802.11n Protection Mechanisms: Part 2

802.11n Protection Mechanisms: Part 2

By CWNP On 08/13/2007 - 8 Comments

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).


CF-End Frames:

This frame type, previously unused due to a lack of PCF implementations, can now be used in contention environments as a NAV reset tool.  When Dual-CTS is enabled and a station doesn't have any data to transmit when it obtains a TXOP, the station can truncate (cut it short, thus giving back its remaining time) its TXOP by sending a CF-End frame.  When receiving a CF-End frame with its BSSID as the destination address, the AP may resond by sending dual CF-End frames - one using STBC, one using non-STBC.  This resets everyone's NAV in the BSS.  Does anyone else notice the fact that with these two mechanisms, you could cause an AP to DoS its own BSS? :-(

L-SIG TXOP protection ("PHY Layer Spoofing")

In OFDM frames (clause 17), the PPDU header consists of short training fields, long training fields, and a SIGNAL field.  Inside the SIGNAL field is the LENGTH subfield.  In a non-HT format frame, this subfield, called L_LENGTH in 802.11n, indicates the length of the PSDU in octets in the range 1-4095.  This value is used by the PHY to determine the number of octet transfers that occur between the MAC and the PHY.

When the 802.11n frame format is HT Mixed (keeping in mind that there are only HT Mixed format (HT_MF) and HT Greenfield format (HT_GF)), the LENGTH subfield is used along with RATE subfield to control the duration that non-HT STAs defer transmission equaling a period of time corresponding to the length of the HT PPDU (or the L-SIG Duration when L-SIG TXOP protection is used).  To restate, when using an HT Mixed format preamble, the Rate subfield in the Legacy OFDM Signal field (L-SIG) field of HT frame headers is always set to 6 Mb/s.  The Length subfield of the L-SIG field of HT frames with an HT Mixed format PHY preamble always contains a value that (together with the Rate subfield) represents a duration corresponding to the length of the rest of the PPDU, with a few exceptions.   This is "PHY Layer Spoofing."

An HT STA must indicate whether it supports L-SIG TXOP Protection in its L-SIG TXOP Protection Support capability field in Association Requests and Probe Responses.
The AP determines whether all HT stations associated with its BSS support L-SIG TXOP Protection and indicates this in the L-SIG TXOP Protection Full Support field of its HT Information Element.  This field is set to 1 only if the L-SIG TXOP Protection field is set to 1 by all HT station in the BSS.

A station is not allowed to transmit a frame using L-SIG TXOP Protection directed to a recipient that does not support L-SIG TXOP Protection.  When the station is associated with an AP, support at a recipient that is associated with the same AP is indicated if the L-SIG TXOP Protection Full Support field is set to 1 in the HT Information element broadcast in Beacons transmitted by the AP with which the stations are associated.  L-SIG TXOP support at the recipient may additionally be determined through examination of HT Capability elements exchanged during association.  

Under L-SIG TXOP Protection operation, the L-SIG field with an HT mixed format PHY preamble represents a duration value equivalent to the sum of:
a) the value of Duration/ID field contained in the MAC header, and
b) the duration remaining in the current packet (from the end of the symbol containing the L-SIG field to the end of the last symbol of the packet).

The maximum value of L_LENGTH is 4095 octets.

Non-HT stations are not able to receive any PPDU that starts during the L-SIG duration.  Therefore, no frame may be transmitted to a non-HT station during an L-SIG protected TXOP.  L-SIG TXOP Protection should not be used and the implementers of L-SIG TXOP Protection are advised to include a NAV based fallback mechanism, if it is determined that the mechanism fails to effectively suppress non-HT transmissions.  How this is determined is outside the scope of the standard.

The figure below (Example of L-SIG Duration Setting) illustrates an example of how L-SIG Durations are set when using L-SIG TXOP Protection.  An L-SIG TXOP protected sequence starts with an initial handshake, which is the exchange of two short frames (each inside a HT MM PPDU) that establish protection.  RTS/CTS is an example of this. Any initial frame exchange may be used that is valid for the start of a TXOP, provided the duration of the response frame within this sequence is predictable.  The term L-SIG TXOP protected sequence includes these initial frames and any subsequent frames transmitted within the protected duration.



