# Congestion Control in CoAP Observe Group Communication
## 5 Performance Evaluation
### Continuous Traffic  
In the continuous traffic scenario, the experiment aimed to explore the efficiency of the congestion control mechanism under typical network conditions. The messages were exchanged continuously by means of the Obs in the five topologies which determined different levels of congestion. The results were recorded following the performance metrics suggested in Section 4.4. Table 3 shows the overall performance metric values in the continuous traffic scenario for different topologies in the default CoAP, compared to those of CoCo-RED. Two metrics, hop count and setting time, served as the indicators for the congestion levels. The dumbbell topology resulted in the highest level of congestion, followed by the cross, the grid, the chain, and the random (as each node only selected direct neighbors within the transmission range in the chain topology, the messages were then exchanged in a unidirectional manner, and the setting time was eventually low in this study).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Topology**} & **Congestion Control** & **Average Setting** & **Average Response** & **Average Packet** \\  & **Mechanisms** & **Time (s)** & **Time (s)** & **Loss (s)** \\ \hline \multirow{2}{*}{Grid} & Default CoAP & 17.74 & 0.67 & 2.92 \\  & CoCo-RED & **14.18** & **0.59** & **1.63** \\ \hline \multirow{2}{*}{Chain} & Default CoAP & 13.91 & 0.65 & 1.30 \\  & CoCo-RED & **13.26** & **0.54** & **1.00** \\ \hline \multirow{2}{*}{Cross} & Default CoAP & 18.09 & 0.74 & 3.31 \\  & CoCo-RED & **17.78** & **0.70** & **3.07** \\ \hline \multirow{2}{*}{Dumbbell} & Default CoAP & 18.97 & 0.78 & 4.11 \\  & CoCo-RED & **18.70** & **0.73** & **3.64** \\ \hline \multirow{2}{*}{Random} & Default CoAP & 13.26 & 0.55 & - \\  & CoCo-RED & **12.77** & **0.44** & - \\ \hline \hline \end{tabular}
\end{table}
Table 3: Overall performance metric values in a continuous traffic scenario for different topologies (the better-performing mechanism is highlighted in bold).  
The response time and packet loss are also important performance metrics, especially for applications that require immediate reactions, short notification periods, and data integrity. The results from the two metrics in all network topologies suggest that CoCo-RED can handle congestion better than the default CoAP for both low congestion levels (i.e., the chain and random topologies) and high congestion levels (i.e., the grid, cross and dumbbell topologies). This was because, in CoCo-RED, the buffer management helped to measure the congestion levels of the network with the AvgQ controlling the packet drop rate prior to buffer overflow. Additionally, the retransmission timeout from FPB was significantly shorter than the BEB used by the default CoAP.  
Overall, according to the performance metric values for the two mechanisms, CoCo-RED achieved a better performance than the default CoAP. Moreover, CoCo-RED can, in particular, effectively deal with high congestion levels in a network. This aligned with the aforementioned experiment on the packet loss and the response time. To summarize, CoCo-RED seems to be a smart choice for a congestion control mechanism in the Obs or in message exchanges in CoAP Group Communication which require quick responses under continuous traffic in a network.