System Architecture Design

pSHIELDSystem Architecture Design3.1.2 Description of Cross-Layer ArchitecturePU3.1.2.1 A definition for Cross-Layer Design (CLD)A layered architecture, like the seven layer Open Systems Interconnection (OSI) model [3], defines ahierarchy of services to be provided by the individual layers. The services at the layers are realized bydesigning protocols for the individual layers, which can be implemented on the target platform to obtain acomplete system.At the protocol design phase, the designer has two choices. Protocols can be designed by respecting therules of the original architecture. In the case of the layered OSI reference model, this would meandesigning protocols such that they only make use of the services at the lower layers and not be concernedabout the details of how the service is being provided. It also implies that the protocols would not needany interfaces that are not present in the reference architecture.Alternatively, protocols can be designed by violating the reference architecture. Since the referencearchitectures in communication and networking have traditionally been layered, its violation is generallytermed as cross-layer design.1.1.1.1 A taxonomy of CLDIn recent times, a large number of cross-layer designs have been proposed. A classification based on thelayers that are coupled by the different proposals can be found in [4]. In this section, we classify theexisting cross-layer design proposals according to the kind of architectural violations they represent. Twopoints should be mentioned here. Firstly, our coverage of the cross-layer design proposals is meant to berepresentative and not exhaustive. Secondly, the reference architecture we assume is motivated from the“best of both worlds” five-layer model proposed in [5]. Thus, we assume that the reference architecturehas the application layer, the transport layer, the network layer, the link layer which comprises the datalinkcontrol (DLC) and medium access control (MAC) sub-layers [3], and the physical layer—with all thelayers performing their generally understood functionalities.We identify the following architectural violations:1) Creation of new interfaces (Figure 3: A, B, C)2) Merging of adjacent layers (Figure 3: D)3) Design coupling without new interfaces (Figure 3: E)4) Vertical calibration across layers (Figure 3: F)A. Creation of new interfacesSeveral cross-layer designs require creation of new interfaces between the layers. These can further bedivided into three categories depending on the direction of information flow along the new interfaces:1) Upwards: From lower layer(s) to a higher layer2) Downwards: From higher layer(s) to a lower layer3) Back and forth: Iterative flow between the higher and lower layerWe now discuss the three sub-categories in more detail and point out the relevant examples.1) Upward information flow: A higher layer protocol that requires some information from the lowerlayer(s) at runtime results in the creation of a new interface from the lower layer(s) to the higherlayer, as shown in Figure 3 A2) Downward information flow: Some cross-layer design proposals rely on setting parameters onthe lower layer of the stack at run-time using a direct interface from some higher layer, asPUD2.3.2Issue 5 Page 19 of 122