insertion Loss/SNR, dependant on conditions at that time. thus, giving varying 'Signal Attenuation' readings.
Line attenuation is taken at just one reading (300Khz), rather than over the whole array of bins/buckets that 'Signal attenuation' is taken from. IE- 256 Bins for ADSL1 ..... 512 Bins for ADSL2+ etc etc.
As already mooted, there is no need for concern whatsoever that the 2 readings are different.
this seems to be confusing me even further lol.. (which isnt hard)
... on mine DMT Tool says @ 300kHz = 21db - but my
Line Attenuation figure in the router is 26-27db (depending on router I use) - isn't the figure 'Line Attenuation' qouted in the router an
'average' of the Line Attenuation across all the tones? (rather than the LA at 300kHz.?) or is it something to do with ADSL2 having 512 tones as to why the LA is slightly higher?
with regards to the differences between the two, an easy way to explain for the OP
(I think) is think of the Line Attenuation figure a representation of your LINE CONDITION, and the Signal Attenuation a representation of the SIGNAL CONDITION over the LINE
- as your line doesnt change then niether should your LINE ATTENUATION , but your signal does vary and therefore so should the SIGNAL ATTENUATION figure ...sorry if I got that totally wrong, but thats my understanding of it