Alvarion BreezeNET PRO.11 Series User Manual page 198

Appendix
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This field has two meanings depending on the frame type:
In Power-Save Poll messages this is the Station ID
In all other frames this is the duration value used for the NAV Calculation.
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A frame may contain up to 4 Addresses depending on the ToDS and FromDS bits
defined in the Control Field, as follows:
Address-1 is always the Recipient Address (i.e. the BSS station that is the
immediate recipient of the packet). If ToDS is set, this is the AP Address; if
ToDS is not set, then this is the address of the end-station.
Address-2 is always the Transmitter Address (i.e. the station which is
physically transmitting the packet). If FromDS is set, this is the AP address; if
it is not set, then it is the Station address.
Address-3 is in most cases the remaining, missing address. On a frame with
FromDS set to 1, Address-3 is the original Source Address; if the frame has the
ToDS set, then Address 3 is the destination Address.
Address-4 is used in special cases where a Wireless Distribution System is
used, and the frame is being transmitted from one Access Point to another. In
such cases, both the ToDS and FromDS bits are set, so both the original
Destination and the original Source Addresses are missing.
The following Table summarizes the usage of the different Addresses according to
ToDS and FromDS bits setting:
To DS
0
0
1
1
User's Guide
From DS Address 1
0 DA
1 DA
0 BSSID
1 RA
Address 2 Address 3
SA BSSID
BSSID SA
SA DA
TA DA
9-42
Address 4
N/A
N/A
N/A
SA
BreezeNET PRO.11 Series