What impact does Annex M have on download sync speeds?
The point of Annex M mode is that because of the asymmetric nature of ADSL2+ speeds (i.e. that the downstream speed is so much higher than the upstream speed), each megabit you gain on upstream gains you more benefit (has a larger proportional impact) in overall terms than the megabit in speed you ‘lose’ on the downstream path. The approximate answer is that the theoretical trade-off is about 1:1 – i.e. whatever increase in upstream speed will result in a matching decrease in downstream speed. That said, there are reasons why the trade-off may be worse than 1:1. This means that you may see a greater drop in downstream sync speed than you gain in upstream speed. In practice, with some modems and lines, we have also observed an improvement in both download and upload speeds in Annex M mode, which in theoretically impossible, but which in practice is likely to be due to Annex M router firmware improvements compared to existing Annex A ADSL firmware. In a nutshell, if your speeds rise in b
