Last Post: June 2, 2004:
This is a popular question about IEEE 802.11 frames:
Ã¢Â€ÂœWhich frames are acknowledged and which are not?Ã¢Â€Â
There are three types of IEEE 802.11 frames -- data, management, and control frames. Each type has multiple subtypes. Frames of any one subtype are either 1) always unicast, 2) always multicast, or 3) sometimes unicast and sometimes multicast.
Remember that a frame is unicast or multicast based on the Receiver Address (RA) stored in the Address1 field, which is not necessarily the Destination Address (DA) shown on many 802.11 protocol analyzers. This is crucial for understanding 802.11 frame exchanges. (See my recent post, Ã¢Â€ÂœACK's for multicast data frames?Ã¢Â€Â.)
Assuming frames are received without error and are intended for the receiver (filtered based on RA and sometimes BSSID), which frames should a receiver acknowledge? The IEEE envelope please...
All "unicast" data frames (including many with multicast DA's in Address3!)
All unicast management frames
All PS-Poll control frames (but not all unicast control frames)
All the rest are not acknowledged.
Have a great day. /criss
I don't know that much more can be said other than, "Yup." ;-) This is correct.
Once a client station has associated with an access point (AP) are all the frames transmitted by the client "unicast" in the IEEE 802.11 sense?
Yes! They all carry the unicast MAC address of the AP in the address1 field, even the frames, such as ARP queries, with a multicast or broadcast Destination Address (DA). A broadcast address in the address3 field does not change the fact that these frames are "directed" to the AP and ACK's are expected for each one.
Well, >>almost<< all frames from an associated client are unicast. In support of roaming, associated stations occasionally transmit probe request frames with an address1 of ff:ff:ff:ff:ff:ff. Instead of being directed to the current AP, these are "broadcast" to discover alternative AP's.
Can you think of any other exceptions?