Forum

Greetings,

Am using several of company X APs. I in digging through the radios and their configuration and in an effort to use the equipment in a theoretical/practical learning environment.

I have a question.

One of the configuration settings is:

Medium Reservation(RTS/CTS) which can be enabled or disabled. if enabled then the threshold can be set. the default is 2347.

What would be the impact on the system between enabling or disabling the setting?

As I say, this discussion is to help in my theoretical/practical learning experience.

Where I live there is virtually no-one that I can sit and have a face-to-face conversation with. Thus I depend on the these forums a great deal to add to the information that I am reading and researching. (I am working on my CWSP currently and would like to try to get CWAP prior to 12.31.06 but I have my doubts.

Thanks for the disscussion forums.

Hi Keith:

It can't hurt much to enable it. I suggest you take some throughput measurements before and after. You will probably see zero difference until you lower the threshold below 1500 octets. Even then the difference will be minor unless you are facing major sources of RF interference in which case your throughput may improve.

Then read up on RTS/CTS.

I hope this helps. Thanks. /criss

Keith,
I know that you work for (or have your own) WISP. As far as enabling RTS/CTS on the AP's, you won't want to do that.

The purpose of RTS/CTS is to "clear the road" per say, prior to data transmission. RTS/CTS is usually discussed when you have "hidden node" problem. Funny enough, a WISP is the worst possible hidden node, because the CPE on your homes will usually only be able to hear the AP, not each other, due to the directional antennas on each CPE.

So, since RTS/CTS is only really used when there is a hidden node problem, why am I not recommending on your AP's? Well, it is because of the flow of traffic. 90%+ of WISP traffic is coming from the AP to the CPE, not the other way around. Since ALL CPE devices can hear the AP, then there is no reason to implement RTS/CTS on the AP. Why would the AP have to notify everyone before it just sent its data?

Now, where RTS/CTS may be useful is on the CPE itself. Most WISP CPE bridges will have this setting. On the WISP's I consult for I haven't done this yet, but I am thinking about running some tests. The bum deal is that you have to change it on all CPE's on the AP to test it.

Keith, my suspicion is that you are having some problems on the network for you to be researching this type of thing. Is there anything I can do to help? I love Cali this time of year. :)

Hi Keith:

I agree with GT that RTS/CTS will probably not help your network performance. I understand your question to be literally, "What would be the impact?" of using RTS/CTS. And I read between the lines that you are interested in learning and at liberty to experiment. Maybe I have it all wrong.

Let's now address the question, "What good is RTS/CTS at an AP, when obviously all associated stations can hear the AP?"

The IEEE 802.11 standard defines how its components work, with little explanation of how they all work together. Most technical authors for good reasons explain the simple case of a single infrastructure Basic Service Set (BSS). Unfortunately many do not also explain the more complex and increasingly more common case of multiple BSSs occupying the same channel and volume of space.

RTS/CTS, like virtually all features of CSMA/CA, is designed to work across multiple BSSs occupying the same channel and volume of space. Your AP is likely out of range of (hidden from) a member of another BSS while at the same time that member is within range of one or more of your stations. In this case enabling RTS/CTS at your AP could help throughput as well as be educational -- maybe even fun.

I hope this helps. Thanks. /criss

I lack any experience with a WISP, but I do have a dissenting opinion on what my good friend GTHill wrote. He is certainly correct that Medium Reservation (RTS/CTS) is designed primarily for use with station (CPE) devices, but it has helped my network when enabled on the AP.

I live in downtown Los Angeles in a densely packed apartment complex. There are currently 11 nearby networks where I look at my available networks list, and when I run Omnipeek I see around 20 networks. Enabling RTS/CTS has really helped my performance on the 802.11g radio of my AP.

If there are a lot of other Wi-Fi networks in the same area that your WISP covers, you may want to consider enabling RTS/CTS by dropping it to a low threshold (I use a threshold of 256). This way if there is going to be a collision from a nearby network, the collision is more likely to happen with the short (20 byte) RTS frame rather than the long (up to about 1500 byte) data frame.

I can just tell you that in my case the 802.11g throughput increased from about 12-13 Mbps to about 15-16 Mbps after setting that RTS threshold on the AP. Again, everything Gene said about only using RTS/CTS on station (CPE) devices is correct in theory, but you may want to give it a try if there are a lot of other Wi-Fi networks nearby.

As Ben said, you aren't really going to hurt a lot by trying it. I would run a throughput test before and after, preferably on two different days of the week but during the same time, just to make sure usage is similar. If it works, great, if it doesn't, it is easy to change it back.

Let us know if you try it and what your results were. Thanks!