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  • CWNP

PCO Operation Overview

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.

 

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802.11n Guard Intervals (GI)

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.

 

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EAP NAK

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

 

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Can My Sniffer Smell 802.11n?

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. 

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Hotspots for Hackers

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.

 

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802.11n 20/40 MHz BSS Mode Rules

Rules for operation in 20/40 MHz BSS:

A 20/40 capable station operating in 20 MHz mode follows the rules for a 20 MHz capable station.  A 20/40 capable station is allowed to operate under Phased Coexistance Operation (PCO) where the AP switches back and forth between 20 MHz and 40 MHz operation.  Indication of the switching of channel width is done in Beacons, and a 20/40 capable station is allowed to use L-SIG TXOP protection.

 

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802.11n 20/40 MHz BSS Mode Operation

The 802.11n draft gives us 20/40 MHz BSS mode, which my Apple Extreme Basestation supports in the 5 GHz band.  Oh, can't you feel the joy welling up inside you at the thoughts of this.  First, let's get some definitions out of the way.

20/40 MHz Operation:

The following terms are used to describe transmitted PPDU formats:

"40 MHz HT" is a Clause 20 transmission using HT Mixed Mode Format (HT_MF) or HT Greenfield Format (HT_GF) frame formats and 40 MHz channel bandwidth

"20 MHz HT" is a Clause 20 transmission using HT Mixed Mode Format (HT_MF) or HT Greenfield Format (HT_GF) frame formats and 20 MHz channel bandwidth

"DSSS/CCK" is a Clause 15 or Clause 18 transmission

 

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Reverse Direction (RD) Protocol

The purpose of the 802.11n RD protocol is to more efficiently transfer data between two 802.11 devices during a TXOP by eliminating the need for either device to initiate a new data transfer.  Before the RD protocol, each uni-directional data transfer required the initiating station to capture (and possibly reserve time on) a contention-based RF medium.  With RD, once the transmitting station has obtained a TXOP, it may essentially grant permission to the other station to send information back during its TXOP.  This requires that two roles be defined: RD iniator and RD responder.  The RD initiator sends its permission to the RD responder using a Reverse Direction Grant (RDG) in the RDG/More PPDU field of the HT Control field in the MAC frame.  This bit is used by the RD initator for granting permission (RDG) to the RD responder, and it is used by the RD responder to signal whether or not it is sending more frames immediately following the one just received (More PPDU).  For a more technical walk-through of this functionality, here are some excerpts from the 802.11n-draft2.00 amendment with my occassional input inserted.

 

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802.11 PPDU Formats

There are three overall PPDU structures possible in an 802.11n network, one of which was previously defined by Clause 17.

 

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The Certification Game

There is a growing list of Wi-Fi industry organizations and vendors that have developed hardware and software certifications.  Let's take a look at some of them.

 

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