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05 Jun 10 Improving WiFi Performance

I have recently started to study for my CWNA and I will be posting some articles, tips, tricks, some explanations on my blog from my personal experience, research and study. Please feel free to comment, I always welcome positive criticism. If any of my work helps you in any way, I’ll be happy about that. If you have something to add here please do let me know. Lets help each other out. cheers

Fragmentation

Weather it is “a/b/g/n” fragmentation and RTS/CTS settings are important to maximize and improve the Wireless connectivity. If there are collisions on the network and there are performance issues fragmentation can actually improve the performance. As we all know that WiFi is a shared medium. So if the packet size is smaller that will lead to less collisions because AP’s can process smaller packets faster as compared to larger packets and time slots to process packets from multiple clients will be smaller hence leading to less collisions. Key thing to remember here is since the packet size is smaller it is more likely that the whole packet is processed before the next transmission came through, which means there is was no collision and no re transmission of frames is needed. However keep in mind that as long as the collisions are not happening that rapidly and they are very low on a scale of 1 to 10, lets just say they are 0.5 (approximately) there is no need to change the settings.

RTS/CTS (Hidden Node)

In general a wireless client is considered a hidden node if it can see the AP but none of the other clients can see it and realize when it is transmitting. Please take a quick look at the attached PDF. As you can see that Client 3 is able to barely communicate with the AP however it is unable to see Client 1 and 2 and vice versa. Therefore if “Client 1 and 2” transmit or “Client 3” transmit they will not know about that transmission hence each one of them will try to transmit at the same time leading to collisions.

Hidden Node Issue

This is where RTS/CTS (Request to send/Clear to Send) comes in handy. With RTS/CTS Client 3 will send RTS frame to the AP and if there is another transmission going on, AP will notify Client 3 not to send data yet sending a CTS frame which will include the time value telling Client 3 to hold off from transmitting. End result, collision will be avoided. RTS/CTS does increase the overhead as there are more RTS/CTS frames so it is critical to only implement it and change settings (lower settings) when its really needed.

RTS/CTS (Hidden AP)

Secondly we have another scenario to deal with “hidden AP” problem. No matter what you do you will have this problem as you can not control the Wireless Signal perfectly. Please take a look at the attached PDF for a quick overview:

Hidden AP issue

As you can see from the picture that the two AP’s aren’t able to see each other even though their signal overlaps because they are to far away. Now imagine if “AP1” transmits to “Station 2” and at the same time “AP2” transmits to “Station 4”. Since neither of the AP’s can see each other and know that there is already a transmission in place data coming from “AP1” to “station 2” will collide with the data from “AP2” to “station 4” (remember when the signal gets transmitted from the AP it gets transmitted in all directions) and there will be a collision.

With proper “RTS/CTS” and fragmentation settings in place this can be avoided. How? Lets see:

  • AP1 sends an RTS packet to st2
  • st2 sends an RTS packet to st4
  • st4 replies with a CTS to st2 (tells st2 that I am transmitting to AP2)
  • st2 sends a CTS packet to AP1 (tells AP1 that wireless medium is not clear)

I really believe that doing at least a hybrid site survey is a good idea and choosing the right equipment is very important for a reliable wireless deployment. Check out the video below, it is an excellent video explaining everything clearly. Thanks to this video I was able to grasp the concept of RTS/CTS, hidden node and AP clearly.

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