## 5 Simulation Results  
In this section, we evaluate and compare the performance of IRDT, RI-MAC and X-MAC by using computer simulation. Also, we clarify the impact of collision avoidance for control packets. We devised a large-scale sensor network system composed of a large number of nodes as an application of the proposed method to our further studies. However, the ns-2 simulator, which is the most general simulation tool, does not scale well for such sensor networks, as discussed in [16]. Therefore, we prepared an event-driven simulation program written in Visual C++ for this experiment. Evaluation by using a general simulator that scales well for sensor networks is under consideration. Here, we use the network model shown in Figure 11, in which one sink node and 49 sensor nodes are deployed over 400x400 m\({}^{2}\). In this figure, the sink node is represented as a square, and other shapes denote sensor nodes. The communication range of each node is 100 m, and the sensor nodes shown in the figure with the same shape and color have the same number of hops from the sink node. When modeling the network, we used the following assumptions:  
* Static network topology
* A disk model is used in order to abstract away from any fluctuations in wireless communication
* The capture effect is not considered  
In order to examine the impact of collision avoidance for control packets, we assume that the network topology is static. Regarding the model of communication between nodes, we employ the disk model, where the strength of the radio signals does not deteriorate, and unless packet collisions occur, a transmitted packet is assumed to be received for certain by the nodes within the communication range. In addition, our evaluation is performed on with conservative settings for the packet collision model in which both packets are always discarded if a packet collision occurs while a packet is being received. Note that when another wireless communication model is utilized, the value of \(T^{*}\) is varied with time, and therefore nodes should frequently exchange information about the network topology for the purpose of calculating \(G(R)\). Also, regarding the capture effect, even though the value of \(P_{SREQ}\) appears to decrease slightly, SREQ collisions are of intrinsic importance in IRDT. In our simulations, sensor nodes other than the sink node in the network generate data packets according to a Poisson process. Each sensor node transmits data to the sink node through a multi-hop relay, where the routing algorithm for IRDT in the simulation is described in Section 3.2. Here, data is collected after completion of the exchange of routing tables. Each node conducts CSMA/CA in order to avoid collisions with other packets. Before a node transmits an ID packet or an SREQ packet, it performs a clear channel assessment (CCA). If the channel is busy, it does not transmit a packet. In the case of other types of packet transmission, a node performs up to five attempts for binary exponential backoff of CSMA/CA. The initial size of the contention window is set to \(W_{min}\) and incremented up to \(W_{max}\). All nodes use a data discard timer for preventing repeated SREQ collisions from occurring, where the timer is set to \(T_{d}\). The parameters are set as shown in Table 1. In particular, the \(TTL\) is set to \(H+3\) (\(H\) is the number of hops from the sink node) since extra relays increase the energy consumption. \begin{table}
\begin{tabular}{l r} \hline Parameter & Value \\ \hline Simulation time & 6 h \\ \(T_{d}\) & 5 s \\ \(T_{si}\) & 300 s \\ \(TTL\) & \(H+3\) \\ \(T_{ws}\) & 2 ms \\ \(T_{wd}\) & 10 ms \\ Contention window size (\(W_{min}\)) & 3 \\ Contention window size (\(W_{max}\)) & 5 \\ Current consumption (TX) & 20 mA \\ Current consumption (RX) & 25 mA \\ Current consumption (Sleep) & 0 mA \\ Packet size (ID, SREQ) & 24 bytes \\ Packet size (DATA) & 128 bytes \\ Packet size (RACK, DACK) & 22 bytes \\ Transmission rate & 100 kbps \\ \hline \end{tabular}
\end{table} 
Table 1: Parameter settings.