Scheduled-based protocols are basically Time Division Multiple Access
(TDMA) protocols. In TDMA protocols a centralized (master) node distributes
the transmission schedule among other nodes in the network during
the initialization period. After the initialization period, no overhead control
packets (RTS or CTS) are required. TDMA protocols are collision free
and perform best in single-hop networks. They require strict synchronization
among nodes in order to coordinate node transmission slots. TDMA based
protocols are not adaptive. Once the transmission schedule is distributed, it
cannot be modified to accommodate newly added nodes. TDMA protocols are
not scalable. They cannot support a large number of nodes because latency
increases significantly with the number of nodes. We present some scheduledbased
MAC protocols in Section 3.
Contention-based protocols are basically Carrier Sense Multiple Access
(CSMA) protocols [12]. In CSMA, wireless nodes are able to sense the communication
medium and defer their transmission while the channel is busy.
CSMA protocols can easily accommodate newly added nodes (adaptive), do
not require strict synchronization among nodes, and can support a large number
of sensor nodes (scalable). Multi-hop communication is easier to handle
in CSMA protocols than in TDMA ones. Several contention-based MAC protocols
are described in more detail in Section 4.
2.3 Energy-E?±ciency in WSNs MAC Protocols
Wireless sensor nodes are energy constrained and therefore they require
energy-e?±cient protocol designs in order to prolong network lifetime.
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