11b and 802.11g stations using the same
channel. Because 802.11b nodes cannot interpret the higher-speed OFDM-encoded
transmissions of 802.11g nodes, additional steps must be taken to minimize contention
between them. The standard refers to these steps as protection, invoked whenever 802.11b
and 802.11g nodes are both associated with the same access point, or part of the same
IBSS.
Guide to Wireless LAN Analysis 24
One protection option is for all stations to use the full RTS/CTS method for every
unicast exchange, but this imposes significant costs to 802.11g throughput. As an
alternative, 802.11g nodes can send a single CTS packet at 802.11b rates addressed to
themselves (CTS to Self) to reserve the medium. This does not solve the hidden node
problem, but it does allow 802.11g nodes to provide all 802.11b nodes within range with
the information they need to defer using the medium until the 802.11g transaction is
completed.
The use of RTS/CTS can be set to be always on, always off, or be invoked
automatically when fragmentation reaches a preset level (for example, a data packet length
of 500 bits). The precise methods are dependent on the implementation of the equipment
vendor.
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