Black Box LW0050A, Руководство пользователя

Страница: 142 / 146

140
PRO 11 SERIES WIRELESS ETHERNET
Duration/ID
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.
Address Fields
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 Access Point
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 Access Point 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 From DS Address 1 Address 2 Address 3 Address 4
0 0 DA SA BSSID N/A
0 1 DA BSSID SA N/A
1 0 BSSID SA DA N/A
1 1 RA TA DA SA