**4. Problem statement**

In solving the smart routing problem via cross-layer design, the goal is to solve the 4-tuple defined by,


6 Wireless Sensor Networks / Book 1

194 Wireless Sensor Networks – Technology and Protocols Cross-Layer Design for Smart Routing in

be used between the application layer and the PHY layer. For example, the application layer may inform the PHY layer of transmission parameters such as transmit power and operating

The design of a cross-layer optimization algorithm for WSNs that consider both performance and energy factors requires efficient communication between protocol stack layers such as the PHY, link, network and application layers. Direct signaling between application layers reduces latency in the communication between multiple layers and is crucial in the design of cross-layer optimization algorithms [9]. The direct signaling scheme for our protocol stack

The goal of direct signaling is to exchange information between important protocol layers for smart routing. This ensures that the required information to perform cross-layer optimization is retrieved, and allocation decisions are sent, with minimal delay. For example, the cluster-head's PHY layer will inform the application layer of the sensor and mesh node state information which includes energy rating information, surrounding interference and more. State information and coordination protocols to provide feedback to the cluster-head are covered in Section 9. The link layer and network layer will also inform the application layer of the channel conditions and available path information, respectively. Furthermore, upon executing the cross-layer optimization policy, the application layer will inform the PHY layer of the necessary resource allocations and the link layer of the next-hop information. Direct signaling enables these interactions with minimal delay for optimized and timely responses

*lx*,*ly* for the direct signaling

(*<sup>n</sup>* <sup>−</sup> <sup>1</sup>) (1)

frequency to use during transmission.

model is illustrated in Figure 4.

in our distributed network.

speed-up factor of (*n* − 1) [9].

method is calculated as,

where *T<sup>L</sup>*

**Figure 4.** Protocol Stack Model to Enable Smart Routing

For any two non-adjacent layers, *lx* and *ly*, the propagation latency *TDS*

*TDS lx*,*ly* <sup>=</sup> *<sup>T</sup><sup>L</sup>*

*lx*,*ly*

*lx*,*ly* is the propagation latency between layers *lx* and *ly* in a traditional layered protocol

stack with (*n* − 1) layers between them. Hence, the direct signaling method provides a

to identify the most suitable path that satisfies both performance needs of an application and energy conservation considerations; this path routes data from a source sensor to a centralized controlling station. In doing so, the cross-layer optimization policy focuses on functions at three layers: path selection at the network layer; channel selection at the link layer; and, transmit power allocation at the physical (PHY) layer. Hence, the 4-tuple defines the operating point for the network in solving the resource allocation for a single transmission.

The 4-tuple forms the basis of utility functions that model the preferences of nodes in allocating various PHY layer resources to throughput; the optimization problem exists since these PHY layer resources also impact network lifetime. Utility functions quantify the performance benefits and power costs associated with the allocation prior to selecting the optimal operating point. In doing so, the criticality of the application plays a major role in determining the necessary tradeoff for the given sensor.
