## 4 Physical Features and Response Characteristics
### Harvesting Physical Features from the Network  
Harvesting information about the network allows the framework to learn the state of the physical deployment with regard to the features necessary for solution comparison. The network manager initiates a probing regime in order to interrogate the deployment. Figure 5 shows how this process is performed in the network. The process is initiated by the network manager by sending a message to the root. On receipt of this message the root must inform the entire network. The root may use the structure of the current communication solution or a flooding technique to proliferate the message to each node in the network. Each node then sends a beacon within its neighbourhood which requests a reply from each neighbour. Addressing a particular node when replying incurs additional communication overhead particularly in dense networks but increases reliability on information retrieved. With the replies received a node will generate a list of the physical features in its neighbourhood. When a node is confident it has received all replies from the network a packet is sent to the root containing the information it has garnered from its neighbourhood. The root passes this information to the network manager from which it can build a virtual network topology from the local physical features. Network wide physical features can be obtained from this virtual topology. The information gathering process is initiated at the outset of the network to first establish which communication solution to use within the deployment. The network manager then initiates this process in a periodic manner throughout the network lifetime to remain reactive to any major changes that might occur within the network. As the framework takes a macroscopic perspective on network control the network manager does not need to respond to small fluctuations within the network. These changes are handled by the more dynamic communication solution deployed in the network. Large changes to the network, however, may necessitate a change of communication solution. Large changes to the physical network occur much less frequently allowing less frequent probing by the framework. Using the same BEMS scenario as described in Section 7, an evaluation of the additional overhead incurred via the information gathering process is undertaken. The results in Figure 6 show the additional overhead incurred during information gathering assuming this process is initiated once every running hour. The data shown in the graph is taken from simulation during the testing phase described in Section 7. The figure shows results over a range of network sizes and densities. In general a higher load is observed in the network at lower densities. This is in part due to larger number of retransmissions in the network as well as observing a larger diameter compared to more dense networks of similar scale. The frameworks probing process shows heavier burden on the network at higher densities. This is due to the addressed reply policy employed by the framework. In dense networks each node will receive many beacons and reply to each of them individually increasing overhead. The data collection overhead as a percentage of the overall load is reduced as the size of the network increases. This is due to the wider diameter of the network at higher scale. While higher network densities will have a large knock on effect on the collection process, the diameter of the network has a lesser impact on the data collection process than that on normal network load. It is important to note that this is the maximum overhead which the network may incur as a result of probing by the framework. There are many ways this overhead can be reduced within the network. Should a communication solution maintain a full list of neighbours including the physical link metrics for neighbours, steps 5c,d from Figure 5 are no longer required, significantly reducing overhead. Allowing local physical information to piggyback on data packets can further reduce network overhead.