Hi list, I am hoping someone here can explain to me (or point to an article that does) what is happening at the lower layers of an ADSL connection. This is an excerpt from the wiki page on ADSL (ANSI T1.413 Issue 2); "Up to 254 sub-carriers are used downstream; each of these 254 sub-carriers can support the modulation of 0 to 15 bits per baud. The baud rate is 4,000 symbols per second on each subcarrier. Thus the maximum theoretical downstream data rate of an ADSL system is 15.24 Mbit/s (254×15×4000). However, because the data is split up into packets (actually Reed–Solomon encoded codewords) of 255 bytes, the maximum achievable downstream data rate is 8.128 Mbit/s (including other overheads)." That is quite a drop in speed and I'm trying to understand where this is happening. Assuming a typical PPPoA set up, the ATM frames are 48 bytes of data payload and 5 header bytes, to make a total frame size of 53 bytes. Somewhere between the physical transfer rate at the bottom of the stack and this ATM layer, we are consuming all that bandwidth with other non-user data. Where is it going? According to that extract, it all disappeared because of RS encoding, which is hugely vague. Are ATM frames those used as the 68 data frames in a superframe? I understand that Reed-Solomon is splitting data into 255 byte codewords because an 8-bit symbol sized has been imposed to give a 32 byte parity block; Is the superframe where RS encoding is applied? If so, I don't understand how that consumes as much overhead as this statement claims. Could someone enlighten me as to what I'm missing between the ATM layer and the wire rate. Kind regards, James.