802.11 Protocol Architecture
messages are carried by a medium devoted to the DS and the messages
do not leave the ESS. Figure 2.2 illustrates this concept.
FIGURE 2.2
Ethernet Distribution System Medium
If the DS medium is 802.11 then access points transport messages to
each other using their radios. This is a wireless DS (WDS). In a
wireless DS single-radio access points use the same radio and channel
for its BSS wireless medium and for its wireless DS medium. Dual-
radio access points may use one radio for the WDS, and another radio
on a different channel for the BSS wireless medium. The wireless DS
is a special case for transporting messages between access points of an
ESS and is supported by the otherwise unused Address 4 field in the
802.11 MAC header. Figure 2.3 illustrates the physical construction of
a WDS.
