Image Processing Reference
In-Depth Information
during the broadcast or unicast period, according to their requirements. However, in order to detect
unicast packets to be received when there are also packets to be sent, toggle snooping and toggle
transmission are used, i.e., both listening and transmission are performed alternately on two different
frequencies with different rates, so that the detection of a packet to be received is guaranteed within a
maximum delay. Simulation results show that MMSN features both reduced energy consumption and
higher performance as compared to the standard CSMA mechanism when the number of available
channels increases.
8.4.2 Multichannel LMAC
In [Dur] the problem of providing multiple channel support to an example single-channel MAC
protocol, the LMAC protocol [Hoe], is addressed. he authors propose to multiplex the time-slot
in the frequency domain in an on-demand fashion, in order to enhance the spectrum utilization.
This approach does not require multiple transceivers, and the switching is performed only when the
network reaches its density limit. The proposed technique works in two phases. In the first phase,
nodes select their time-slot according to the classic LMAC protocol, while in the second phase nodes
select the radio channels. In fact, while in LMAC a time-slot can be reused only after at least two
hops, in the multichannel approach proposed in [Dur] the same time-slot may be reused on a dif-
ferent radio channel. hus, a node which finds its time-slot already occupied by a two-hop neighbor
can use the same time-slot but on a different channel. Bridge nodes are used to maintain the whole
network connected. Simulation results shown that applying a similar approach to a classical MAC
protocol, such as LMAC, denser networks can be supported. In addition, the number of collisions is
significantly decreased.
8.4.3 Multichannel MAC
In [Che] another Multichannel MAC (MCMAC) protocol for WSNs is presented. Unlike MMSN,
this protocol does not use a fixed channel assignment for nodes, but they are dynamically selected.
This protocol introduces network clusterization and exploits the existence of cluster-head nodes that
collect request messages from the cluster members, select the radio channels and communicate them
to both source and destination nodes. In this way, node pairs can communicate using the received
schedule and the designated radio channel. Although this approach can increase the sleeping times
of nodes, thus lowering their power consumption, it introduces a significant overhead, due to the
high number of signaling messages sent from/to the cluster-head.
8.5 Summary and Open Issues
This chapter addressed MAC layer protocols for WSNs from a broad perspective, ranging from
static, single-channel MAC protocols to multichannel approaches and protocols able to support
mobility. While the MAC protocols are usually grouped into two main categories, schedule-
based and contention-based, a number of MAC protocols for WSNs actually try to combine the
advantages of both classes in order to cope with the conflicting requirements of WSNs, such
as energy efficiency, scalability, timeliness, adaptability, and fault tolerance. Several MAC proto-
cols in the literature are overviewed in this chapter, to give an idea of the problems addressed,
of the techniques used and the performance obtained by these solutions. Classical approaches
as well as novel trends, such as the use of spatial correlation to enhance energy efficiency, are
discussed.
Despite the large number of existing MAC protocols, there is still room for further improve-
ments, in order to achieve better trade-offs between timeliness and energy efficiency. For example, the
