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21 Wireless Local Area Networks
BO
backoff time (
s)
DIFS
distributed inter-frame space (50
s)
PLCP
time required to transmit physical layer convergence protocol (
s)
RTS
request to send frame size (bits)
CTS
clear to send frame size (bits)
D
frame size (bits)
A
acknowledgement frame size (bits)
R
channel bit rate (bits per second)
SIFS
short inter-frame space (10
s)
We refer to Figure 21.15b and assume that node B and C intend to transmit
data only without receiving data. When node C is transmitting data to node D,
node B is aware of the transmission. This is because node B is within the radio
coverage of node C. Without exchanging RTS and CTS frames, node B will not
initiate data transmission to node A because it will detect a busy medium. The
transmission between node A and node B, therefore, is blocked even if both of
them are idle. This is referred as the
exposed node problem
. To alleviate this prob-
lem, a node must wait a random backoff time between the two consecutive new
packet transmission times.
21.5.5 IEEE 802.11 MAC Sublayer
In IEEE 802.11, the MAC sublayer is responsible for asynchronous data service
(e.g., exchange of MAC service data units (MSDUs)), security service (confi denti-
ality, authentication, access control in conjunction with layer management), and
MSDU ordering.
A
B
1. RTS
1. RTS
2. CTS
2. CTS
C
D
CTS : Clear to Send
RTS : Request to Send
(b)
