84 J. Borms, K. Steenhaut, and B. Lemmens
without any coordination overhead. An example of such a protocol is the
xRDT protocol described in [7].
– Co ordinated channel selection. Nodes coordinate channel assignment in such
a way that the available bandwidth is distributed efficiently amongst nodes in
the network. Protocols like HyMAC [9] and MC-LMAC [8] combine FDMA
and TDMA techniques to assign a unique timeslot-channel combination to
each node in a two-hop neighborhood. Another example is TMCP [10] which
divides a dense WSN in several aggregation trees and assigns a different
channel to each tree.
Both type of schemes increase the bandwidth available for communications by
increasing the number of channels for unicast transmissions. Simple receiver-
fixed assignment offers less advantage in terms of bandwidth, but imposes no
coordination overhead on the network. Such a simple scheme could be an elegant
solution to increase the bandwidth of uncoordinated MAC protocols such as
X-MAC with little implementation overhead.
In general, fixed schemes consider mostly unicast. This restriction can be an
issue since broadcast is a commonly required operation used by many WSN rout-
ing protocols and applications. Fixed schemes only reduce internal interference
and do not consider external interference, so we believe they do not fully exploit
the possibilities of multi-channel operation.
2.2 Dynamic Channel Selection
Several techniques have been proposed to allow communication while nodes
switch their channels more frequently.
– Common Hopping. This is a class of protocols where nodes all listen to the
same channel, but “hop” between available channels frequently. Data can be
exchanged on a different channel after a handshake on the common channel.
Unlike other channel selection schemes, broadcast is easy to support. This
type of scheme offers some robustness to external interference, but since
packets are quite short in a WSN, the possible bandwidth gain compared
to single-channel schemes is limited. To the best of our knowledge, common
hopping has not been explored in WSNs for this reason.
– Independent Hopping. With independent hopping each node will frequently
change its channel, according to its own individual schedule. When a node
has data for another node, it switches its radio to the target node’s channel
and initiates the data transfer (with or without prior handshaking, depending
on the protocol). An example of such a protocol is McMAC [11]. This type
of scheme has the advantage over common hopping in terms of interference
since it protects against both internal and external interference. However,
it may be difficult to support broadcast (for example, the McMAC protocol
does not explicitly support broadcast transmissions). Additionally, a node
must store the hopping schemes of its neighbors. These type of schemes
require tight synchronization so nodes can accurately compute the channel
of their neighbors.