I use DSLREPORTS.COM go to the FAQ section and search for "MTU" also if you are using a DSL modem and running it to a router consider a Netgear combination Modem/Router. I think they come in Cable and DSL versions. If you tweak the MTU, set it the same in all devices. If you pay for 3M ADSL and arent getting it, report the trouble. at&t is upgrading lots of the backhaul carriers this year at a rate I have never seen before.