How to increase ADSL line speed on a typical Telkom’s 4meg line from 2211 down stream to 3899.

Most routers will keep dropping line speed to get as close to 6db noise margin as possible. You can’t change this. Routers aim for a target noise margin of 6db (for our adsl, 12db for adsl2+) and alter the connection accordingly to reach this target via the SNR’s.

Attenuation is pretty important too yes, but according to this diagram (Accurate):

With G.DMT modulation (referred to as ADSL (up to 8Mbps) on the chart, maximum downstream speed are:
4.6Mbps when the downstream line attenuation is 51dB
3.8Mbps when the downstream line attenuation is 57dB

The attenuation is perfectly fine for a 4mb connection, just not capable of more. Using our copper wires, you take your attenuation, and divide it by 13.81 (because standard gauge copper (0.4mm) has a loss of approximately 13.81db/km).

So in this case being 3.4km’s away from the nearest exchange, confirmed with Telkom, the stats were pretty straight forward :
down up with filter
speed 2259 507
atten. 47 33.5
n/m 6.3 6.6

So the router is doing the right job and trying to get to a noise margin as close to 6 as possible but the speed is half of what it should be according to the attenuation. Now testing internals, doing an isolation test :
down up
speed 2315 507
atten. 47 33.5
n/m 6.1 9.4

In this case the phone socket was somewhat close to a stove and toaster and kettle, so turning off the wall socket for the kitchen and rebooting the modem resulted in the following :
down up
speed 3345 507
atten. 47 33.5
n/m 6.6 11

Instantly it was realised that the wiring creates a lot of noise on the line, so requested telkom to move the line out of my kitchen, putting the router far away from any electronical devices, and making sure that the 50″ telephone cable does not go past any speakers (NB), fridges, or electronic devices, resulting in :
down up
speed 3849 507
atten. 47 33.5
n/m 5.9 12.1