Figure 21.15a).
To solve this problem, 802.11 specifi es an optional
request to
send/clear to send
(RTS/CTS) protocol at the MAC layer. When this feature is in
use, a sending station transmits an RTS and waits for the AP to reply with a CTS.
Since all stations
in the network can hear the AP, the CTS causes them to delay
any intended transmissions, allowing the sending station to transmit and receive
a packet acknowledgment without any chance of collision. Since RTS/CTS adds
additional overhead to the network by temporarily
reserving the medium, it is
typically used only on the largest-sized packets, for which transmission would be
expensive from a bandwidth standpoint. This mechanism
reduces the probability
of a collision on the receiver area by a station that is hidden from the transmitter
to the short duration of the RTS transmission, because all stations hear the CTS
and make the medium busy until the end of the transaction.
The duration infor-
mation on the RTS also protects the transmitter area from collisions during the
ACK (from stations that are out of range of the acknowledged station). It should
also be noted that, due to the fact that
RTS and CTS are short frames, the mecha-
nism also reduces the overhead of collisions, since these frames are recognized
faster than if the whole packet were to be transmitted.
The mechanism is con-
trolled by a parameter called RTS threshold, which, if used, must be set on both
the AP and the client side.
The time required to transmit a frame, taking into account the RTS/CTS
four-way handshake is given as:
BO
DIFS
4PLCP
RTS
CTS
D
A
R
3SIFS
s
where:
