What is "Down Poll Rate", "Rx Checksum Offload" and "Tx Checksum Offload"?

Hi Everyone!

I've got a 3Com 3C920 NIC based PCI Ethernet Card.
When you double click on "3Com 3C920 Integrated Fast
Ethernet Controller (3C905C-TX)" i.e the name of your
network card that is listed on the "Configuration" tab on
the Network properties window in Windows 98 SE, and go to
the "Advanced" tab, you see a lot of entries in
the "Property:" box. I want to know what the following
properties mean.

1. Down Poll Rate
2. Rx Checksum Offload
3. Tx Checksum Offload

I presume that "Rx Checksum Offload" and "Tx Checksum
Offload" means offloading the FCS CRC generation/checking
for every Ethernet frame to the system processor.

If this is so, is(are) there any particular use(s) of
offloading the CRC computations to the main processor?
Is there any real advantage at all?

Warm Regards,
Jerry