## 3 Simulation Framework  
Our proposed simulation framework for wireless real-time mesh networks exploits common design restrictions that render such networks significantly more predictable than contention-based wireless networks. The most important common design features of state-of-the-art real-time mesh networks include the following:  
* Static, conflict-free resource scheduling: the physical layer resource usage is coordinated centrally, so that collisions are explicitly avoided, and thus, there is no interference (or only a very low amount) within nodes belonging to the same network. The spatial reuse of frequency resources is not allowed, and the nodes are assigned fixed transmission opportunities in time and frequency, depending on their pre-arranged transmission needs. Furthermore, limiting the maximum number of nodes within the network prevents problems arising from the lack of frequency reuse.
* Frequency division multiple access by coordinated frequency hopping: to increase the number of simultaneous transmissions, the nodes perform coordinated frequency hopping over multiple channels; thus, multiple nodes, up to the number of available channels, can transmit simultaneously. The frequency hopping also mitigates narrowband external interference and effectively randomizes the effects of the multipath fading in the wireless channel.
* Centrally-coordinated, redundant multipoint routing to avoid failures due to problems with single nodes within the network.  
This static, mostly contention-free operation allows building simulators that are significantly less complex than typical simulators for wireless _ad hoc_ networks and allows for significantly increased simulation accuracy by eliminating the intra-system interference issue. On the other hand, some design features of such real-time networks, such as channel hopping, are usually not present in other wireless systems.  
System-level simulations focus on the network-wide aspects of complex wireless systems that are intractable by analytic means and/or do not lend themselves to define a single figure of merit to characterize the performance of the system. Such aspects in a typical wireless mesh, _ad hoc_ or sensor network might include algorithms for user and resource scheduling, resource allocation, interference management, _etc_. In addition to these pure networking aspects, in wireless cyber-physical systems, the design and verification of the underlying physical system pose an important additional challenge.  
Using realistic system simulators, the system-level effects of physical and link layer improvements can be evaluated without the need to implement the improvements on actual devices in the early design stages. Very detailed packet-level simulators, even if they incorporate accurate physical layer models, often yield to excessive simulation times to gain statistically significant results. For this reason, computationally efficient abstract evaluation techniques are preferred.  
In this section, we present our system simulation framework for wireless real-time control networks, an approach that can yield accurate system-level application performance metrics, while keeping the computational complexity reasonably low by exploiting the inherent restrictions present in typical wireless real-time control networks.