## 5 Rate Adaptive MAC (RA-MAC)
WLAN rate adaptive algorithm [22], the sender increases transmission data rates after ten consecutively successful packet delivery, and decreases transmission data rates after one failed packet delivery. We used an experimental log file, which records packet data rates, packet delivery status (success or failure), RSSI values, node radio states, and time-stamps, collected by the TinyNodes to calculate the packet delivery ratio, energy consumption and energy consumption per received packet (same metric as Section 6.3.). Based on packet delivery status and RSSI values recorded in the log file, ARF and RA-MAC uses different strategies to choose transmission data rates. The probability of successful delivery given a certain RSSI value, and the average amount of energy consumed given a specific transmission data rate, are based on the statistics over the whole experiment. Although ARF achieved higher link packet delivery ratio compared to RA-MAC (Figure 9(a)), it consumed higher network energy (Figure 9(b)). Overall, Figure 9(c) shows that RA-MAC, compared to ARF, achieved significantly higher transmission energy consumption efficiency by consuming less energy for one successful delivery. The rate-adaptive algorithms in WLAN (e.g., ARF) has focused on optimizing network throughput which is important to users of WLANS. However this may not be applicable to the context of WSN, in which energy is usually a more important metric. The link quality in WSNs is typically more dynamic and lossy than that in WLAN. WSN nodes are usually equipped with inexpensive radio transceivers, with poorer gain and SNR. Many WSN nodes use narrow band FSK communications which is more susceptible to fading than spread spectrum modulation techniques. WSN nodes have a significantly smaller transmission energy budget compared to WLAN nodes because of network lifetime concerns. Finally, WSNs are often deployed in harsh, or even hostile, environments with significant attenuation and multi-path fading. Consequently, it is difficult to achieve 10 consecutive successful transmissions, and ARF had to use low transmission data rates most of the time. ## 8 Conclusion

We have introduced RA-MAC, an energy-efficient Rate Adaptive Media Access Control protocol for long-lived WSNs. Previous research has shown that the dynamic, lossy, and asymmetric nature of wireless communications is one of the major challenges to reliable data delivery in WSNs. This is due to

Figure 9: RA-MAC _vs._ ARF. nodes operating within the transition zone where the received SNR is highly variable and various noise processes contribute to the unreliable behavior. RA-MAC increases the link quality by raising the SNR, trading it off against data bit rate which generally far exceeds the requirement of the link. We have used an analytical model of energy consumption at different data rates and channel conditions to _minimize network energy consumption_. Finally, we have implemented RA-MAC in TinyOS for an off-the-shelf sensor platform (the TinyNode), and compared its performance with the state-of-the-art WSN MAC Protocol (SCP-MAC) in both simulation and experiment, showing the real benefits of our protocol. ## References

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