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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Bandwidth and Information Capacity of Cables
We may have noticed from Table 1.2 that CAT5e can support 1000Base-T (1 Gbps), even though it is only rated 100 MHz. We realize that we are using all four pairs of the 100 MHz cable for doing 1 Gbps, but we still can’t seem to explain this rather big jump to 1 Gbps. There seems to be no obvious math here. And does that mean 100 MHz is really not equivalent to 100 Mbps as often assumed? Yes, there is really no obvious relationship between bandwidth and maximum data rates.
Historically, especially when used for RF purposes, the usable bandwidth (maximum frequency range) of a cable was supposedly related to the relative attenuation of different sine-wave frequencies as they passed through the cable. ...

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