WAP Question

[killumanati]

I have a Linksys WirelessG WAP (802.11G 54mbits). There are 2 settings; RTS threshold & Fragmentation Threshold, these settings are on the WAP and on the NIC. What are the optimal settings for these. The values I can input for these 2 settings range from 256-2346. Thanks for any help...

 

[ktwebb]

From 802.11planet RTS and RTS threshold

"One of the best ways to determine if you should activate RTS/CTS is to monitor the wireless LAN for collisions. If you find a large number of collisions and the users are relatively far apart and likely out of range, then try enabling RTS/CTS on the applicable user wireless NICs. You can activate the function by clicking "enable RTS/CTS" somewhere in the user setup screens. You don't need to enable RTS/CTS at the access point in this case. After receiving a RTS frame from a user's radio NIC, the access point will always respond with a CTS frame.
Of course, keep in mind that user mobility can change the results. A highly mobile user may be hidden for a short period of time, perhaps when you perform the testing, then be closer to other stations most of the time. If collisions are occurring between users within range of each other, the problem may be the result of high network utilization or possibly RF interference.
After activating RTS/CTS, test to determine if the number of collisions is less and the resulting throughput is better. Because RTS/CTS introduces overhead, you should shut it off if you find a drop in throughput, even if you have fewer collisions. After all, the goal is to improve performance.
The method for enabling RTS/CTS on access points is different than with NICs. For access points, you enable RTS/CTS by setting a specific packet size threshold (0 -- 2347 bytes) in the user configuration interface. If the packet that the access point is transmitting is larger than the threshold, it will initiate the RTS/CTS function. If the packet size is equal to or less than the threshold, the access point will not kick off RTS/CTS. Most vendors recommend using a threshold of around 500. The use of 2347 bytes effectively disables RTS/CTS for the access point.
In most cases, initiating RTS/CTS in the access point is fruitless because the hidden station problem doesn't exist from the perspective of the access point. All stations having valid associations are within range and not hidden from the access point. Forcing the access point to implement the RTS/CTS handshake will significantly increase the overhead and reduce throughput. Focus on using RTS/CTS in the NICs to improve performance.

 

[ktwebb]

Also from 802.11 planet


<H3>Fragmentation Implementation Tips</H3>
The use of fragmentation can increase the reliability of frame transmissions. Because of sending smaller frames, collisions are much less likely to occur. The fragment size value can typically be set between 256 and 2,048 bytes. This value is user controllable. In fact, you activate fragmentation by setting a particular frame size threshold (in bytes). If the frame that the access point is transmitting is larger than the threshold, it will trigger the fragmentation function. If the packet size is equal to or less than the threshold, the access point will not use fragmentation. Of course, setting the threshold to the largest value (2,048 bytes) effectively disables fragmentation.
Similar to RTS/CTS, a good method to find out if you should activate fragmentation is to monitor the wireless LAN for collisions. If you find a relatively large number of collisions, then try using fragmentation. This can improve throughput if the fragmentation threshold is set just right.
If very few collisions (less than 5 percent) are occurring, then don't bother. The additional headers applied to each fragment would dramatically increase the overhead on the network, reducing throughput. That, you want to avoid.
If significant numbers of collisions are occurring, try setting the fragmentation threshold to around 1,000 bytes first, then tweak it until you find the best results. After invoking fragmentation, follow-up with some testing to determine if the number of collisions is less and the resulting throughput is better. You should try a different setting or discontinue using it altogether if the throughput drops (even if you have fewer collisions).
The use of 802.11 simulation tools (e.g., Opnet) can aid in determining optimum fragmentation threshold sizes, but you'll need to develop a simulation model that mimics your network. It will be difficult to accurately portray the RF interference on the actual network. As a result, fine-tune the actual WLAN in conjunction with real testing results.
As a final note, keep in mind that the use of RTS/CTS could be a better way to reduce collisions if hidden nodes are present. It's best to jointly consider the use of RTS/CTS and fragmentation before settling on which one to use. As with any 802.11 tuning mechanisms, the goal is to improve performance. If what you do improves throughput, then you're doing the right thing.