4.2. MEDIUM ACCESS CONTROL (MAC) 47
decrementing their counters and B generates a new random backoff B. In this example, the counters
of A and B expire at the same time and these stations start transmitting at the same time, which
results in a collisions.The stations do not listen while transmitting, so the channel becomes idle only
at the end of the longest transmission (of B in this example). After the channel is idle for DIFS,the
stations A and B pick a new random backoff in CW
1
since they already had one attempt for their
current packet. Station X resumes decrementing its counter which expires before that of A and B.
At that time, station X transmits, after SIFS, A sends an ACK, and the procedure continues.
Time
A
Previous
Transmission
DIFS
50μs
B
X
Random backoff times
uniform in {0, 31}x20μs
B X
SIFS
10μs
ACK
A
X
B
Residual waiting
times of A and X
B
A
X
A
B
X A
Random backoff times
uniform in {0, 63}x20μs
Figure 4.1: Steps in WiFi MAC.
4.2.2 ENHANCEMENTS FOR MEDIUM ACCESS
The MAC protocol described above is a Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA) protocol. In general, wireless networks based on CSMA can suffer from two po-
tential issues: (i) Hidden terminal problem, and (ii) Exposed terminal problem. The hidden terminal
problem refers to the situation where two sending devices cannot hear each other (i.e., they are
hidden from each other), and hence one of them can begin transmission even if the other device is
alreadytransmitting.This can result in a collision at the receiving device.Ininfrastructure mode WiFi
networks, this issue can manifest itself in the situation where two devices hidden from each other
end up having overlapping transmissions, and the AP failing to receive either of those transmissions.
The exposed terminal problems refers to the situation where a device refrains from transmitting as
it senses the medium to be busy, but its intended destination is different from that of the ongoing
transmission, and hence could have in fact transmitted without any collision. In infrastructure mode
WiFi networks, since all communication takes place via the AP, the reader can convince oneself that
the exposed terminal problem is not really an issue.
To overcome the hidden terminal problem,WiFi networks can make use of the RTS and CTS
messages.The idea is that a sender wanting to transmit data, first sends out an RTS message obeying
the rules of medium access described above. This RTS message carries the address of the receiver
to whom the sender wishes to send data. Upon receiving the RTS message, the intended receiver

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