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Which of these devices cause all-band interference in the 2.4 GHz ISM band? (Choose all that apply.)

A. Bluetooth

B. FHSS cordless phones

C. DSSS cordless phones

D. 802.11a access point

E. All of the above

I chose ABC. The correct answer is supposedly ABD, with the explanation: "All-band interference is typically associated with frequency hopping spread spectrum (FHSS) communications that can disrupt 2.4 GHz Wi-Fi radio communications. While hopping and dwelling, an FHSS device will transmit in sections of the frequency space occupied by an 802.11 a/b/g/n radio. The frame transmissions by the 802.11 a/b/g/n radios can be corrupted from the all-band transmission of the FHSS interfering devices. Bluetooth uses frequency hopping, and some cordless phones also use frequency hopping."

How is D one of the correct answers?? Is this another typo?

Thats what I said.

That's an error.

Was this question in one of the online practice tests?   There have been occaisions in the past where the test mechanism lost sync with the questions and the answers.

Yes, this was in one of the online practice tests.

All band interference is associated with FHSS technology *ONLY*. So, the only correct answers are Bluetooth (FHSS technology) and FHSS cordless phones. Rest all the answers are incorrect.