Image Processing Reference
In-Depth Information
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Clear channel assessment (CCA) and packet back-off, for channel arbitration
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Link layer ACKs, for reliability
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LPL, for low-power communication
CCA is performed using a weighted moving average on a set of samples taken when the channel is
idle to assess the ground noise, thus enabling a better detection of valid transmissions and collisions.
The approach has proven to outperform thresholding methods used in a variety of wireless protocols
which produce a large number of false negatives that reduce the channel bandwidth.
Packet back-off is configurable and is chosen from a linear range instead of the usual exponen-
tial back-off approach. Optional packet-by-packet link layer ACKs are provided to reliably transfer
important data. Finally, low-power operation is achieved through an adaptive preamble sampling
scheme able to reduce duty cycle and minimize idle listening. A node cycles between awake and
sleep states. While awake, it listens a long preamble to assess whether it has to remain awake or can
go back to sleep. RTS/CTS packets, due to their high overhead, are not used.
B-MAC supports on-the-fly reconfiguration and provides bidirectional interfaces for services to
optimize performance in terms of throughput, latency, or power saving. While S-MAC and T-MAC
are not just link protocols, but also network and organization protocols, the lightweight Berkeley
MAC protocol only contains a small core of media access functionalities. Network services such as
organization, synchronization, and routing build upon it. As B-MAC is very configurable, other MAC
protocols for WSNs (e.g., S-MAC and T-MAC) may be implemented efficiently upon B-MAC and
could benefit from its flexibility.
B-MAC has a set of interfaces that allow choosing various services to tune its operation in addi-
tion to the standard message interfaces. These interfaces allow network services to adjust CCA,
ACKs, back-offs, and LPL. hrough such interfaces, protocols built on B-MAC can make local policy
decisions to optimize power consumption, latency, throughput, fairness, or reliability.
8.2.6 WiseMAC
WiseMAC [Hoi] is a CSMA-based protocol which uses the preamble sampling technique [Hoi]
to reduce the cost of idle listening. In the preamble sampling technique, nodes regularly sample the
channel for a short interval to assess whether there is on-going activity or not. All nodes in the
network sample the channel with the same constant period, which is independent of the actual traf-
fic, while their relative sampling schedule offsets are independent. If a node finds the channel busy,
it will keep on listening until either a data packet is received or the channel becomes idle again. he
transmitter transmits a wake-up preamble in front of every message to ensure that the receiver will
be awake when the data portion of the message will arrive. Such a preamble, however, introduces a
power consumption overhead, both at the transmitter and at the receiver. In WiseMAC the length
of the wake-up preamble is dynamically chosen by a node exploiting the knowledge of the sampling
schedule of its direct neighbors, which is received at every data exchange as it is piggybacked in ACK
messages. Every node therefore maintains an updated table with the sampling schedule offset of its
direct neighbors. Another factor which is taken into account when dynamically sizing the preamble
length in WiseMAC is the potential clock drift between the sender and the receiver. In [Hoi] the
calculationforthelowerboundispresented.
The varying preamble length makes the WiseMAC protocol adaptive to the traffic load. WiseMAC
achieves energy efficiency and also provides for scalability and mobility support, as only local syn-
chronization information is used. A performance comparison in [Enz] showed that WiseMAC
is able to provide both low average power consumption in low traffic conditions and high energy
efficiency in high traffic conditions, while T-MAC does not have this property. he main shortcom-
ing of WiseMAC is the decentralized sleep-listen scheduling, which determines different sleep and
