Someome pointed me to this info from the cable modem maker ...

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Q. Why do the RD and TD lights on my modem flash, even when my computer is off?

A. The flashing is caused by ARP (Address Resolution Protocol) packets sent out by the BPA routers. ARP is used to map the MAC address (which uniquely identifies your network card) to your IP address. The packets cost nothing (for users on volume plans) and do no harm.

RD stands for 'receive data'; TD stands for 'transmit data'.
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Doug M