An HT station is allowed to transmit a CF-End when the TXOP is not completely used by the TXOP owner, in a BSS whose beacon contains an HT Information element with the Operating Mode field set to 0.  This will reset the NAV at the HT station.  An HT STA using L-SIG TXOP protection should use an accurate prediction of the TXOP duration inside the Duration/ID field of the MAC header to avoid inefficient use of the channel capability.  If the initial frame handshake succeeds (i.e., upon reception of a response frame with L-SIG TXOP Protection addressed to the TXOP holder), all HT mixed format PPDUs transmitted inside an L-SIG TXOP Protection protected TXOP must contain an L-SIG Duration that extends to the endpoint indicated by the MAC Duration/ID field.

8 Responses to 802.11n Protection Mechanisms: Part 2

Subscribe by Email
ylq ylq Says:
06/15/2018 at 05:27am
On the way home Cheap Marlboro Cigarettes, we started to set off firecrackers. The elder brother said: "We put the cannon in the morning Newport Cigarettes Price, put the cannon in the afternoon, and fireworks and spring thunder in the evening."n the morning, my brother and I went to the river to play the artillery battle. Each of us had a box of guns, and both of us wiped it hard, rubbed it and threw it into the river, rubbing the cannon in the water with a bang Wholesale Cigarettes, and the splashes splashed with my brother.n the afternoon, my brother and I played in the sky Carton Of Cigarettes. I took the cannons upside down in the river Marlboro Cigarettes Price, hitting them with a little artillery, and only heard a sound of ��ju��. The sky cannons flew up into the sky. Then there was a loud noise of ��bang�� and the splashes in the river could be seen. . I saw my brother put a cannon gun on the surface of the water. He pointed it up and the cannon blasted the water in the river into a raindrop.n the evening, we put fireworks and spring thunder on the doorstep. My brother and I decided to put the fireworks along with the spring thunder. We put four fireworks on the soil together, and after a while, I saw a colored fireball that was one by one fired out of the barrel and flew to the sky. The air crashed and the night was very beautiful.

Ralph Raymond Says:
05/21/2018 at 14:55pm
Its right that organizations ought to have the flexibility to pick when and where information moves keeping in mind the end goal to enhance for execution, nimbleness, costs, recover ability, and consistence. However, I try to get rush my essay and its really good piece of content have this site.

crothermbeme crothermbeme Says:
04/18/2018 at 02:50am
On Monday,NFL jersesys shop US local time, the New England Patriots began their first voluntary activity in the offseason. Many media have focused their attention on the absence of quarterbackShop NFL Jerseys By Team Tom Brady and near-fielder Rob Gronkowski. However, the return of a key player deserves the same level of attention. He is the wide receiver - JulianBuy Customized NFL Jerseys Edelman. On Monday, Edelman appeared in the Gillette Stadium. This was his first step toward a $500,000 training bonus.Shop NFL Hats
The amount Shop NFL T-Shirtsof this bonus ranks first in the Patriot team. The following is the comparison of Edelman's total training bonus with other players in the team: Edelman:Shop NFL Hoodie $500,000 in Glonnowski: $250,000 Kickback Goalbacker Kodarel Patterson: $250,000 in defense Endpoints Lawrence Guy: $200,000 kicker Stephen Gestkowski: $100,000Pittsburgh Steelers Jerseys security guard Patrick Bell (Clock family): $85,000 In addition to the bonus, Edelman's appearance is also A positive signal was Minnesota Vikings Jerseysreleased: He was recovering systematically from the anterior cruciate ligament tear injury on August 25 last year. With Brandine Cooks Customized Seattle Seahawks Jerseysbeing traded by the team, Danny Amundola joined the Miami Dolphins again through the free agency market. As Brady’s numberCustomized Carolina Panthers Jerseys one goal, Edelman’s return to health in the new season is The Patriots' top priority. As a rule, Brady and Edelman hadIndianapolis Colts Hats practiced many passes during this offseason.
In addition, Edelman’s move Tampa Bay Buccaneersto Gillette Stadium on Monday was also a balance between strength training and Brady’s personal trainer Alex Guerrero. Edelman, who willSeattle Seahawks Hoodie soon turn 32, is a frequent guest of the TB12 Sports Therapy Center founded by Guerrero.
It is reported that Brady may reduceLos Angeles Chargers Hoodie the frequency of participating in the Patriots training during this offseason, and will pay more attention to his training withBaltimore Ravens T-shirts Guerrero. From the current situation, Edelman does not seem to have similar ideas with Brady. (TB12 is Tom Brady's English initialsAtlanta Falcons T-shirts and jersey number.) If you don't remember what Elderman's Patriots lost last season, perhaps the following three facts can remind Denver Broncos T-shirtsyou of Edelman’s Importance: 1. He collected data in 89 postseason games, ranking third in NFL history, second only to Jerry Rice's 151Jacksonville Jaguars Hoodie and Reggie Wayne's 93.
This also means that as long as Edelman completed Los Angeles Rams Hatsfive postseason catches, he will surpass Wayne and rank second in history. 2. He was one of 14 players who completed at least 1000 yardsCustomized New England Patriots Jerseys of NFL postseason advancement and ranked 13th in NFL history with 1024 yards of data. As long as Edelman finished San Francisco 49ers Jerseyscatching the ball in the 39-yard playoffs, he will be ranked in the top ten in history. 3. In his career, a total of four postseasonArizona Cardinals Jerseys games have completed single-field catches and advanced over 100 yards. In this data, he and Dion - Blanche together, ranked first in the history of the Patriots.

