[SI-LIST] Length matching on Gb lines

Hi all
 
This is a question about matching Gb signal lengths in the primary-side and 
secondary-side of magnetics.
 
Spec sheets for PHYs usually have very tightly-matched lengths within a Gb port 
between the PHY and the magnetic.  A ethernet cable on the other side of the 
magnetic (in the outside world) can have huge length mismatching between the 
wires. 
 
Why is the primary PHY side of a magnetic more sensitive to length matching on 
the secondary ethernet side?  Is it due to the center-tap on the PHY side?
 
My next question would be:  how close do lines have to be matched within the 
diff pair and between diff pairs within the same port?
 
Any help or links would be great.
 
Regards,
Richard

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