How is 802.11n "backwards compatible" with 802.11b?

[Sam Brynes]

I am reading the CCNA Wireless official cert guide from Cisco by David Hucaby. The copyright date is 2014.

On page 56, it says that "802.11n was designed to be backward compatible with 802.11b, 802.11g, and 802.11a." How can this be true if 802.11b only supports DSSS, and 802.11n only supports OFDM? I am assuming that 802.11n only supports OFDM since appendix B on page 474 of the book only lists 20-MHz and 40-MHz channels, which are OFDM (since DSSS channels are 22-MHz wide)?

Don't two 802.11 wireless amendments need to be using the same modulation type to be able to communicate with each other?

[Rasika Nayanajith]

OFDM and DSSS are not compatible with each other, but they could coexist in same environment where 802.11g (OFDM) devices should use protection mechanism (eg RTC/CTS) when DSSS devices seen in the network.

Similar concept goes with 802.11n where they have to use protection IE in order to non-HT station to understand 802.11n frame is going over the air & wait for that transmission to finish, before they tx a frame.

Here is link for some of my notes on that topic which may help you.

http://mrncciew.com/2014/11/02/cwap-802-11-protection-mechanism/

HTH

Rasika

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[Sam Brynes]

Manannalage,

Thanks. I get what you're saying, which is why I posted the original question. If 802.11n devices cannot use DSSS to communicate with 802.11b clients, how can 802.11n be backwards compatible with 802.11b?