802.11n 20/40 MHz BSS Mode Operation

802.11n 20/40 MHz BSS Mode Operation

By CWNP On 08/28/2007 - 40 Comments

The 802.11n 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

 

Now how does an AP and station declare what they support in the way of channel widths?

An HT AP declares its channel width capability (20 MHz only or 20/40 MHz) in the Supported Channel Width Set subfield of the HT Capabilities element and its BSS channel width in the BSS Channel Width subfield in the HT Information element.  An HT client station declares its channel width capability (20 MHz capable or 20/40 MHz capable) in the Supported Channel Width Set in the HT Capabilities element.  A 20/40 MHz capable HT station, 20 MHz capable HT station, or non-HT station may be associated in a BSS with BSS Channel Width equal to 20/40 MHz.

If a station associates to a BSS using the 20 MHz channel width, how does it switch over to 20/40 MHz operation?

A non-AP HT station may switch between 20/40 MHz capable and 20 MHz capable operation by disassociation and association or reassociation.  20/40 MHz capable and 20 MHz capable HT stations must use the 20 MHz primary channel to transmit and receive 20 MHz HT frames.  The Notify Channel Width action frame may be used by a non-AP station to notify another station that the station wishes to receive frames in the indicated channel width.   

The PPDU Formats for Beacon transmission are defined below:

Band and BSS channel width                             PPDU Format for Beacon transmission
5 GHz band BSS 20/40 MHz                              Non-HT OFDM
5 GHz band BSS 20 MHz                                   Non-HT OFDM
2.4 GHz band BSS 20/40 MHz                           DSSS/CCK or non-HT OFDM.
2.4 GHz band BSS 20 MHz                                DSSS/CCK or non-HT OFDM.

The HT AP indicates protection requirements in the BSS through the "Operating Mode" field and "Non-greenfield STAs Present" field of the HT information element as well as the "Use Protection" field of the ERP information element.  

Phased Coexistence Operation (PCO) is an optional BSS mode with alternating 20 MHz and 40 MHz phases controlled by a PCO capable AP.  A PCO capable AP advertises its BSS as 20/40 MHz BSS Channel Width and PCO Capable.  A non-PCO capable station regards the BSS as a 20/40 MHz BSS and may associate with the BSS without regard to this field.
A station may associate with a PCO BSS as a PCO capable station by setting the PCO field in the HT Extended Capabilities element to 1.  If the PCO transition time of the station does not meet the time indicated by the PCO AP in the PCO Transition Time field, then the PCO capable station regards the BSS as a 20/40 MHz BSS and may associate with the BSS as a 20/40 MHz station.  We will talk about the details of PCO operation in another article.

What if we need to use DSSS/CCK transmissions with 40 MHz channel widths?  Keep in mind that OFDM (clause 17), ERP-OFDM (part of clause 19), and HT (clause 20) all use OFDM.

Support of DSSS/CCK in 40 MHz:

Transmission and reception of PPDUs using DSSS/CCK by 20/40 MHz capable HT stations is managed using the "DSSS/CCK Mode in 40 MHz" subfield of the HT Capabilities Info field.

If the "DSSS/CCK Mode in 40 MHz" subfield is set to 1 in Beacon and Probe Response frames, an associated HT station in a 20/40 MHz BSS may generate DSSS/CCK transmissions. If the subfield is set to 0:

— associated HT stations must not generate DSSS/CCK transmissions;
— the AP must not include an ERP information element in its Beacon and Probe Response transmissions;
— the AP must not include DSSS/CCK rates in the Supported Rates information element;
— the AP must refuse association requests from any station that includes DSSS/CCK rates in its Supported Rates information element;
— the AP must refuse association requests from an HT station that has the DSSS/CCK Mode in 40 MHz subfield set to 1;
— the Beacon must be transmitted using a non-HT OFDM PPDU.

An HT station declares its capability to use DSSS/CCK rates through the DSSS/CCK Mode in 40 MHz subfield of its Association and Reassociation Request transmissions.  If this subfield is set to 0, the station must not use DSSS/CCK rates.  If it is set to 1, the station may use DSSS/CCK rates.

Next up: 802.11n 20/40 MHz BSS Mode Rules.

Editor's Note: This post was originally published in August 2007 and has been updated for accuracy and comprehensiveness.

Blog Disclaimer: The opinions expressed within these blog posts are solely the author’s and do not reflect the opinions and beliefs of the Certitrek, CWNP or its affiliates.