Image Processing Reference
In-Depth Information
Traditional performance parameters such as throughput and bandwidth utilization play a sec-
ondary role in WSNs MAC protocols as compared to energy efficiency and scalability. Similar
considerations hold for latency, so MAC protocols for WSNs may trade latency for energy efficiency.
However, in the cases in which latency is an issue (e.g., in real-time WSNs or in wireless sensor/actor
networks, WSANs), solutions to bound end-to-end latency are needed. Some works provide specific
frameworks implementing solutions in which the MAC and Routing layers cooperate to achieve both
energy efficiency and real-time support, e.g., the Real-Time Power Aware Routing protocol [Chi].
Finally, fairness among sensor nodes is generally not an issue, since all of them cooperate to accom-
plish a common task. It may happen that, at any given time, one node has more data to send than
other nodes. As long as application performance is not degraded, allowing such a node to transmit
more than other nodes is acceptable.
While designing a MAC protocol for WSNs the traffic pattern to be dealt with has to be taken
into account. The communication patterns typically found in WSNs can be classified as broadcast,
convergecast, local gossip, and multicast [Kul] [Dem].
•
Broadcast
: A base station (or a sink node) sends data to all the sensor nodes in the WSN.
•
Local gossip
: Neighboring nodes communicate with each other locally, following the
detection of an event.
•
Convergecast
: A group of sensors communicate what they perceived to a specific sensor
(e.g., the sink, a cluster-head, etc.).
•
Multicast
: A sensor sends a message to a specific subset of sensors. An example of
multicast communication can be found in cluster-based protocols where cluster-heads
may need to send a message to a subset of their neighbors, i.e., the members of their
cluster only.
As the primary goal of MAC protocols for WSNs is energy efficiency, in Section .. the main causes
of energy waste in WSNs are identified and discussed.
8.1.1 Causes of Energy Waste in WSNs
MAC protocols for WSNs have to cope with the main causes of energy waste in wireless networks
that are collisions, overhearing, idle listening, protocol overhead, and overemitting.
•
Collisions
:Whenatransmittedpacketiscorruptedduetoacollision,ithastobediscarded
and, after a random back-off procedure is run, retransmissions occur. These activities
increase energy consumption. Moreover, when the workload approaches the available
bandwidth, sensing for a clear channel consumes a significant amount of energy, although
the number of successful transmissions will be extremely low.
•
O
verhearing
: It occurs because the radio channel is a shared medium and thus a node
may receive packets that are not destined for it.
•
Protocol overhead
: It is due to sending and receiving control packets (such as, ACKs,
RTS/CTS, etc.) and MAC headers. Such an overhead may be significant, as in typical
WSN applications small data are exchanged, thanks also to the in-network processing
that allows for condensing multiple raw sensor readings into short information.
•
Idle listening
: Listening to receive possible traffic that is not sent. his happens because a
node does not know when it will be the recipient of a message from one of its neighbors,
so it must keep its receiver switched on at all times.
•
Overemitting
: It occurs when a node sends a message and the destination node is not
readytoreceive(e.g.,asthenodeisinsleepstate).
