QoS and Non-QoS Data Frames

The 802.11 standard defines 15 different data frames, as you could see from the following table.

above pictures from CWAP

As you could see above table, the most significant bit (MSB) of the subtype field (Bit b7) is defined as the quality of Service subfile, specify that the frame is a QoS data frame. 

QoS stations are capable of transmitting both QoS and Non-QoS data frames. It is  common to have a wireless network that consists of both QoS and non-QoS station. In this type of mixed environment, it is likely that QoS devices will transmit both QoS data frames and non-QoS frames depending upon the capabilities of the receiving station. 

The strange things for Data frames do not actually carry any data. In fact, almost half of the data frames defined do not carry any data. The frames that do carry data are as follows:

• Data (simple data frame) Data + CF-Ack
• Data + CF-Poll
• Data + CF-Ack + CF-Poll QoS Data
• QoS Data + CF-Ack
• QoS Data + CF-Poll
• QoS Data + CF-Ack + CF-Poll

 And the frames that do not carry any data re follows:

• Null
• CF-Ack
• CF-Poll
• CF-Ack + CF-Poll
• Qos Null
• QoS CF-Poll
• QoS CF-Ack + CF-Poll 

 So, you may ask why have a data frame that does not carry data ? Sometimes a station needs to transmit special control information to an access point or another station, without actually transmitting any data. Client stations sometimes use Null data frames to enable or disable power save mode, which is indicated by a bit in the frame control field. Using the Null data frame allow a station to communicate with another device without requiring it to transmit data.