Image Processing Reference
In-Depth Information
A
B
C
D
FIGURE .
Hidden terminal scenario.
A
B
C
FIGURE .
Exposed terminal scenario.
range of C and vice versa. If C starts to transmit to B, A cannot detect this by its carrier-sensing
mechanism and considers the medium to be free. Consequently, A also starts packet transmission
and a collision occurs at B.
The exposed terminal problem is also a result false prediction of the channel state at the receiver.
An example scenario is shown in Figure .. ..The four stations A, B, C, and D are placed such that the
pairs A/B, B/C, and C/D can hear each other, all other combinations cannot. Consider the situation
where B transmits to A, and one short moment later C wants to transmit to D. Station C performs
carrier sensing and senses the medium busy due to B's transmission. Consequently, C postpones its
transmission. However, C could safely transmit its packet to D without disturbing B's transmission
to A. his leads to a loss of efficiency.
Two approaches to solve these problems are busy tone solutions [] and the RTS/CTS protocol.
In the busy tone solution two channels are assumed: a data channel and a separate control channel
for the busy tone signals. he receiver of a packet transmits a busy tone signal on the control channel
during packet reception. If a prospective transmitter wants to perform carrier sensing, it listens on
the control channel instead of the data channel. If the control channel is free, the transmitter can start
to transmit its packet on the data channel. This protocol solves the exposed terminal problem. The
hidden terminal scenario is also solved except those rare cases where A and C start their transmissions
simultaneously. However, if the busy tone is transmitted only when the receiver detects a valid packet
header, the two colliding stations A and C can abort their transmissions quickly when they perceive
the lack of a busy tone. he busy tone solution requires two channels and two transceivers.
