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As I understand, a 10Gb Ethernet card is capable of putting 10Gb every second on (say) a fibre optics cable. Now naively, for this to happen in hardware, one will need a 10GHz clock running the network card.

It is possible to half that frequency by clocking on both edges, but 5GHz is still awefully high for transistors to support. For 100Gb Ethernet, 50GHz seems completely unreasonable.

What is the clock frequency of clocks running (say) a 10Gb Ethernet card? Are there tricks used to cut down this frequency from the "naive" 10GHz frequency?

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2 Answers

up vote 10 down vote accepted

You are correct that frequencies that high would be completely unmanagable. Sending one bit per frequency would cause problems for various types of radio transmissions as well. So we have modulation techniques which allow more than one bit to be send.

A touch of terminology: baud, most people will remember that term from the days of telephone modems, is the symbol rate at which a communications medium is operating. A symbol can contain more than one bit however.

1 GbE uses a symbol rate of 125 MHz (125 Mbaud), uses all 4 pairs for both directions at all time, and uses a trellis coding to enable 2 bits of data to be sent in each symbol (along with some error detecting information). Thus 125M/s * 4 * 2b = 1Gbps.

100MbE used the same symbol rate of 125MHz, which is why only a minor improvement was needed for the existing Cat 5 standard to carry 1Gb signals, specified as Cat5e

10GbE uses 500Mhz, 4 pairs, and codes 5 data bits into each symbol; 500M/s * 4 * 5 = 10Gbps. 40GbE and 100GbE have no copper standards at this time (there is a rudimentary draft that uses 1000 Mbaud, 4 pair, with 10 data bits and 25 data bits codings, respectively). There is also a proprietary "backplane" specification and non-standardized fiber specifications.

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Chris S already gave the correct answer: bauds, not bps.

But besides, 5GHz is not "awfully hight for transistors to support". There are teraherz transistors commercially available.

Of course, a GHz signal on a transmission line would be incredibly hard to shield from noise for more than a few millimeters. Optical signals, on the other hand....

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