Pulse transformers, why?

Hi,

I'm putting together my own SPI based LAN interface using a cheap Microchip
ethernet chip.

Why are circuits interfaced to Ethernet have pulse transformers? What make
such transformers special? I saw one example schematic for a
pulsetransformer were it was grounded via 2000V capacitors. Is data
transmitted on an Ethernet's twisted-pair cables by generating high voltage
pulses to facilitate longer distance and higher speed because of increased
S/N or something? I tried to find voltage specs for Ethernet signals on
google but couldn't find any.

And then what about such transformers used on the phone lines? I figure they
are necessary to step down the "high" phone line voltages?

erik






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