How do the lowest layers of the DSL stack work?

James Bensley jwbensley at
Sun Jul 1 19:01:26 UTC 2012

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,

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