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Occasional Contributor I

Request to Send (RTS) frames are sent by the AP towards station (scanner)

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:
RTS_png.JPG

Thanks,

Myky

Aruba Employee

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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?


Charlie Clemmer
Aruba Customer Engineering
Occasional Contributor I

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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

Aruba Employee

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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.


Charlie Clemmer
Aruba Customer Engineering
Occasional Contributor I

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

Hello Charlie,

 

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

 

Thanks,

Myky

Frequent Contributor I

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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

Occasional Contributor I

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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:

 

rts.JPG

 

Thanks,

Myky

Frequent Contributor I

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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

Aruba Employee

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)


@Myky wrote:

 

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

 


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


Charlie Clemmer
Aruba Customer Engineering
Occasional Contributor I

Re: Request to Send (RTS) frames are sent by the AP towards station (scanner)

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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