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  • CWNP

Channel Planning with 40 MHz Channels

Let's not even consider 40 MHz channels in the 2.4 GHz ISM band.  There can only be two non-overlapping HT MIMO channels in the 2.4 GHz band.  Considering the mess of protection mechanisms, competing 20 MHz channels, and the dozens of interfering non-WiFi devices in that band, 802.11n in 2.4 GHz is a lost cause from day 1.  Instead, let's consider the only reasonable (IMHO) enterprise-class 802.11n deployment methodology: 5 GHz.

 

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Is 802.11n Worth the Money?

I'm thinking that most 802.11n deployments will initially be in the 5 GHz UNII band using 40 MHz channels.  Otherwise, why would anyone roll it out - right?  There are a few enhancements other than just throughput with 802.11n equipment, but you should consider if they will be enough to justify the costs.  Throughput enhancements will likely have to be a large part of a WLAN upgrade to 802.11n to get organizations to spend the money.  Starting with an assumption that using 40 MHz channels in 5 GHz bands will be the norm due to more available channels and less congestion in each channel, let's consider the ramifications of backwards compatibility with 802.11a devices. 

 

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802.11w - Management Frame Protection

The 802.11w amendment offers three new security pieces: Data Origin Authenticity, Replay Detection, and Robust Management Frame Protection.  

The data origin authenticity mechanism defines a means by which a station that receives a data or robust management frame can determine which station transmitted the data or management frame.  This feature is required in an RSNA to prevent one station from masquerading as a different station.  Data origin authenticity is only applicable to unicast data frames, or unicast Robust Management frames, and Deauthenticate or Disassociate frames with Robust Management protection.  The protocols do not guarantee data origin authenticity for broadcast/multicast (bc/mc) data frames or broadcast/multicast Robust Management frames.

 

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40 MHz Channelization

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.

 

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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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