We have already discussed the frame control field format in the previous lesson that the Frame Control field contains multiple subfields. The figure below illustrates the subfields of the frame control field.
Protocol Version – It is a 2-bit field that indicates the current version of the protocol. The receiving devices use this value to determine if the version of the protocol of the received frame is supported.
Frame Type and Frame Subtype – The frame type is a 2-bit frame that determines the function of the frame i.e management(00), control(01), or data(10). The value 11 is reserved. The subtype is a 4-bit long field showing the sub-type of the frame like 0000 for association request, 1000 for the beacon. There are multiple subtype fields for each frame type. Each subtype determines the specific function to perform for its associated frame type.
ToDS and FromDS – Both frame field size is 1 bit long. The ToDS fieldset indicates that the destination frame is for DS(distribution system) and From DS field indicates the frame coming from DS. Both frames are only used in data frames of wireless clients associated with an AP.
More Fragments – It is a 1-bit long field Indicates whether more fragments of the frame, either data or management type, are to follow. If set to 1 means the frame is followed by other fragments.
Retry – The retry field is also 1 bit in the size. If the frame is set to 1, which indicates that the current frame is a retransmission of an earlier frame.
Power Management – The 1-bit long field Indicates whether the sending device is in active mode or power-saver mode. If the frame value is set to 1, it is mean that the station goes into power-save mode. If the field is set to 0, the station stays active.
More Data – The 1-bit long field indicates to a device in a power-save mode that the AP has more frames to send. It is also used for APs to indicate that additional broadcast/multicast frames are to follow.
Security – The 1-bit long field indicates whether encryption and authentication are used in the frame. It can be set for all data frames and management frames, which have the subtype set to authentication.
Reserved – It is a 1-bit long field that indicates that all received data frames must be processed in order. If the field value is set to 1 the received frames must be processed in strict order.