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Hii everybody ..

I have one doubt regarding the CRC and FCS.I think PHY is having separate CRC(16 bit) and for MAC we have FCS(that is also 32 bitCRC).
so when i capture frame using airopeek it says that
CRC error frame, so is that means frame with PHY CRC or the MAC FCS.

is FCS will be incorrect if we have CRC for PHY incorrect?

how the two are realated and how an analyzer says this frame is corrupted.

Plz help me in understanding the thing.

With thanks

Vinay Saini

vinay_tech4u,

You are correct that the PHY and MAC have separate CRCs (CRC-16 and CRC-32 respectively). The PHY CRC (PLCP CRC) protects the IEEE 802.11 SIGNAL, IEEE 802.11 SERVICE, and LENGTH fields. After the Physical Medium Dependent layer detects RF energy and/or begins demodulating data it detects the sync pattern and then receives the Signal, Service, and Length fields. Next it receives and and tests the CRC-16. If the CRC check fails then the PMD goes back to a receive idle state. Otherwise, it continues processing the frame- the MPDU bits are assembled into bytes and presented to the MAC.

The MAC Sublayer Management Entity is charged with validating the FCS (CRC-32) against MAC header and data.

Airopeek's documentation says that it does not capture the PLCP header. Airopeek begins a capture with the MAC header. So the CRC error indicates an invalid MAC level FCS.

Regards,

moe

Thanx Moe

I got ur point , thanx for such a nice explanation.

Vinay