How to use RTS/CTS?

[RicN]

Hello, I have some questions about the RTS/CTS (request to send/clear to send) feature.

If you have the answer for just one/some of them I would be very grateful!

1. On the Procurve networking products (AP420, 530 and WESM) can you configure a threshold value for frame size when the AP/WES should send a RTS before the frame, but am I correct when I think that this only affect the AP/WES behavior?

(Is the wireless stations still free to use RTS if they wish regardless the settings on the AP/WES?)

2. If I set the value real low, like 0, will this mean that EVERY frame will be "RTS:ed", that is - even ACK-frames and beacons and similar? Or is it only used for Data-frames?

3. Is it mandatory for wireless stations to understand a RTS frame and respond to it, i.e. is it a part of the 802.11 standard?

4. To work correctly, would it need to be set on both wireless stations and AP/WES? That is, to work in both directions, since the RTS is only sent when a frame shall be transmitted?

5. Is it common to actually use this feature?

6. If so, what threshold value should be a good recommendation to use?

7. If enabled, would I see better performance through lower collision rates or would the "gain" be destroyed by more frames be sent? (That is two more frames for each ordinary frame.)

[Manfred Arndt]

RTS/CTS is a mechanism to prevent collisions caused by a node that is hidden from the transmitter (but both nodes can hear the AP). It is most often used in outdoor deployments when using directional antennas.

1. The RTS threshold is a per station setting. Every station and AP can have a different threshold value.

2. RTS/CTS is only used for unicast data frames, when the packet size exceeds threshold. It does not affect management or control frames (e.g. beacons, ACKs, etc).

3. RTS/CTS support is mandatory, but setting the threshold is not supported on many clients.

4. It does not have to be set on all stations, since it is mandatory for a station to respond to an RTS request.

5. It is not common to turn this on in a well designed indoor Enterprise wireless network.

7. RTS/CTS performance depends on how much collisions a network has. But for most well designed networks, it is best to leave this alone. This is because it introduces a lot of additonal overhead and latency (2 additional control packets for every transmission) and as such reduces the maximum possible throughput.

[RicN]

Thank you for the answers!

>It is most often used in outdoor
>deployments when using directional antennas.

What is the reason for this? In a WDS bridge situation or with the APs operating normaly?

Does by the way any of the Procurve AP/RP models allow outdoors installment or is it just the antennas that could be mounted outside?

>4. It does not have to be set on all
>stations, since it is mandatory for a
>station to respond to an RTS request.

That is, so even if the AP/RP has the feature disabled (set to a very high value) different stations could still send RTS and the AP/RP will respond with CTS frames?

Is it always the would-be-reciever of the frame that should respond with the CTS? That is, if the AP/RP is configured to use RTS/CTS for all frames above for example 500 bytes, and it shall send a frame to station X, will the RTS frame itself be addressed to X and the X station will send the CTS?