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certain uncontrolled network condition. Second, efficient mechanisms to make a timely
and accurate assessment of the state of the network needs to be built into the stack, and
the corresponding overheads must be taken into account.
8.6.4 Standardization of Interfaces
The one thing layering achieved was to present standardized boundaries and interfaces
between modules of the system—the protocol layers. Now that the layered architecture
is being violated in different ways, finding the new reference architecture becomes a
challenge. As a result, the question to ask is, What should the boundaries be between
modules? In addition, Should we stick to traditional layer boundaries and determine the
new interfaces from there, or should we look at completely new boundaries, or a combi-
nation thereof? What should the interfaces between modules look like? Addressing this
challenge requires greater synergy between the performance viewpoint and implementa-
tion concerns than is seen in the literature today. In general, organization of the mod-
ules (layers or otherwise) and the interfaces between them determines how efficiently
information can be shared between them and what kinds of overheads and delays occur?
This, in turn, determines how effective cross-layer design proposals, which rely on shar-
ing dynamic information between the modules, can be. Hence, proposers of cross-layer
design relying on back-and-forth information flow between layers or dynamic vertical
calibrations need to consider the impact of delays in the retrieval/updating of informa-
tion on protocol performance. They also need to quantify the overheads associated with
their cross-layer design proposals.
8.6.5 Physical Layer in Cross-Layer Proposals
In wired networks, the role of the physical layer has been rather small: sending and
receiving packets as initiated by the higher layers. Advances in signal processing at the
physical layer can allow it to play a bigger role in wireless networks. Hence, the question
of how much of a role the physical layer should play. This is relevant to the cross-layer
design effort because first, layered architectures like the OSI reference model do not
allow much of a role for the physical layer besides providing a bit pipe, and second,
enhancements in the physical layer will have to be balanced by corresponding changes
to the higher layers. Hence, figuring out the role to be played by the physical layer is
an important question. Cross-layer designs relying on advanced signal processing at the
physical layer can be an interesting research framework for the future.
8.6.6 Ideal Model for Secure Communication
The last open challenge relates to the communication model assumed. Wired networks,
by their very nature, are essentially a collection of well-defined point-to-point com-
munication links. The same cannot be said about wireless networks because the wireless
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