Image Processing Reference
In-Depth Information
As compared to standard MAC protocols, multichannel protocols also raise new problems
[Mah]. As an example, the multichannel hidden terminal node is harder to solve than the classical
hidden terminal node, as control packets (e.g., RTS/CTS) could be sent on different channels, while a
network interface is able to work only on one channel at a time. Another problem that causes energy
and bandwidth waste on multichannel communications is deafness, which occurs when a node A
transmits to node B while the latter is transmitting to node C on a different radio channel. Finally,
even the co-existence of unicast and broadcast transmissions can be problematic in a multichannel
environment [Zhob], as it needs either different priorities for broadcast and unicast traffic or the
alternation of carrier sensing on the broadcast and unicast channel, to be stopped when a signal is
overheard. his technique is called toggle snooping.
The channel assignment may be computed either following a distributed approach or by the sink
node in a centralized way and then forwarded to nodes. The majority of protocols use distributed
approaches. he assignment may be ixed, as in [Zhoa], or dynamic, as in [Che]. In the following,
some multichannel MAC protocols are briefly discussed.
8.4.1 Multifrequency Media Access Control for WSNs
The Multifrequency MAC for Wireless Sensor Networks (MMSN) protocol consists of two different
parts, i.e., a frequency assignment schema and a medium access protocol. he frequency assignment
schema aims to assign different frequencies to each neighbor or, if the number of frequencies is not
large, to efficiently assign the available frequencies in order to limit the potential communication
conflicts. In [Zhoa] four different assignment strategies are proposed, i.e.,
•
Exclusive frequency assignment
: he nodes exchange their IDs with the two-hop neighbors
and then make the frequency decision in the increasing order of ID values, so that nodes
with lower IDs have the lower frequencies.
•
Even selection
: The exclusive frequency assignment is extended to the case in which all
frequencies have been already used by at least one node within two hops. In this case a
random frequency is selected among the least used ones.
•
Eavesdropping
: Each node broadcasts its frequency decision and waits for a back-off inter-
val. During such an interval the other decisions are recorded. When the back-off timer
expires, a frequency is randomly selected from the least chosen ones. This mechanism
only uses one-hop neighbor information, so it features less overhead, but also more
conflicts.
•
Implicit-consensus
:hetwo-hopneighbors'IDsarecollectedasinexclusivefrequency
assignment, then each node calculates its frequency locally, using a pseudorandom num-
ber generator seeded by node IDs and a defined algorithm. his schema provides different
channels for all two-hop neighbors, but it is effective only when the number of available
frequencies is large.
One of these schemes can be used to assign the frequency for data reception. Nodes are synchronized
and the medium access divides the time into time-slots. A time-slot consists of a broadcast contention
period
T
bc
,inwhichnodescontendforthesamebroadcastfrequency,andatransmissionperiod
T
tran
, in which nodes contend for shared unicast frequencies. he problem of supporting unicast and
broadcast transmissions at the same time is solved by using a dedicated radio channel and prioritizing
broadcast traffic. Nodes first check the broadcast frequency for receiving or transmitting broadcast
packets. If there are no broadcast packets, nodes can transmit or receive a unicast packet. A time-slot
canbeusedeithertotransmitortoreceiveonepacket.Nodesthathavenodatatotransmitsimply
listen for broadcast packets, then switch to their receiving frequency and listen for unicast packets.
Nodes that have packets to send use a CSMA approach with random back-off to access the medium
