DELL laptop broadcasts CF-End frames

 

Problem description

Some DELL laptops with the Wi-Fi adapter Dual Bund Wireless-AC 8265 and the driver version 19.60.0.7 from 02.04.2017 are broadcasting CF-End frames.

Troubleshooting   

 Well, Who Is This Guy?

The CF-End Frame belongs to the PCF (Point Coordination Function). The PCF is a kind of Wi-Fi with the centralized access control, where AP takes roles of central coordinator.

PCF method and position of the CF-End frame

Some facts about the CF-Frame:

• it announces the end of the Contention Free Period. In contention free period (CFP), the Point Coordinator (AP) solicits the transmission of a specific STA with CF-Poll (Contention Free Polling)
• this frame can be sent from AP only.
• It is the broadcast frame.
• It is a control frame. So, it is transmitted with the lowest possible MCS
• It is the short frame and it is 38 bytes long only

·     The PCF as well as HCCA (QoS version of the PCF Access) have not be implemented. The only exception is proprietary iPCF method from SIEMENS.

The first thing you have to check if you see some PCF or HCCA Frame is the signal strength field of the frame. Most likely the signal was to weak. So, it was a malformed frame and Wireshark wrongly classify it as a CF frame.

In our case the signal strength was about 35-39 dBm. So, it was not the issue.  

It was not only one frame. There were grate number of CF-End frames. It looked very strange.

I found out that the CF-End frame flow depends on the activity of the laptop. Sometimes the number of frames jumps up to 320 frames per second. It happens for example after a rebooting of the laptop.

Blue – all frames from the STA; Red – CF-End frames from that STA

To filter out CF-End frame only use the Wireshark filter:

wlan.fc.type_subtype == 30

To see all PCF and HCCA frames type in the display filter field:

wlan.fc.type_subtype in {30 31 33 .. 35 37 .. 39 41 .. 43 46 47}

Conclusion

 If the CF-end frame is so small, why it is so bad?

Well, because it cases the interference in the standard CSMA/CA Distributed Coordination procedure. As all 802.11 Control Frames the CF-end uses SIFS (Short Interframe Spacing). It is much shorter as the DIFS or AIFS. So, this Frame has higher priority in comparison to a data frame. There is no issue from one or few CF-End Frames but is not good if several clients transmit 100 of packets simultaneously.

 As I told at the very beginning affected are DELL laptops with the Wi-Fi adapter Dual Bund Wireless-AC 8265 and the driver version 19.60.0.7 from 02.04.2017

 It is the case when the driver update helps.

After the updating to the 20.70.4.2 driver version the issue was solved.