A cable running upstairs to a desktop from the router is only carrying 100Mb/s Full Duplex, even when both endpoints are gigabit. When tested with a cable tester, wire 4 is blank and does not connect.
XDA Developers on MSN
Don't blame your router for slow speeds until you've tested your Ethernet cables
The physical layer is often at fault ...
Microcontroller-based circuits for testing cables, sometimes in conjunction with a PC, are easy to use and very flexible. For the hobbyist, however, the complication of such devices is not justified.
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