This will introduce some latency in the system. Fairness
among nodes in the same vicinity can be tolerated since all these nodes
93
The IEEE 802.11 Distributed Coordination Function (DCF) [23] is a contention-
Fig. 2. Periodic listen and sleep.
are expected to serve the same application. Therefore, a node with a long
data message will not give up the medium to other nodes until its whole
data message is transmitted. Thus, shorter messages waiting on the queues
of neighboring nodes might have to wait longer to get access to the medium.
This can be justified considering that in WSNs, application level latency is
usually favored over the per-node fairness.
The SMAC protocol has the following four features:
1. Periodic Listen and Sleep
Each node in the network turns o?® its transceiver (sleeps) and wakes up to
listen to the medium periodically, as shown in Figure 2. The parameter that
governs the percentage of the wake-up period to the sleep period is called the
duty cycle and is given by :
Duty cycle = listen time/cycle time
2. Synchronization
SMAC introduces a new packet type (SYNC) to accomplish the synchronization
task. At the deployment time, all nodes keep listening to the medium
until one node broadcasts a SYNC packet containing its schedule (the time
left to sleep). Neighboring nodes, upon receiving this packet, will set their
schedule to the new schedule and broadcast a SYNC packet to their neighbors
as well.
Pages:
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172