Image Processing Reference
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switching between CSMA and TDMA depending on the current level of contention (CSMA under
low contention, TDMA under high contention).
Two different modes of operation are defined, the low contention level (LCL) and the high
contention level (HCL).
Under LCL nonowners are allowed to compete in any slot with low priority, while under HCL a
node does not compete in a slot owned by its two-hop neighbors.
Z-MAC is robust to topology changes and clock synchronization errors. A local synchronization
scheme, where each sending node adjusts the synchronization frequency based on its current data
rate and resource budget, is used. In the worst case, i.e., when clocks are completely unsynchronized,
Z-MAC performance is comparable to that of CSMA. Under high contention, as more transmis-
sions occur, time synchronization becomes more accurate and Z-MAC performance approaches that
of TDMA.
8.2.13 Pattern MAC
The Pattern MAC (PMAC) protocol defined in [Zhe] is another adaptive MAC protocol for WSNs
that tries to enhance the performance of fixed duty cycle protocols, such as S-MAC [Ye] [Ye].
Here, the determination of the duty cycle for a node is based on its own traffic and on the traffic
patterns of the neighboring nodes. PMAC is a time-slotted protocol but, unlike classical schedule-
basedMACprotocolsforWSNs,itisnotbasedonschedules.Instead,itisbasedonpatterns.Apattern
is a binary string that indicates the sleep-wake-up schedule of a node planned for several frames.
The basic idea of this protocol is that nodes get information about the activity of their neighbors,
so that they can go to sleep also for several frames if there is no activity, whereas in the case that
any activity is present, a node knows when it has to wake-up thanks to the patterns. Hence, a node
runningthePMACprotocoladjuststhesleep-wake-upschedulebasedonitsownpatternandthe
patterns of its neighbors. In order to achieve high energy saving, the number of sleep times in a
pattern is increased exponentially when the network is under light traffic conditions, whereas the
sleep sequence is interrupted when a node has data to send. In order to efficiently support the pattern
exchange, time is divided into super frame times (SFT), each divided into two parts: pattern repeat
timeframe(PRTF)andpatternexchangetimeframe(PETF).Bothpartsaredividedintoslots.During
the former, nodes transmit data repeating their current patterns. In addition, at the end of the PRTF
there is a time-slot during which all nodes stay awake, to speed up communications as well as to
support broadcast traffic. On the other hand, the PETF is used for performing pattern exchange
between neighboring nodes. To allow every node to send its pattern, the PETF features as many slots
asthemaximumestimatednumberofneighborsanodemayhave.
The PMAC protocol achieves very low energy consumption when the network load is low, as only
the sensor nodes involved in the communications will wake-up frequently, whereas the other nodes
stay asleep for longer times. his reduces the energy waste due to idle listening as compared to other
approaches such as, S-MAC or T-MAC that periodically wake-up nodes. his protocol requires time
synchronization, but loose synchronization schemes may be used, e.g., through periodical SYNC
packets from neighbors.
8.2.14 Crankshaft
The Crankshaft protocol proposed in [Hal] is especially designed for WSNs featuring high node
density. Under such scenarios, protocols featuring communication grouping, such as S-MAC [Ye]
[Ye] or T-MAC [Dam], suffer from degradation in both performance and energy efficiency due
to contention and collisions. Moreover, the adaptive duty cycle in T-MAC may prevent nodes to
go to sleep. Nevertheless, TDMA-like protocols such as LMAC [Hoe] would require frames with a
large number of time-slots, thus causing a significant delay increase. Moreover, most of the time-slots
