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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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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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