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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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802.11 Fast BSS Transition (FT) Part 2 of 2

The IEEE 802.11r amendment introduces a new 3-tier AKM architecture and some new terminology such as Mobility Domain, Key Holders, RICs, and two tiers of Pairwise Master Keys (PMKs).  A Mobility Domain is a set of BSSs, within the same ESS, identified by a Mobility Domain Identifier (a numerical value).  Fast BSS Transition (FT) is not specified between Mobility Domains.  The definition of an authenticator is, under the new amendment, split into two pieces – each being responsible for certain tasks.  These two pieces are called the PMK-R0 Key Holder (R0KH) and the PMK-R1 Key Holder (R1KH).  These could, in many instances, be considered the WLAN controller (R0KH) and the lightweight AP (R1KH) though this is not a requirement of the amendment.

 

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802.11 Fast BSS Transition (FT) Part 1 of 2

The 802.11i amendment gave us Preauthentication and Pairwise Master Key (PMK) Caching.  Nothing fancy, just the basics.  Preauthentication enables supplicants (stations) to authenticate with authenticators (APs or WLAN controllers) to which they may roam.  Preauthentication always happens through the AP to which the station is currently associated – over the distribution system (typically an Ethernet network).

 

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Opportunistic PMK Caching - Complaints

Having just written a whitepaper on Fast BSS Transition, I decided to look into the nuances of configuring supplicants for Opportunistic PMK Caching (OPC).  Holy smokes batman - what a pain it is to find documentation on this.  Microsoft says that OPC is supported when you have the KB893357 update loaded: http://support.microsoft.com/kb/893357

 

 

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802.11n Primary and Secondary Channels

The 802.11n draft gives us 20 and 40 MHz wide channels.  This is no secret and has been widely publicized for months.  What hasn't been publicized is how 40 MHz channels work.  The draft explains the concepts of "primary" and "secondary" channels - each 20 MHz wide using OFDM modulation.  A Secondary Channel is defined as a 20 MHz channel associated with a primary channel used by HT stations for the purpose of creating a 40 MHz channel. Continue reading...

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802.11n Throughput Testing Methodology: Hopeless with Existing Equipment

I was a little perturbed at 802.11n when I first started testing it, and now, after discovering the truth, I'm just saddened. 

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Another Interframe Space: RIFS

RIFS (Reduced Interframe Spacing) is a means of reducing overhead and thereby increasing network efficiency.  RIFS may be used in place of SIFS to separate multiple HT format transmissions from a single transmitter when no SIFS-separated response transmission (like an ACK) is expected.  The RIFS is the time from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface.  The value of RIFS for the MIMO PHY is 2us.  

 

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Ramblings on Infrastructure Design

You hear all the time about how 802.11n is going to change everything - especially the design infrastructure.  I thought I'd ramble about that a bit.

 

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802.11n Protection Mechanisms: Part 2

Dual CTS bit in the HT IE:

When this feature is used, beacons in a BSS have the "Dual CTS Protection subfield" set to 1.  Stations will then start every TXOP with an RTS frame addressed to the AP.  The AP responds to this RTS with two CTS frames.  If the RTS is an STBC frame, then the first CTS is an STBC frame back to the station and the second CTS is a non-STBC frame back to the station.  This assures that all STBC and non-STBC stations receive the CTS and set their NAVs accordingly.  NAVs are set to cover the entire transmission process (as always), including both CTS transmissions (which is new).

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