How to Limit Your Network Traffic on Ubuntu

First you should download wondershaper programme.

sudo apt-get install wondershaper


sudo wondershaper eth0 500 1000

Replace wlan0 with your interface, 500 with your downlink speed, and 1000 with your uplink speed. (Values are Kbps)

Check the status of wondershaper:

sudo wondershaper ifacename

Howto disable shaping on specified interface:

sudo wondershaper clear ifacename

Resource : http://www.ubuntu-unleashed.com/2008/05/howto-limit-uploaddownload-speeds-and.html