# Design and Analysis of a Low Latency Deterministic Network MAC for Wireless Sensor Networks
## 7 Simulation Results  
In this section, we describe our simulation results and validate the models using the OMNeT++ [34] simulator. We conduct the simulation to compare the performance of our protocol with IEEE 802.15.4e. We have analyzed the performance of our proposed protocol under the assumption that nodes are active in their allotted slots and go for power saving mode in the rest of the beacon interval. In our simulation environment, the star topology is considered, and the nodes are deployed randomly, keeping the coordinator in the center of the circle. We set the simulation parameters according to the IEEE 802.15.4e standard, which are given in Table 2.  
In Figure 4, let the \(X\) axis be the different offered loads for each node and the \(Y\) axis be the corresponding packet success probabilities. It is observed that the packet success probabilities decrease with respect to the loads per node increase irrespective of the number of nodes in the network attached to a shared slot. When 10 nodes per shared slot are considered and the offered load is of 500 bits, we find that the success probability is 0.77. However, we got the success probabilities for 20 and 30 nodes per shared slots as 0.71 and 0.68, respectively. It is also seen from Figure 4 that the simulation result matches with the analytical one. From Figure 5, it is observed that the packet success probabilities decrease with respect to the number of nodes attached to the shared slot with or without hidden terminal problem under the offered load 200 bits/node. The comparison of our proposed protocol with IEEE 802.15.4e for the packet success probability under ideal channel condition with 20 nodes per shared slots is shown in Figure 6. It is observed that the packet success probabilities decrease, if the offered loads to a shared slot increase. According to the IEEE 802.15.4e standard, the initial contention window for the stations is \([0,8]\). Hence, the expected number of nodes choosing the same slot will be \(20\times(1/8)\geq 2\). Accordingly, the chance of transmission by two stations at the same time is higher Fortunately, in our proposed protocol, the contention window is based on the active number of nodes per shared slot such that the expected number of nodes choosing the same slot is always less than or equal to one. Hence, the packet success probability of our MAC protocol outperforms, as noticed in Figure 6. Even if, two nodes choose the same random backoff value and access the channel in the same slot, our carrier sensing mechanism lets the nodes go to the backoff state. For example, as shown in Figure 2b, \(A\) and \(B\) find the channel busy due to simultaneous CCAs and stop the packet transmissions, and \(C\) can transmit its packet successfully. As a result, the packet success probability is enhanced. \begin{table}
\begin{tabular}{c c} \hline \hline
**Notation** & **Meaning** \\ \hline Parameters & Value \\ Radio band & 2.4 GHz \\ Channel bandwidth & 250 kbps \\ Carrier sense sensitivity & \(-85\) dBm \\ Channel number & 11 \\ Superframe duration & 31.2 ms \\ Number of nodes per shared slot & 20 \\ Shared slot duration & 1.95 ms \\ Number of transmission slots in one superframe & 10 \\ Number of retransmission slots in one superframe & 5 \\ Unit backoff period & 20 symbol \\ PHY overhead & 6 byte \\ MAC overhead & 3 byte \\ Short address & 8 bit \\ Transmission current consumption & 9.1 mA \\ Receiving current consumption & 5.9 mA \\ Sleep current consumption & 0.001 mA \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Simulation parameters. Figure 4: Validation for packet success probability with different offered load. In Figure 7, we consider the offered loads along the \(X\) axis and the corresponding throughput along the \(Y\) axis. The data payload size is a maximum of 127 bytes including the physical and MAC layer overhead and address. We consider the physical, MAC layer and address to be 6 bytes, 3 bytes, and 8 bits, respectively. The throughput initially increases with respect to the offered load.