Last Post: September 28, 2006:
Non-QoS STAs assign sequence numbers from a single counter while QoS STAs have multiple counters, one per traffic identifier (TID) per receiver plus one more for multicast. Sequence numbers are used for frame fragmentation and reassembly, and duplicate frame filtering.
IEEE P802.11REVma 18.104.22.168.1 Sequence Number field
"The Sequence Number field is a 12-bit field indicating the sequence number of an MSDU or MMPDU. Each MSDU or MMPDU transmitted by a STA is assigned a sequence number. Sequence numbers are not assigned to control frames, as the Sequence Control field is not present.
"Non-QoS STAs, as well as QoS STAs operating as non-QoS STAs because they are in an non-QoS BSS or non-QoS IBSS, assign sequence numbers, to management frames and data frames (QoS subfield of the Subtype field is set to 0), from a single modulo-4096 counter, starting at 0 and incrementing by 1 for each MSDU or MMPDU.
"QoS STAs associated in a QoS BSS maintain one modulo-4096 counter, per TID, per unique receiver (specified by the Address 1 field of the MAC header). Sequence numbers for QoS data frames are assigned using the counter identified by the TID subfield of the QoS Control field of the frame, and that counter is incremented by 1 for each MSDU belonging to that TID. Sequence numbers for management frames, QoS data frames with a broadcast/multicast address in the Address 1 field, and all non-QoS data frames sent by QoS STAs are assigned using an additional single modulo-4096 counter, starting at 0 and incrementing by 1 for each MSDU or MMPDU. Sequence numbers for QoS (+)Null frames may be set to any value.
"Each fragment of an MSDU or MMPDU contains a copy of the sequence number assigned to that MSDU or MMPDU. The sequence number remains constant in all retransmissions of an MSDU, MMPDU, or fragment thereof."
I hope this helps. Thanks. /criss