System Architecture Design

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pSHIELDSystem Architecture DesignPU3.1.2.4.1 Physical LayerIn wired networks, the role of the physical layer has been rather small—that of sending and receivingpackets when required to do so from the higher layers. As we have seen, advances in the signalprocessing at the physical layer can allow it to play a bigger role in wireless networks. Consider, forinstance, multi-packet reception capability at the physical layer.3.1.2.4.2 MAC LayerThe MAC layer’s functionality is intimately connected to the network layer (and hence to the rest of thestack), we see that signal processing advances at the physical layer promise to have a significant impacton all aspects networking protocol design. Cross-layer designs relying on advanced signal processing atthe physical layer are an interesting research ground for the future.3.1.2.4.3 Cross-Layer PrinciplesThe solutions for cross-layer adaptation seek to enhance the performance of the system by jointlyoptimizing the performance of single or multiple cross-layers. The uncertainty is to what extent the layeredarchitecture needs to be modified in order to introduce co-operations among protocols belonging todifferent layers. At one end, solutions based on triggers between the layers implement interdependenciesbetween protocols while maintaining compatibility with strict layering. A full cross-layer design representsthe other extreme; this implies introducing stack-wide layers’ interdependencies that enable theoptimization of each protocol’s performance by exploiting the full knowledge of the network statusabstracted at different layers of the protocol stack.However, in a multiple-objective optimization scenario, care should be taken to avoid undesirable (andunpredictable) interactions across parameters in various layers, leading to conflicts or even loops betweenthe layers. There also exists a design trade-off between the multiple optimization goals and the effect ofthe increased processing and interactions to achieve these goals. Unfortunately by doing cross-layerdesign in an undisciplined way, it is likely to end with a poorly structured system and to greatly increasethe complexity of an already complex system [10][11].Figure 6 illustrates the main ideas of cross-layer adaptation and optimization in a hybrid terrestrial andsatellite network. Cross-layer optimization may be implemented locally (intra-node) or globally (internode).A number of cross-layer methods and architectures have been proposed in the literature[12][13][14] [15][16]. They all share some common features and diverge notably in the way the cross-layerprinciple is implemented, on what kind of application they focus, in the capacity the architecture has andwhere the actual adaptation intelligence is located.PUD2.3.2Issue 5 Page 24 of 122
pSHIELDSystem Architecture DesignPUFigure 6 - Cross-layer adaptation and optimization in satellite network.These architectures mostly fit out into one of the two categories: direct or explicit cross-layercommunications and indirect or implicit cross-layer communications via a common entity, see Figure 7.PUD2.3.2Issue 5 Page 25 of 122
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