Image Processing Reference
In-Depth Information
Normal
Active state
Sleep state
S-MAC
Normal
Active time
Sleep time
T-MAC
TA
TA
TA
FIGURE .
Duty cycles comparison between S-MAC and T-MAC. The arrows represent message transmissions
and receptions.
tothereceivedRTSinsteadoftheusualCTS.hissolutionmaycauseperformancedegradationunder
high loads, so it has to be used carefully.
8.2.4 Data-Gathering MAC
MAC protocols for multi-hop WSNs that utilize listen/sleep duty cycles suffer from the so-called data
forwarding interruption problem, because not all the nodes on a multi-hop path to the sink are aware
of the forthcoming arrival of on-going data. As a result, some of them go to sleep, thus introducing a
significant latency due to sleep delay.
The Data-Gathering MAC (DMAC) [Lu] is an energy-efficient low-latency protocol designed
and optimized to solve the data forwarding interruption problem in convergecast WSNs. he key idea
of DMAC in order to enable a continuous packet flow from sensor nodes to the sink is to stagger the
wake-up scheme by giving the sleep schedule of a node an offset, which is a function of its level on the
data-gathering tree. During the receive period of a node, all of its child nodes have transmit periods
and contend for the channel. To reduce collisions during the transmit period of nodes on the same
level in the tree, every node backs-off for a period plus a random time within a fixed contention
window before packet transmission. DMAC can be viewed as an extension of the Slotted Aloha
algorithm [Rob] in which slots are assigned to the sets of nodes based on a data-gathering tree.
Low node-to-sink latency is obtained by assigning subsequent slots to the nodes that are consecu-
tive in the data transmission path. Similarly to T-MAC, DMAC also adjusts the duty cycles adaptively
accordingtothenetworkworkload.
DMAC fits well the scenarios in which data transmission paths are known in advance (so it is
possible to build the data-gathering tree) and outperforms S-MAC in terms of latency (thanks to
staggered schedules), throughput and energy efficiency (thanks to the adaptivity). However, when
collisions are likely to occur, i.e., in event-triggered WSNs, DMAC performance degrades.
8.2.5 Berkeley MAC
The Berkeley MAC (B-MAC) protocol [Pol] is a carrier-sense configurable MAC protocol for
WSNs that provides a flexible interface to combine low-power operation, effective collision avoid-
ance, and high channel utilization. B-MAC consists of four main components, i.e.,
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