brock lesner Says:
03/07/2018 at 03:36am
If you have the time spend boring so get the latest fun zone here the most game,app and the movies just go the site here zxcvbnm,. and crater the full fun forever thanks for the share the information with your friends.

Rajavel Selvakumar Says:
08/07/2017 at 10:00am
Nice article.

When we have a HT STA and a Aonly STA in same channel connected to the same BSSID, HT device can use any one of the protection (as ERP IE will not present)
- RTS/CTS or
- CTS-Self or
- Using Non-HT preamble or
- L-SIG TXOP protection or
- Using a mixed format preamble

I have seen Broacomm STA use RTS-CTS mechanism and Qualcomm STA uses one other that RTS-CTS and CTS-Self.

How we can say using sniffer captures that the device use
- Using Non-HT preamble or
- L-SIG TXOP protection or
- Using a mixed format preamble

As far as i know sniffer will filter the pre-amble. Is there a way to find which protection mechanism is used?

Tom Carpenter Says:
08/27/2013 at 09:00am
Great question. The 802.11g radio must support the detection and processing of all preambles supported by earlier 802.11 PHYs operating in the 2.4 GHz band not using frequency hopping. This includes DSSS (1 and 2 Mbps) and HR/DSSS (5.5 and 11 Mbps). So that's step one - detecting the presence of an 802.11b communication on the same channel as the AP. This detection can be through the association with the 802.11g AP or simply detected on the medium as an 802.11b device communicating (remember, 802.11g radios can process the earlier PHY communication methods). Next, protection is implemented. This is signaled in the ERP information element of the Beacon frame. Basically, the Beacon frame screams to the BSS, "Hey! Everyone start using protection. I just heard one of those old guys on the network and he's a little slow!" Ok, that might be a mean way to talk about 802.11 and 802.11b clients, but it's reality :-)

Says:
08/22/2013 at 07:38am
How 802.11g networks find the presence of 802.11b networks. In which field of the packet indicates the presence of 11b network.


Regards,
Ram


Says:
11/02/2008 at 17:33pm
Hi Devin ,
i've found many different articles according protection modes of 11n but this one is the best in the net , especially for the L-SIG TXOP protection part. Thank you.

Cheers ,
Straight Edge

<< prev - comments page 1 of 1 - next >>

Leave a Reply

Please login or sign-up to add your comment.
Success Stories

I literally just came out of the testing centre having taken the CWDP exam. The certification process opened my mind to different techniques and solutions. This knowledge can only broaden your perspective. Great job, CWNP, you have a great thing going on here.

-Darren
Read More

Working through the CWNP coursework and certifications helped not only to deepen my technical knowledge and understanding, but also it boosted my confidence. The hard work it took to earn my CWNE has been rewarding in so many ways.

-Ben
Read More

I want to commend you and all at CWNP for having a great organization. You really 'raise the bar' on knowing Wi-Fi well. I have learned a ton of information that is helping my job experience and personal career goals, because of my CWAP/CWDP/CWSP studies. Kudos to all at CWNP.

-Glenn
Read More