I have a desktop computer that will connect to our network but is getting a large amount of frames received with CRC errors. What is best for fixing CRC errors? I'm using a PCI 802.11n card and our router is just downstairs.
Fixing CRC errors is not uncommon, especially as the distance between devices (like your PC and router) increases and the number of objects in between (like walls and floors) in your wireless router coverage area grows.
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802.11 is designed to overcome errors by retransmitting errored frames. As the rate of errors gets higher, 802.11 devices automatically adjust their data rate down to reduce error rate. This is kind of like speaking more slowly to someone who is having trouble hearing you. Note that what causes this rate adjustment is not the absolute count of errors (which always grows over time) but how frequently errors are occurring right now.
This is why, when you walk away from a router with a laptop, you see the data rate drop. Close to a router you may see the data rate reported as 300 Mbps. As you near the outer edge of the wireless router coverage area, that rate may fall to 15 Mbps before disconnecting altogether. (These data rates are for a 40 GHz channel - depending on devices and their configuration, you may see rates ranging down to 6.5 Mbps).
If your desktop's connection is reliable and fast enough for your purposes, don't worry about fixing CRC errors. If your PC's connection is too slow or too unreliable, you can try to improve things by (1) moving your PC closer to your router and decrease distance in your wireless router coverage area, (2) eliminating any obvious and unnecessary sources of interference around your PC (see below), (3) reconfiguring your router - for example, choosing a different channel or enabling 11a/g compatibility in case there are older devices nearby causing frequent errors.
Here's a link to a trouble-shooting article that might help you. Even if you don't have the tools mentioned here, at least you'll understand a bit more about possible sources of RF interference: http://searchnetworking.techtarget.com/tip/WLAN-troubleshooting-with-spectrum-analyzers. If you're using a 2.4 GHz channel (1-11), common culprits are microwave ovens, cordless telephones, Bluetooth peripherals, baby monitors, wireless video cameras - and 802.11networks owned by your neighbors. Moving to 5 GHz channel and/or letting your router automatically choose its own (best) channel is the easiest way to avoid most of these.
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