Cryptography Reference
In-Depth Information
8.6.1 Cross-Layer Couplings
Although many cross-layer proposals exist that are disparate in nature, identifying pro-
posals that are pertinent to a particular task has been a challenge. For example, the design
of a cross-layer scheme between a network and the MAC layers of a WSN protocol
stack should involve a cost-benefit analysis of different cross-layer design proposals with
reference to complexity versus performance. In addition, adequate security should be
considered while selecting a cross-layer proposal. Furthermore, design proposals should
incorporate explicit notifications by new interfaces to the transport layer so that they
improve end-to-end performance and make use of channel knowledge at the MAC layer
that would allow opportunistic usage of the channel and improve performance. In addi-
tion, energy-, delay-, and security-related issues need to be handled across the layers in a
holistic manner. This would require comparative, quantitative study of the different cross-
layer design proposals and is currently an open challenge for the cross-layer community.
8.6.2 Coexistence of Cross-Layer Design Proposals
One of the prime challenges with cross-layer security is with the coexistence of different
cross-layer security schemes. For example, the MAC layer in the WSN stack responds
to variations in the channel by adjusting the data rate. Is it then possible to adjust the
frame length at the link layer to provide further help? How will an overriding control
from the transport layer, attempting to control the link-layer parameters, interact with
these adaptation loops?
The question of the coexistence of cross-layer design ideas is pertinent when it comes
to determining whether some cross-layer design proposals can stifle further innovation.
Let us say that the physical and link layers are optimized for a certain performance metric
in a cross-layer design scheme. If this scheme is deployed first, can other schemes that also
rely on some (other) cross-layer couplings, or those that assume no coupling between the
link and physical layers, be deployed, as well, at a later time? Apart from presenting new
cross-layer design proposals, designers need to start establishing which other cross-layer
design interactions may or may not be employed together with their proposal.
8.6.3 Particular Cross-Layer Design
The network conditions in a wireless network are usually time-varying. In such a situ-
ation, one of the stated motivations behind cross-layer design is to achieve the network
equivalent of impedance matching. The idea is to make the protocol stack responsive
to variations in the underlying network conditions so that an optimal operating point
is always maintained. The pursuit of achieving such optimal operation presents two
complementary challenges. First, designers need to establish the network conditions
under which the proposed cross-layer designs will result in performance improvements.
Therefore, research should be conducted to illustrate how a cross-layer design involving
an iterative optimization of throughput and power leads to a loss in performance under a
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