Image Processing Reference
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mobility information about all the current and potential two-hop neighbors, and how to perform
synchronization between nodes, as they could independently calculate frame times different from
each other. To overcome these issues, this protocol uses a cluster-based approach, in which nodes
are grouped into clusters. A cluster-head node for each cluster is elected in rotation, with an election
schema similar to the one used in low energy-adaptive cluster hierarchy (LEACH) [Hei]. Time
is divided into rounds, and a different cluster-head is elected at each round. he predicted mobility
information is put by nodes into the header of MAC packets. he cluster-head is always on, so it col-
lects all the information and it broadcasts all the mobility information to the member nodes during a
dedicated time-slots at the end of the frame. his provides the nodes with a best-effort knowledge of
the mobility information of the current and potential two-hop neighborhood. A similar solution is
used to provide synchronization between nodes. hey calculate independently their frame time, but,
instead of setting-up the frame by themselves, nodes communicate their frame time to the cluster-
head, which collects the value for all its member nodes and calculates the average value. At the end
of each round, cluster-head nodes exchange all the average values between themselves and the global
meanvalueisdisseminatedalongthewholenetwork.So,attheendofeachround,allthenodes
adjust the frame time as well as the scheduled access slots and the random-access slots according to
the global frame time. While the frame time remains the same for the whole round, at the end of each
frame the number of random-access slots within a cluster may be increased or decreased, according
to the mobility patterns of cluster nodes.
8.3.3 Mobility-Adaptive Hybrid MAC
Another approach to handle mobility in WSNs is the Mobility-Adaptive Hybrid MAC (MH-MAC)
presented in [Raj] that tries to combine the advantages of schedule-based and contention-based
protocols. The proposed solution is a hybrid approach in which the WSN nodes are differentiated
in mobile nodes and static nodes, and the most suitable medium access mechanism is used for each
type of nodes. So, for static nodes a schedule-based channel access mechanism is adopted, while a
contention-based approach is used for mobile nodes. Time is divided into time-slots and two dif-
ferent types of time-slots, i.e., static and mobile, are defined. Each node uses a mobility estimation
algorithm to determine its mobility. Based on its mobility, a node uses the static or mobile slot. Static
slots are assigned in a LMAC-like fashion, while for the contention-based time-slots a Scheduled
Channel Polling mechanism [Ye] is used to limit the duty cycle in order to decrease energy con-
sumption. According to such a mechanism, a sender node sends a short wake-up tone followed by
the receiver ID, so that all the other nodes but the receiver can go to sleep. he MH-MAC protocol
adapts to different levels of mobility by dynamically adjusting the ratio static/mobile slots as well as
the frame time. When less nodes are mobile, more slots are reserved for schedule-based allocation and
vice-versa.
8.4 Multichannel Protocols for WSNs
Multichannel protocols are usually hybrid approaches that combine FDMA with TDMA and/or
CSMA and use different frequencies for parallel communications. Typically, these approaches try
to maintain all the benefits, in terms of energy consumption, of TDMA-based protocols by avoiding
contentions and lowering the nodes' duty cycles. In addition, thanks to the use of multiple channels,
these protocols enhance the network capacity and solve the scalability problems that affect schedule-
based protocols when applied to large and dense WSNs. However, providing multichannel support
introduces some overhead. he first overhead is represented by the signaling required to enable com-
municating nodes to agree on the radio channel to use. The second source of overhead is channel
switches, which cause energy consumption and therefore should be limited.
