How is the upload speed of my ADSL connection measured?

To perform an ADSL upload test, we use a default amount of UDP packets of uniform size at a rate slightly higher than the nominal tested ADSL. In order to perform a test with a slightly higher rate than the one that is recommended by the default, it is necessary to saturate the broadband test.
The processes in our servers dedicated to test ADLS speed, return an echo of the packets sent as they reach the server. The time difference, T, is measured between the arrival from the first to the last packet the user receives.