Image Processing Reference
In-Depth Information
It is worth mentioning that the minimum output power required to transmit a radio
signal over a certain distance is directly proportional to the distance raised to a power
between two and four (the coefficient depends on the type of the antenna used and its
placement relative to the ground, indoor-outdoor deployment, etc.). In these conditions,
it is more efficient to transmit a signal using a multihop network composed of short-range
radios rather than using a (power consuming) long-range link [].
The communication subsystem usually needs a hierarchy to create the abstraction for
the other layers in the protocol stack (we are referring to the device hardware character-
istics and the strict timing requirements). If a simple transceiver is used, some of these
capabilities need to be provided by the main processing unit of the sensor node (this can
require a substantial amount of resources for exact timing execution synchronization,
cross-layer distribution of the received data, etc.). We notice an increasing complexity
in the radio transceivers being used, with more and more functions moved to hard-
ware and offered to the device via specialized interfaces (see, for example, the CC
transceiver [] offering hardware capabilities as a link quality indicator, a security engine,
a localization engine, etc.).
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Data-link layer
:Itisresponsibleformanagingmostofthecommunicationtaskswithin
one hop (both point-to-point and multicasting communication patterns). The main
research issues here are the MAC protocols, the error control strategies and the power
consumption control.
The MAC protocols make the communication between several devices over a shared
communication medium possible by coordinating the sending-receiving actions func-
tion of time and/or frequency. Several strategies have already been studied and imple-
mented for the mobile telephony networks and for the mobile ad-hoc networks but
unfortunately, none of them is directly applicable. Still, ideas can be borrowed from the
existing standards and applications and new MAC protocols can be derived—this can be
proved by the large number of new schemes that target specifically the WSNs.
As the radio component is probably the main energy consumer in each sensor node,
the MAC protocol must be very efficient. To achieve this, the protocol must, first of all,
make use of the sleeping modes of the transceiver (e.g., turn the radio of) as much as
possible because of the reduced energy consumption is these states. he most important
problem comes from the scheduling of the sleep, receive, and transmit states. The tran-
sitions between these states also need to be taken into account as they consume energy
and sometimes take large time intervals. Message collisions, overhearing, and idle lis-
tening are direct implications of the scheduling used inside the MAC protocol which, in
addition, influences the bandwidth lost due to the control packet overheads.
A second function of the data-link layer is to perform error control of the received data
packets. The existent techniques include automatic repeat-request (ARQ) and forward
error correction codes (FEC). he choice of a specific technique comes down to the trade-
off between the energy consumed to transmit redundant information over the channel
and the energy and high computation power needed at both the coder/decoder sides.
Additional functions of the data-link layer are creating and maintaining a list of the
neighbor nodes (all nodes situated within the direct transmission range of the node in dis-
cussion); extracting and advertising the source and destination as well as the data content
of the overheard packets; supplying information related to the amount of energy spent on
transmitting, receiving, coding, and decoding the packets, the amount of errors detected,
the status of the channel, etc.
Usually, several partially overlapping communication channels are available (the exact
details depend on the transceiver being used). An interesting research topic is the usage
