With the various DSL flavours such as ADSL2+, which allows the PHY line to re-sync as conditions worsen and improve at higher and lower sync rates (which is effectively Tx and Rx rate), does the MTU size ever change? I know that it is using fixed size ATM frames here, but will the MTU size ever drop below the minimum ATM frame size?
Also, what about Wireless (802.11)? As the wireless signal becomes weaker and the error rate increases, is the MTU ever shrunk so smaller frames are sent?
I understand that at an the IP level the window size can be shrunk/grown as with the MSS at the TCP level for example. But what about at OSI layer 2, using Ethernet and ATM for example, are there any scenarios where the MTU size would ever change over the course of a link's life? I don't mean over 20 years either, I mean within a reasonable time span, such as days or weeks?