After receiving the data packet,
the receiving node will reply with an ACK packet to confirm the reception of
the data packet and to inform the transmitting node about its next sampling
time. Thus, next time the transmitting node has data to send to this node, it
101
Fig. 11. Packet exchange in WiseMAC protocol.
will send a preamble of length Tp ?‰? Tw just before the receiver samples the
medium. This mechanism causes significant energy savings in heavy tra?±c.
WiseMAC uses the more bit technique to allow burst transmission of data
(transmitting more than one packet at a time). This feature is similar to the
message passing technique used in SMAC.
Unfortunately, WiseMAC does not solve the interruption in forwarding
data packets over multiple hops. This is because nodes on the next hop are
not aware of the data transmission of the downstream hop since their wakeup
times are random (not synchronized).
4.6 Energy and Latency Control in Low Cycle MAC Protocols
In SMAC-like protocols, all nodes follow the same schedule or they may follow
di?®erent schedules. Nodes following the same schedule form a virtual cluster
and nodes on the borders of two or more virtual clusters are called border
nodes (see Figure 12). Although it is thought that there are usually only few
such border nodes in the network, experiment shows that more than 40% of
the nodes in the network adopt two or more schedules. This is because of
the unexpected radio propagation paths [15, 18].
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