When 40 MHz channels are used in 802.11n networks, two 20 MHz channels are bonded together.  The two 20 Mhz channels are designated as primary and secondary and are designated by two fields: (Primary, Secondary) where the Primary is the number of the primary channel and the Secondary is a positive or negative integer indicating whether the secondary channel is one channel above or one channel below the primary channel).  40 MHz channels MUST consist of immediately adjacent 20 MHz channels allowed within the regulatory domain.

Phased Coexistence Operation (PCO) is an optional coexistence mechanism in which an AP divides time into alternating 20 MHz and 40 MHz phases.   Although PCO improves throughput in some circumstances, PCO might also introduce jitter.

The 802.11n draft specifies two guard intervals: 400ns (short) and 800ns (long).  Support of the 400ns GI is optional for transmit and receive.  The purpose of a guard interval is to introduce immunity to propagation delays, echoes, and reflections to which digital data is normally very sensitive.

While it's often not a topic of discussion because EAP types are usually manually configured, supplicants and authentication servers can "negotiate" an EAP authentication protocol type.

In EAP, the initial portion of the frame exchange works like this:

EAPoL-Start (an optional frame that's almost always present) ..... Supplicant > Authenticator
EAPoL-Request/ID (The Authenticator requests the ID of the Supplicant) ..... Authenticator > Supplicant
EAPoL-Response/ID (The Supplicant sends either its real username or a bogus username) ..... Supplicant > Authenticator

With 802.11n certified devices popping up all over the place (most due to the Wi-Fi Alliance's new certification testing), how long will it be before 802.11n APs become rogues?  Well, that's already happened.  How do we detect them?  Fortunately, backwards compatibility is mandatory in 802.11n devices.  DSSS/CCK (when using 2.4 GHz) or clause 17 OFDM rates (when using 5 GHz) are used for Beacons when either 20 MHz mode or 20/40 MHz mode is used.  While Space-Time Block Coded (STBC) Beacons are supported (called Secondary Beacons), legacy Beacons still must be transmitted as the primary Beacon. 

With the introduction of Apple's iPhone (and all of those other converged cellular/Wi-Fi phones), use of public WLAN hotspots is about to massively increase.  Making wVoIP phone calls, instant messaging, browsing, email, and connecting to the corporate office over VPN are just a few things that users will be doing en mass shortly.  Certainly hotspots are already a pretty big deal - including those hotspots that aren't really meant to be hotspots - for staying connected.  But with the oh-so-sought-after Apple iPhone, all of those skype phones from SOHO vendors, Internet tablets like Nokia's N800, and now all of these new converged phones recently showing up in the market, hotspots are going to be busy busy.  Busy hotspots mean busy hackers.  It'll be tough for those guys though...you know, deciding between hacking your Wi-Fi phone, tablet PC, or laptop over your bluetooth connection, Wi-Fi connection, infrared port, or any number of other wireless interfaces.