Hey im having airtel broadband at 128kbps unlimited download...when i check my downstream it shows

Downstream Upstream
Rate (Kbps) 576 576
SNR Margin (dB) 30.2 16.0
Attenuation (dB) 28.0 4.0
AttainableRate(Kbps) 11296 1380
Output Power (dBm) -6.3 0.3
Super Frames 387039 387037
Super Frame Errors 1 0

when i download any files it starts at 90 sometimes even 110kbps and gradulaly it decreases to 20kbps

how to download these at above 50kbps