## 7 Cross-Layer Design Schemes
### Pragmatic Approach to Cross-Layer Design  
Wired networks are effectively a set of well defined reliable communication links over a hard wired transmission medium. Wireless communications, however, are broadcast in nature, subject to interference and signal attenuation and are inherently unreliable, especially in the context of on body communications. The current layered protocol stack was developed with wired communication in mind and has been adapted for use in the wireless context as a best effort use of existing architecture [73]. The new modalities inherent in the wireless medium are not optimally catered for in the current layered protocol stack and inevitably require at least some level of cross-layer design to develop the best solution for wireless communications. Cross-layer designs usually improve performance but can do so at the expense of higher complexity in communication and computation, making complexity reduction an important issue. [PERSON] and [PERSON] [101] argued that layered architecture can be optimal in most cases. Cross-layer design can potentially work at cross purposes and if designers are not careful, a negative effect on system performance is possible [79]. Issues that cross-layer design can introduce are:  
_Additional system complexity & lack of flexibility_. Enhanced performance of cross-layer schemes is often demonstrated via simulations or prototypes, however actual implementation of these schemes highlights issues of introduced complexities and constraints due to modifying a number of protocols in different layers.  
_Protocol incompatibility_. As the traditional protocol stack is violated with cross-layer design, the inherent compatibility and interoperability of protocols that operate in the protocol stack is breached. The integration effort required to adapting a cross-layer protocol to effectively connect to a standard protocol cannot be underestimated and may negate the proposed benefits of any cross-layer design effort.  
_Network protocol evolution constraints_. The in-built independence of protocols operating within the traditional layered protocol stack means that they can be maintained and changed without disrupting other protocols in the stack. This is more problematic with cross-layer design where any change to a cross-layer protocol would need to be potentially coordinated across a number of different layers.  
The trade-off between performance and architecture needs to be considered in the overall context of any network design. Cross-layer designs need to view the totality of design including interactions with other layers and consider the long-term architectural benefit of the proposed cross-layer approach. The law of unintended consequences is key here, cross-layer solutions that show performance gains in the minutiae but result in complexities at other levels of the protocol stack do little to further the advancement of network design [101]. In order to further advance the evolution of wireless communications, the traditional protocol stack architecture needs to be challenged, but only within a framework that sustains and balances improvements against any engineered complexities. The guidelines set out in [101] serve as a set of rules for cross-layer design, namely: (a) map out the interactions between the parameters as part of a dependency graph for the entire protocol stack, (b) consider timescale separation and stability to avoid conflict where two parameters are contending for the same resource at the same time, (c) ensure any implemented cross-layer design is structured,stable, maintainable and able to demonstrate significant benefits that justify the move away from the traditional architecture.