Doesn??™t this sound like what humans do in group conversation? When a person wants to speak, the person
listens for a break in the conversation, and then begins. If the speaker hears interference from another voice,
both people stop talking, and wait an interval of time before trying again to speak: CSMA/CD.
130 NETWORKING [CHAP. 7
The Wi-Fi connections popular today use standards specified in IEEE 802.11. IEEE 802.11 is actually
a family of standards using different encoding and transmitting techniques, and different speeds, with a common
protocol. Wi-Fi uses CSMA/CA, where CSMA still stands for carrier sense multiple access, but CA stands for
collision avoidance.
With wireless networks, it can be harder to detect collisions. If two computers are sharing a common
Wi-Fi network, for example, both may be able to contact the access point, but one may not be able to detect the
other, making collision detection difficult, even though simultaneous messages are garbled at the access point.
Also, the radio environment may be naturally noisy, with cell phones and microwave ovens in the vicinity, for
example, so collisions may be difficult to distinguish from normal noise.
For these reasons, 802.11 standards adopt the CA approach where a sender first waits for the clear radio
channel, and then sends a short request-to-send (RTS) message to the destination computer.
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