Node A,
sensing no activity on the channel, might then begin transmitting, jamming the access
point??™s reception of node B??™s transmission already under way.
Figure 2.3: Basic Service Set (BSS), showing the hidden node problem.
To solve the hidden node problem, the standard specifies an optional method in which use
of the medium is reserved by an exchange of control packets called request to send (RTS)
and clear to send (CTS). A station sends an RTS to its intended unicast recipient. If the
recipient receives the RTS and can accept the proposed transmission, it replies with a CTS.
When it receives the CTS, the first station begins to send. This has two advantages and one
drawback. First, the packets are small, and any collision caused by the transmission will be
brief. Second, both parties to the proposed communication send a packet whose
Duration/ID field covers the whole proposed transaction. That allows all stations within
range of either station to defer use of the medium until the transaction is complete. The
disadvantage, of course, is that the overhead represented by the RTS/CTS exchange must
be added to each transaction.
A special case can occur between 802.
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