# A Cooperative MAC Protocol for a M2M Heterogeneous Area Network
## 6 Simulation Model and Results  
To evaluate the effectiveness of the proposed algorithm, a discrete event-based OPNET model has been developed to simulate the network architecture shown in Figure 6. The key element of this model is the MFDRR that relays packets to and from end devices to the data sink. Figure 9 shows the OPNET model structure of the MFDRR. The left side of the figure shows the implementation of the 6 LoWPAN protocol stack. Currently no 6 LoWPAN model exists in the OPNET library, so we have developed a new 6 LoWPAN OPNET model. The 6 LoWPAN protocol stack has been developed utilizing the open-zb model available publicly [27]. The open-zb model implements the PHY and MAC layers of the IEEE 802.15.4 slotted CSMA/CA protocol. The adaptation layer developed is responsible for the IPv6 header compression and de-compression as required by RFC 4944. The IP and application layers are implemented on top of the adaptation layer. The application layer of the MFDRR is split into two parts where the 6 LoWPAN protocol forwards payloads to the aggregation buffer which is used to generate 802.11 packets. In the reverse direction, when a WLAN  
Figure 8: Blank burst (BB) packet transmission timing relationship.  
packet is received from the data sink, the 802.11 protocol stack strips all headers and delivers the combined payload to the 802.15.4 application layer. The combined payload is de-aggregated by the 6 LoWPAN application layer and individual payloads are sequentially forwarded to the respective MAC layers for the downlink transmission. The 802.11 protocol stack was implemented using the OPNET library. The key simulation parameters are also listed in Table 3. End devices are located 300 meters away from the coordinator. In the heterogeneous network, a three-hop transmission link is used, whereas six hops are used in the case of a comparative homogeneous network model when only 6 LoWPAN nodes are employed. For the heterogeneous network, the MFDRR to the data sink uses a 6 Mbits/s link that significantly reduces the packet transmission time compensating for the packet aggregation delay introduced by the buffer.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline
802.11 and 6 LoWPAN Tx Power & 100 mW and 1 mW \\ Simulation duration & 10 s \\ MFDRR to Data sink distance & 150 m \\ End device to the MFDRR & 100 m \\ M2M area network service area & 0.196 sq. km \\ M2M node density: & \\ Heterogeneous network & 326 nodes/sq. km \\ Homogeneous network & 163 nodes/sq. km \\
6 LoWPAN and 802.11 channel nos: & 11,12 (6 LoWPAN) and 1 (802.11) \\ Path loss model & Free space \\
802.15.4 packet size (bytes) & 128 \\
802.11 packet size (bytes) & 1200 \\ Superframe Order (SO) & 3 \\ Beacon Order (BO) & 4 \\
6 LoWPAN data rate (kbps) & 250 \\ WLAN data rate (Mbps) & 6 \\ Aggregation factor & 25 \\ Blank burst duration \(T_{BB}\) & 2.7 ms \\ End device packet inter-arrival time, & 2, 1, 0.66, 0.5 \\ exponentially distributed (sec) & 2, 1, 0.66, 0.5 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Key simulation parameters.  
Figure 9: OPNET modeler architecture of the proposed heterogeneous network.