# An Efficient SS-MAC Protocol for IEEE 802.15.4-Based WSNs of Cluster Tree Topology
## 5 Performance Evaluation
### Performance Comparison of the Four Protocols  
Different from the CSMA/CA technique adopted by IEEE 802.15.4 MAC, in SS-MAC, the data collection process within a cluster employs a slot-based reservation transmission mechanism, which avoids CCA and backoff operations before packet transmission. The SSMA protocol [8] also adopts this transmission mechanism, but it requires the receiver to reply an ACK to the sender for each packet received, which will affect the protocol's performance. The LELLMAC [9] make use of three algorithms to ensure that two source nodes do not send packets to the same recipient at the exact same moment. Each node needs to send a Request to Send (RTS) to the target node before sending a packet, and wait for a Clear to Send (CTS) after the packet has been sent. The protocol adopts low-duty cycle technology to reduce node energy consumption, and the maximum amount of time allowed to data transmission per wake-up cycle is 60 ms. For the LELLMAC, frequent RTS and CTS packets transmission and a 60 ms transmission duration limit may have a significant impact on the protocol's performance. So far, there are few MAC protocols proposed specifically for WSNs with cluster tree topology. For a better performance comparison, we simulate the data collection process using the four protocols above in a WSN composed of one sink and \(N_{m}\) member nodes, and explore their average packet delay and throughput performance. The average packet delay versus the arrival rate is shown in Figure 11. The average packet delay of the four protocols increases with increase in the packet arrival rate. The curve of IEEE 802.15.4 MAC is the steepest; the average packet delay of IEEE 802.15.4 MAC increases faster than the other three protocols. For SS-MAC, when the packet arrival rate is 8 p/s, the average packet delay with \(N_{m}=10\) is almost twice that of \(N_{m}=5\), which are 0.13 s and 0.22 s, respectively. Compared with SS-MAC, SSMA has similar simulation curves and the average packet delay under the same conditions is larger, which is related to the transmission mechanism that SSMA requires nodes to reply to an ACK for each received packet. Because of the staggered scheduling method, the average packet delay of the LELLMAC is lower than that of the SSMA under the same conditions, but it is still higher than that of SS-MAC, as the nodes need to send RTS packets before transmitting data packets and reply with CTS packets after completing data packets transmission. For IEEE 802.15.4 MAC, the average packet  
Figure 10: Energy consumption versus \(N_{m}\) for SS-MAC. delay is least affected by \(N_{m}\). Moreover, in the case of a low packet arrival rate, IEEE 802.15.4 MAC has the lowest average packet delay and the best performance among the four protocols. Because the channel is basically idle under low load, it is easy for the nodes to obtain channel resources for sending data packets by performing backoff and CCA operations. However, with a continuous increase in packet arrival rate, the average packet delay of SS-MAC is gradually lower than that of IEEE 802.15.4 MAC, and its superiority becomes obvious. Figure 12 shows throughput per node under different load. The throughput of SS-MAC is significantly higher than IEEE 802.15.4 MAC, SSMA and LELLMAC under the same conditions. In a network using IEEE 802.15.4 MAC and \(N_{m}=5\), the throughput of each node reaches a peak at about 1200 b/s when the load per node is about 2600 b/s; if \(N_{m}=10\), the throughput peaks at about 550 b/s when the load is about 1200 b/s. For a network using IEEE 802.15.4 MAC, an increase in load will increase the number of nodes competing for the channel and the number of backoff operations, resulting in throughput degradation, which will not occur in SS-MAC. If the data rate of PHY is 19,200 b/s, for the SS-MAC protocol, it can be predicted that the throughput per node of a network with \(N_{m}=5\) will tend to \(19,200/(2\times 5)=1920\) b/s (the payload of the SS-MAC data frame is half the frame length).