It is true that Gig Ethernet is only defined as full duplex. In terms of fibre, it is clear why that should be - the transmit and receive paths are independent and point-to-point, so it is inherently full duplex. In terms of 1 Gbps copper, the paths are also independent - in fact the transmission and reception happen along all four pairs in both directions at the same time, due to a clever technique of balancing out your own transmitted signal.
Your 10/100/1000 ports, on the other hand, do support half-duplex, because half duplex is defined in the 100 and the 10 standards. It is defined in the 10 Mbps standard because at the time there were hubs rather than switches, and therefore the media were shared, so half duplex was a must. By the time 100 Mbps came along hubs were diappearing, but still supported. In fact, both 10 and 100 can be used in half-duplex (i,e, shared-media) mode, or full-duplex (point-to-point) mode.. Both 10 Mbps and 100 Mbps use only two pairs of the cable: one pair for transmit and one for receive.
The 10 Mbps copper standard uses a carrier of 20 MHz, while the 100 Mpbs uses a carrier of 125 Mz. The clever clever 1000 Mbps standard also uses a carrier of 125 MHz, and gets the requisite number of bits by using all four pairs in both directions at the same time, then doing some clever 5-state encoding on each pair. It also has autonegotiation of which pair does which job, so a cross-over cable and a straight-through cable are the same thing.
The CLI table ... the "a" means it is autonegotiating, and the 1000 is the actual speed the ends have agreed upon.
Hope this helps
Kevin Dorrell
Luxembourg
