CTS Protection mode 101 and 102.

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stephensuley
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PostPosted: Mon May 04, 2009 17:54    Post subject: CTS Protection mode 101 and 102. Reply with quote
Would anyone liek to review this infomration about CTS Protection before I add it to the guide?


CTS Protection
The default value is set to Auto.


The help file says...
When set to Auto, a protection mechanism will ensure that your Wireless-B devices will connect to the Wireless-G router when many Wireless-G devices are present. However, performance of your Wireless-G devices may be decreased.


How it works:
CTS Protection mode is a is a protection mechanism that operates on the physical (PHY) level frame. In its default configuration dd-wrt uses it to provide a way of ensuring coexistence between the legacy and the new wifi devices. Adding to that by using CTS and modifying RTS threshold value you can tweak operation of the CTS protection mechanism this can then be combined with the Fragmentation Threshold tweak to help troubleshoot\fix connectivity and\or performance issues etc. Remember tweaking the CTS\RTS process and the Fragmentation Threshold value often comes at a price usually by decreasing the overall throughput to the WLAN.

At a very high level summary of the process when multiple devices are connected to an access point, they can occasionally be transmitting data to the access point at the same time because neither one can see the other client well enough to determine if it is transmitting on the channel or not. When this happens, the AP will discard both pieces of colliding data, thus contributing to error rates. CTS (clear to send) protection skirts this issue by delegating which device gets to transmit at a given time.

Once CTS \RTS Protection is configured correctly within your WLAN environment there are specific scenarios that occur and the software will activate the CTS\RTS protection mechanisms;

Here are a few examples of what triggers CTS to be employed by the dd-wrt router software.

CTS Protection trigger 1; - NON-default dd-wrt behaviour -This is the trigger that a user is impacting by changing the RTS threshold default to a lower value.
A client that wants to use the radio channel to send data packet(s) of a size that is equal or above the defined RTS threshold value.

CTS Protection trigger 2; -default dd-wrt behaviour
A 802.11g client attempts to connect to a SSID that is using channel bonding. Transmissions using a 40 MHz channel in the presence of 802.11a or 802.11g clients require using CTS protection mode. This will apply the CTS protection mechanism on both 20 MHz halves of the 40 MHz channel, to prevent interference with legacy devices and allow proper operation of the 802.11a and 802.11g clients.

CTS Protection trigger 3; -default dd-wrt behaviour
If you are running in mixed wireless mode on a 802.11n router and you have 802.11b clients in your environment. CTS Protection is used to allow the 802.11b client to operate correctly and also not to interfere with the operation of the 802.11a,g, and N client transmissions.
Also;
- An 11b device associates to the AP. - same as trigger 3
- An 11b AP on the same channel can be heard by the AP - variant on trigger 3
- The AP hears an 11g AP that is in protection because of an 11b device associated. -another variantion of the trigger 3 event.



**NOTE: Trigger 1 will never happen on the dd-wrt default configuration due to the the value of the RTS Threshold value being 2347 and the the Fragmentation Threshold value being 2346. Based on the data packet fragmentation threshold default value at 2346 in size dd-wrt will apply fragmentation to all packets. Becasue of this fragmentaion process the 2347 packet size needed to trigger the RTS threshold is never reached.


For a closer look at what happens in one of these cases lets look at a case like trigger number 1 list above and the steps in the CTS\RTS protection process.


example. A client that wants to use the radio channel to send data packet(s) of a size that is equal or above the defined RTS threshold value.


Steps in the CTS\RTS Protection process

1.) The client wishing to send date over the channel first sends an RTS (request to send) packet to the AP.

2.) In turn the AP responds with a beacon broadcast packet to the WLAN and within the beacon packet information it declares to all the clients on the entire WLAN that the specific client who sent the RTS packet as the "owner" of the channel and then the AP listens only to that client until it is done transmitting; its in protection mode.

3.)The process is repeated for all request to transfer data,which for whatever reason triggers the CTS\RTS protection mechanisms, on a first come first serve basis.


Implementing CTS\RTS Protection on top of the default dd-wrt configuration;

1.) The AP running dd-wrt has the CTS Protection mode is set to Auto by default and then you could adjust the RTS threshold value to something lower then 2346(Which is the default fragmentation threshold value on dd-wrt) on the AP.

2.) All clients connected to the dd-wrt AP are configured for CTS\RTS mode as apposed to disabled or CTS-Self mode.

***NOTE Typically the The RTS Threshold value on dd-wrt is lowered to address or troublehsoot some sort of connectivity or preformance issue with a client or all clients on a WLAN. Adjusting the value is a balenacing act between getting your problem fixed and lossing overall WLAN speed. The more times CTS procetion mode is triggered in a period of time "its frequency" the more impact it will have; good or bad. So start with 2340, then 2320, 2300 etc...


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-If your trying to tweak out every drop of preformance in an ideal setup then you can disable this on the AP and clients. Test again to see if it helps for better results, it should.

-If you want to try 40MHz with your 802.11n clients you might want to start with CTS Protection set to Auto. You do this incase you have 802.11a or g or even some N clients that do not support 40GHz transmissions or "channel bonding" as it is sometimes called.

-If you have a 802.11n based router running in mixed wireless mode that you want to connect 802.11b clients you need to have CTS protection mode set to Auto meaning enabled.

-If CTS Protection makes things faster a network redesign might be needed.

-"CTS to self" based protection - an alternate implementation method of CTS; where by the device willing to send frames over the WLAN first sends a CTS frame to itself. "CTS to self" based protection has less overhead, but it must be taken into account that this only protects against devices receiving CTS frame (e.g. if there are 2 "hidden" stations, there is no use for them to use "CTS to self" protection, because they will not be able to receive CTS sent by other station - in this case stations must use RTS/CTS so that other station knows not to transmit by seeing CTS transmitted by AP).If you have set the CTS protection mode to disabled on the dd-wrt AP, then this is a good choice for the client configuration.

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