Wireless Access

Hello All,

Is it expected to see RTS frames initiated by the AP towards station (barcode scanner)? I am troubleshooting wireless connectivity and in monitor mode PCAP I am seeing a flood of the RTS frames initiated by the AP:

Thanks,

Myky

It sounds like the BSS is operating with 802.11b protection mode in effect. What are the PHY types for the 2.4GHz clients? Do you have a mix of 11n and 11g/n clients on the radio?

Hello Charlie,

Thanks for your response. I was testing only with one client (scanner), and one AP (with no other clients connected). So the scanner connects fine and stays connected. It then initiates the TELNET session to the remote server, works fine for ~1 minute. Same time l am running continuous ping from the AP against scanner`s IP address. Suddenly TELNET connection drops to the server (SYN, ACK retransmit packets are seeing on the LAN interface of the AP, sent from the TELNET server). Taking monitor PCAP, can see tons of the Request to Send frames initiated from the AP towards scanner MAC address and connection never restores itself until scanner reboot. The same issue happens again (loop). What is 802.11b protection mode?

Thanks,

Myky

Because 802.11b and 802.11g use different modulation techniques (DSSS vs OFDM), an 802.11g BSS will enable protection mechanisms if an 802.11b client is heard in order to ensure the legacy client is aware of the transmissions that it can't decode.

While testing a single client with an AP can be helpful, because 802.11 wifi is shared medium, that testing also needs to understand any other rf activity taking place at the same time.

Hello Charlie,

Thanks again for your explanation. Do you think it is expected behaviour to see those frames?

Thanks,

Myky

Hi,

That sounds a bit strange. APs usually generate a CTS-to-self frame (when RTS/CTS mechanism is used). I wonder if you could check the following:

- 802.11 standards supported by the scanner

- frame size. If the frame is larger than a certain size, RTS/CTS will be enabled

Kevin

Hello Kevin, 

It is odd, l agree.

Will get all tech specs of the device, but this is what I am seeing when the device stops responding to the ping and hangs:

Thanks,

Myky

Fully agree, it is odd.

I wonder what would happen if you disable the 802.11g data rates (6, 9, 12, 18, 24, 36, 48, and 54 Mbps) from the Mandatory rates in order to force 802.11n (if the scanner supports it)

Furthermore, do you think that there are 802.11b clients nearby? This would also trigger the protection mechanism (even if they are not connected to your network)

Kevin

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@Myky wrote:

 

Thanks again for your explanation. Do you think it is expected behaviour to see those frames?

 

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Yes, expected behavior if legacy devices are heard in the spectrum.

Thanks! l think my knowledge of wifi is not that deep to understand this :D Do you have any good KB explaining this process? If l disable "b" data rates, will it stop AP to use protection mechanism?

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@Myky wrote:

Thanks! l think my knowledge of wifi is not that deep to understand this :D Do you have any good KB explaining this process? If l disable "b" data rates, will it stop AP to use protection mechanism?

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Check out this earlier Airheads post for more information:

https://community.arubanetworks.com/t5/Controller-Based-WLANs/How-to-control-access-points-from-enabling-protection-against/ta-p/179892

You can get rid of 802.11b clients by disabling the rates of 1, 2, 5.5, and 11 Mbps from the list of Mandatory rates. The following two articles are a good read about this topic (at least in my opinion). The give a good overview about overhead and how to reduce it:

- https://www.cwnp.com/wi-fi-overhead-part-1-sources-of-overhead

- https://www.cwnp.com/wi-fi-overhead-part-2-solutions-to-overhead

HTH

Kevin