3. Collision and Overhearing Avoidance
SMAC protocol uses a mechanism similar to the IEEE 802.11 for medium access
contention (fixed contention window) and hidden terminal avoidance (use
of RTS/CTS packets). All immediate nodes of both the sender and receiver
will go to sleep upon receiving RTS or CTS packets.
4. Message Passing
Transmitting a long message as a whole typically uses only one RTS/CTS
and ACK packets. However, if some bits are not received properly, this will
require retransmission of the whole packet, which will cause energy waste. On
the other hand, dividing a message into smaller packets and transmitting them
independently will increase the overhead. The IEEE 802.11 protocol avoids
this overhead by using only one RTS/CTS packet exchange to reserve the
medium for the first packet. The first packet and its ACK reserve the medium
95
for the second packet. The second packet and its ACK reserve the medium
for the third packet and so on. If the sender did not receive the ACK of one
packet, it will give up the medium and contend for it with its neighbors. In
contrast, the sender in SMAC will not give up the medium and will retransmit
the packet. The reason is that SMAC promotes application level fairness while
802.11 promotes per-node fairness.
Fig. 4. UCB Rene motes [21].
The performance of SMAC was tested using the UCB Rene Motes shown
in Figure 4. The authors compared the energy consumption performance of
the SMAC with full features and SMAC with message passing and overhearing
avoidance but without periodic sleep, against a simplified version of the IEEE
802.
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