It's a question a couple of people have asked me now. It is even the case that Intel and Alcatel have suggested they may have a line-conditioning chip that will allow it over sub-10m distances, but my response is;
The problem become apparent when you consider that 10gig over cat7 runs at 600Mhz (strictly speaking you need 22Ghz to carry 10gig data - Nyquist limit and all that) but 10gigE uses QAM and OFDM modulation techniques to achieve this. Now, cat5e cable is flat'ish to 100Mhz and cat6 to 250Mhz.
By the time gigE came along it was cheap enough to incorporate a QAM16 modulator and OFDM encoder on the network card and so they could start getting away with sub-Nyquist bandwidths. 10gigE takes this to another level with QAM64 modulation (similar to aDSL and DVB-T) and a verterbie decoder. But, even then it really does need the 650Mhz of bandwidth to achieve 10gig speeds over 100m of cable - the guys at Tyco reckoned that doing 10 gigE over cat5e would only ever be feasible over sub-10m distances, even with heavy-duty line-conditioning chips.
The strength of an IP stack is that it will tolerate lots of line noise and packet failures - the network card may reports that it's seeing the 10gig heartbeat but if you're dropping half your packets is it worth it?
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