However, this process does not guarantee that all nodes in the
network will follow the same schedule. This might happen because two or more
nodes outside of each other??™s transmission range could start broadcasting their
schedules simultaneously. Nodes at the border of two di?®erent schedules will
adopt both schedules and therefore will sleep less than other nodes. However,
border nodes may choose to adopt one schedule and only wake up on the other
schedule if they have data to transmit. Note that, by using this choice, broadcast
messages are transmitted twice, once for each adopted schedule. There
is still a possibility that neighbor nodes may not discover each other if they
adopt di?®erent schedules. To enable nodes to discover their neighbors, nodes
discovery process is initiated periodically (every 50 cycles for example). In order
to maintain synchronization with neighboring nodes, every node transmits
a SYNC packet periodically (every 10 cycles for example). The listen period
in SMAC is divided as shown in Figure 3. When a node wants to transmit a
SYNC packet, it must contend for the medium before transmitting. If a node
has data to transmit, it has to contend for the medium before transmitting
94 Ali Abu-el Humos et al.
Chapter 4 Medium Access Control Protocols for Wireless Sensor Networks
Fig. 3. Listen period in SMAC for three senders and a receiver.
the RTS packet as well. Upon receiving a CTS packet, the node can start
transmitting its data and go back to sleep after receiving the ACK packet.
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