802.11n Protection Mechanisms: Part 1

802.11n Protection Mechanisms: Part 1

By CWNP On 08/10/2007 - 28 Comments

When 802.11g was introduced, we had RTS/CTS and CTS-to-Self protection mechanisms.  What do we get with 802.11n so that it's backwards compatible with 802.11a and 802.1b/g?  First, there's a couple of new things I'd like to introduce, and then we'll get to the protection rules.

In an ERP Beacon, ERP stations look at the "ERP Information Element" to determine whether or not protection is necessary in the BSS.  See my whitepaper here:
 http://www.cwnp.com/learning_center/search_details.php?doc_id=l7w2 for further information.

In an HT Beacon, HT stations use the "Operating Mode" and "Non-greenfield STAs Present" fields in the HT Information Element to determine whether or not to use protection.

Section I: Operating Mode

The Operating Mode field has 4 possible settings: 0-3.  I'll paraphrase what they entail, but keep in mind that some would call this over-simplification:
Mode 0: If all stations in the BSS are 20/40 MHz HT capable, or if the BSS is 20/40 MHz capable, or if all stations in the BSS are 20 MHz HT stations in a 20 MHz BSS
Mode 1: (called HT non-member protection mode) - used if there are non-HT stations or APs using the primary or secondary channels
Mode 2: if only HT stations are associated in the BSS and at least one 20 MHz HT station is associated.
Mode 3: (called non-HT mixed mode) - used if one or more non-HT stations are associated in the BSS.
From lab work, I've found that you can basically call mode 0 "pure mode" just like we used to have with just 802.11g.  If the 802.11n AP hears nearby APs and STAs that are non-HT capable, then it flips over to mode 1.  If a non-HT station associates to an 802.11n (HT) AP, it flips over to mode 3.  Mode 2 is for when all HT stations are associated (e.g. pure mode) and at least one 20 MHz slow-poke connects to the BSS.  Somebody smack that 20 MHz guy and tell him to get it in gear!  When non-HT stations leave the BSS, the AP, after a preset time, will switch back from mode 3 to mode 0.  The same is true of when the AP ceases to hear nearby non-HT stations.  It will switch from mode 1 back to mode 0.
When the Operating Mode field is set to 3, HT transmissions are protected.  The type of protection that is required depends on the type of transmission as well as the type of the non-HT STAs that are present in the BSS.
When using a 20 or 40 MHz HT channel, operating modes 1 or 3, and the Use_Protection field is 1 in the Beacon's ERP IE, all HT transmissions must be protected using RTS/CTS or CTS-to-Self sent at a clause 15 (DSSS) or clause 18 (HR/DSSS) rate.  This is a very normal scenario where 802.11b, 802.11g, and 802.11n are all being used together on the same AP.
When using a 20 or 40 MHz HT channel, operating mode 3, and the Use_Protection field is 0 in the Beacon's ERP IE (or the ERP IE isn't present in the beacon), the implementer gets a choice of how to protect the HT transmission:
a) Control frames such as RTS/CTS or CTS-to-self prior to the HT transmissions: 1) For 20 MHz transmissions, use the rates defined in Clause 17 or Clause 19 2) For 40 MHz transmissions, use the non-HT duplicate frames defined in Clause 20.
The rate for protection frames are selected based on the rules described in (Rate selection for control frames that initialize a TXOP) and (Rate selection for control frames that are not control response frames).
b) Using a non-HT preamble, transmit first a PPDU that requires a response frame.  The remaining TXOP following the first PPDU exchange may contain GF and/or RIFS sequences.
c) L-SIG TXOP protection (don't worry, we'll cover it in Part II)
d) Using a mixed format preamble, transmit first a PPDU that requires a response that is sent as a non-HT frame or non-HT duplicate frame.  The remaining TXOP following the first PPDU exchange may contain HT greenfield format and/or RIFS sequences.

Section II: Non-greenfield STAs Present

There are two kinds of HT stations: those capable of using greenfield format and those that are not.
The "Non-greenfield STAs Present" bit is set to 0 if all HT STAs that are associated are greenfield capable.  The same bit is set to 1 if one or more HT STAs that are not greenfield capable are associated.  HT APs use this bit to tell greenfield capable STAs to use protection when non-greenfield-capable STAs are present.  This bit only applies to HT stations since the entire HT IE is only processed by HT STAs.  Greenfield transmissions are to be protected by setting the duration/ID value in all frames sent during a TXOP to the time remaining in the TXOP.
Section III: OBSS Non-HT STAs Present bit in the HT IE
The OBSS Non-HT STAs Present field allows HT devices to report the presence of non-HT STAs that cannot interpret HT transmissions correctly.  A second HT AP that detects a first HT AP’s beacon with the "OBSS Non-HT STAs Present" field set to 1 may cause HT greenfield format and RIFS sequence transmissions of the second AP’s BSS to be protected by setting the Operating Mode field to 3 (HT mixed mode).
If the NonERP_Present bit in the ERP IE is set to 1 in the first AP’s beacon, then the Use_Protection bit may also be set to 1 by the second AP.  This is how ERP-OFDM transmissions are protected when DSSS and HR/DSSS stations are associated to an ERP AP.   HT stations may also scan for the presence of non-HT devices either autonomously or after the station's AP transmits a HT Information element with the Operating Mode field set to 1.  Non-HT devices may be detected as follows:
— one or more non-HT STAs are associated , or — a non-HT BSS is overlapping (a non-HT BSS may be detected by the reception of a Beacon where the supported rates only contain Clause 15, 17, 18 or 19 rates), or — reception of a management frame (excluding a Probe Request) where the supported rate set includes only Clause 15, 17, 18 and 19 rates, or — reception of a Beacon containing an HT Information element with the OBSS Non-HT STAs Present field set to 1.
When non-HT devices are detected, the STA may enable protection of its HT greenfield format and RIFS sequence transmissions.
In summary, separate fields and bits are used for status reporting (OBSS Non-HT STAs Present and NonERP_Present) and for protection (Operating Mode and Use_Protection) so that protection extends to the BSS directly detecting the non-HT STAs and to the nearby BSSs, but not beyond.  A first AP has non-HT STAs associated to its BSS or directly observes a non-HT BSS, and reports this status to its neighbors.  A second AP’s BSS overlaps the first AP’s BSS, and may respond to the status report by requiring protection.  The second AP does not relay the status report further.  A third AP’s BSS overlaps the second AP’s BSS but not the first, and so the third AP sees no report of non-HT STAs and so requires no protection.

Did you remember that this was just Part 1? ;-) Part 2 can be found here.

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

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