[Q}differences between 1000BASE-PX20 and BX20

ieee802.3-2005 1000base-px20-u

I read from IEEE802.3ah-2004. There are lots of standards, so are the receiver sensitivity and overload power values.

For example:1000BASE-PX20-U and 1000BASE-BX20-U have different value for receiver sensitivity and overload power.

Anybody knows the difference between them?

I tend to build a preamplifier, so anybody knows the guide value for receiver sensitivity and overload power value in IEEE802.3-2005?

I thought it was free because it is said that the standard would be released after six months once created. But so far it is June 2006, I still cannot download the IEEE802.3-2005

Added after 43 minutes:

I have got the idea for the BX and PX,
BX for bidirectional xxx
PX for passive optical network

U is for upstream, D is for downstream

Still curious about the lastest value for receiver sensitivity and overload power for 1000BASE-PX20 in IEEE802.3-2005, any one knows?