The ideal amount is 0 packet CRC errors.
I am yet to see a real radio link with 0 CRC errors though.
There is no hard and fast rule as to what is aceptable.
I look for 2 things when looking at CRC errors.
1. The % of packet CRC errors compared to the Input packets, I use the same rule of thumb that you use for a serial line so look for less than 1% of the total input.
2. As the counters are incremental you may have a transient interference issue that will still fall in below the <1% rule so look at the rate of change. If you are seeing the CRC errors increment at a quick rate then you have a problem that needs to be address.
Something to remember with CRC errors is that they are a packet that will be discarded at a hardware level and as such TCP will have to retransmit this packet, UDP will see that packet lost forever. If you have a lot of CRC errors in TCP traffic then your link utilization may also go up due to the retransmitted packets compounding the issue. This is the same on a serial line, it is not unquie to WLAN.
At the end of the day there is no hard and fast rules, if you are happy with the performance of the link then it is ok, and you may have to take into account the results of the site survey, on some links due to length or the environment you may just have to live with CRC errors, but do everything you can minimise them
David
