Hello Sarah,

the limitation does not come from keeping the ability to detect collisions, at least not only from that.

You can model the host to switch port as a point to point link, but it uses multiple wires (important point also this).

So it becomes a multi wire waveguide.

Signal attenuation, differential delay on wires play a role too.

Ethernet is baseband and uses pulses. To transmit a bit a Manchester code is used it is not simply an on/off modulation.

The difference between bit 0 and bit 1 is associated to signal transitions during the symbol interval.

Actually 4 bits are sent inside 5 bits sequences using an additional bit to  detect errors. (4B/5B)

So on wire the speed is 125 Mbps not 100 Mbps.

when receiver fails to detect a symbol 4 bits are potentially errored in one single event.

noise and inter-symbol interference put a limit to the distance can be reached without receiving and regenerating the signal.

802.3z is fiber based and made of two unidirectional light waveguides = 2 fibers

802.3z is not baseband but an optical carrier identified by its wavelength is modulated by a Non return to zero modulation

8 bits are sent together into a 10 bit symbol

All this combined with the use of single mode fiber ( no dispersion of power over multiple modes) contributes to have a much greater reach.

you can read this tutorial

http://en.wikipedia.org/wiki/Manchester_code

http://en.wikipedia.org/wiki/4B5B

a more electronic point of view:

http://www.citap.com/documents/tcp-ip/tcpip021.htm

Little disgression:

once we connected printers to the parallel port and the cable used for this connection could not be very long for the same reason: a true serial communication can be faster then parallel communication. This was something that surprised me when I was still studying.

Hope to help

Giuseppe