# Novel Extensions to Enhance Scalability and Reliability of the IEEE 802.15.4-DSME Protocol
## 5 Performance Assessment
### Packet Loss Ratio Assessment  
To evaluate the improvements introduced by the Mirror D-DSME extension a comparative assessment was performed using the OMNeT++ framework, with the simulation parameters shown in Table 2 (i.e., the same ones used in Section 5.2).  
The assessed metric is the Packet Loss Ratio (PLR), defined as the percentage of lost messages over the number of messages generated at the application level, i.e.,  
\[PLR=\left(1-\frac{\textit{ReceivedMessages}}{\textit{Gen Messages}}\right) \times 100 \tag{7}\]  
where _ReceivedMessages_ is the number of messages received at the sink application layer and _Gen Messages_ is the number of messages generated at the source application layer.  
The simulated scenario includes one _PAN-Coordinator_ that receives real-time messages from 7 end nodes. Each End node generates a flow \(f\) made up of 59-byte messages with a period \(P_{f}=4T_{SF}\).  
To assess the reliability under different channel conditions, the Log-normal shadowing channel model with \(\alpha=3.44\) and \(\sigma=7\) was adopted, as these values provide a very harsh channel environment, similar to a realistic industrial context [31]. Results were obtained increasing the distance of the nodes from the _PAN-Coordinator_ that acts as the sink.  
Two configurations (one for DSME and one for D-DSME) were considered. In the DSME configuration, the messages are confirmed and the Group ACK is enabled, while in the D-DSME configuration the messages are unconfirmed and the Group ACK is disabled. The simulation parameters (excluding the above discussed Group ACK option) are shown in Table 2. In D-DSME  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Parameter** & **Value** \\ \hline Superframe Duration (\(T_{SF}\)) & 61.44 ms \\ GroupAck & Disabled \\ CAP reduction & Enabled \\ macMinBE & 5 \\ maxMaxBE & 5 \\ macMaxCSMABackoffs & 4 \\ macMaxQueueLength & 590 bytes \\ \hline \hline \end{tabular}
\end{table}
Table 2: Simulation parameters.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Min.\_Max Supported RTNs (\(n_{nd}\))** & \multirow{2}{*}{\(n_{SF}\)} & **Avg. Queueing** \\ \hline
**Std. DSME** & **D-DSME** & & **Delay \(\mathcal{D}\)** \\ \hline \(1\ldots 3\) & \(1\ldots 4\) & 1 & \(0.32\pm 0.005\) s \\ \(4\ldots 11\) & \(5\ldots 14\) & 2 & \(0.79\pm 0.01\) s \\ \(12\ldots 26\) & \(15\ldots 34\) & 4 & \(1.60\pm 0.02\) s \\ \hline \hline \end{tabular}
\end{table}
Table 3: Simulation results.  
each multi-superframe contains only one superframe, therefore \(T_{SF}=T_{MSF}\). As the flow period is \(P_{f}=4T_{SF}>T_{MSF}\), the _miGTS_ in the first multi-superframe can be used to transmit the message. The _miGTS_s in the following three multi-superframes are used for retransmissions, and so on.  
In the D-DSME configuration, the same (unconfirmed) message can be transmitted (or retransmitted) four times as a maximum. Conversely, with the standard DSME approach the (confirmed) message is transmitted twice as a maximum.  
Figure 5 shows the measured PLR values. They are significantly lower when the Mirror D-DSME extension is adopted. This is a reasonable result, as in the D-DSME scenario an end node has more chances of transmitting the same message than with the standard DSME protocol.  
In particular, the lower the GTS use, the higher are the advantages in terms of PLR that the Mirror D-DSME extension provides. Clearly, if no flow has period \(P_{f}>T_{MSF}\) there will be no unused slots. In such a case, the Mirror D-DSME extension does not improve the standard DSME protocol.