For the scheduled-based protocols, since nodes know ahead of time when
they will be active, they can go to sleep mode for the rest of the time when
they are inactive.
Four major sources of energy waste in contention-based schemes are identified
[16, 17]:
1. Collisions. If node A starts transmitting a message to node B and B
is within the communication range of another transmitting node C, then
there will be collisions at node B. Collisions require retransmission of data,
which results in waste of energy resources.
2. Overhearing. This happens when a node hears a transmission intended
to other nodes. As a result, the node consumes energy even if the packet
was not intended for it.
3. Control packet overhead. Control packets such as RTS, CTS and ACK
used for example in IEEE802.11 MAC protocol, consume energy when
transmitted and received, and consequently reduce the network bandwidth
available for data packets.
4. Idle listening. If a wireless sensor node is not transmitting or receiving,
it is in idle mode. Idle listening is a major energy waste in WSNs. It
usually consumes 50??“100% of the energy required for receiving [11, 4].
Designing MAC protocols that avoid or reduce these energy waste sources are
of critical importance and will have a direct impact on prolonging network
lifetime.
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3 Scheduled-based MAC Protocols
Scheduled-based schemes assign collision free channels to each node, using
Time Division Multiple Access (TDMA), Frequency Division Multiple Access
(FDMA) or Code Division Multiple Access (CDMA).
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