[altis]

Sir Robin doesn't agree with you there...

It has been verified by experiment that certain auto-negotiating ethernet cards (both in PCs and in Macs), when connected to certain models of cable modem, sense the ethernet duplex setting incorrectly. Such a cable modem has ethernet hardware which is capable of operating only in half-duplex mode, at 10 Mbps, yet some auto-negotiating ethernet cards incorrectly sense the cable modem as full-duplex. If an auto-negotiation card incorrectly starts operating in full-duplex mode, there can be collisions between ethernet packets being transmitted in both directions at the same time, leading to packet loss, and repeated re-transmissions. There is no visible evidence of this apart from poor performance. This problem can be avoided by manually configuring your ethernet card to be 10 Mbps half-duplex.

taken from:
http://homepage.ntlworld.com/robin.d.../ethernet.html