## 6 Conclusions  
This paper analyzes end-to-end QoS in synchronized low duty cycle networks. The contributions of this paper are summarized as follows  
* We define two metrics to measure the WSN performance: reliability and availability. The reliability is the traditional metric that measures the probability of successful transmission. The availability measures the time that a node has been active, which makes it well suited for data centric WSNs.
* An analysis on the effect of synchronization with beacons in a low duty cycle network shows that significant amount of network capacity is lost due to missed beacons. The capacity loss is equal to the PER, when PER is between 0%-20%. On higher error rates, synchronization losses become common and the capacity loss increases. In a typical WSN MAC, the reduction is 31% and 44% for 30% and 40% PERs, respectively. The result motivates developing more robust beaconing schemes to increase the beacon reception probability, such as sending multiple successive beacons.  
Figure 27: Average end-to-end latency with different packet error rates in ZigBee with and without the reliable data forwarding algorithm.  
In addition, to reduce the downtime after a synchronization failure, association procedure should be fast and robust e.g. by using only short message exchange.
* We analyze the hidden node problem in IEEE 802.15.4 contention-based channel access and note that the hidden nodes cause a significant reliability problem in practical low duty cycle networks. Even with few competing nodes, the end-to-end goodput drops by 20%. As a result, we propose adjustments to the backoff exponent and access cycle lengths that decrease the collision probability significantly.
* In our analysis on network load against packet errors, we found that the typical 3 retransmissions per link are enough to provide good availability when PER is less than 15%. With higher packet error rates, an end-to-end reliability scheme is required.
* We show that preparing buffer space only based on the expected traffic is not enough. Packet errors significantly increase the buffer requirements. In low duty cycle networks, sufficient buffer space is required to prevent data loss as a node cannot forward its data due to beacon misses and synchronization losses. In our simulations, compared to the network without packet errors, 30% PER doubled the buffer requirements.
* We present a simple but efficient reliable data forwarding algorithm suitable for resource constrained WSNs. The benefit of the reliable data forwarding algorithm is its ease of implementation and applicability to several existing MAC and routing protocols. The algorithm buffers a packet in intermediate nodes along a route until the packet is successfully forwarded. The buffering scheme guarantees end-to-end reliability, assuming that network errors are caused only by link failures (node failures do not occur).  
This paper identifies and evaluates the factors affecting low duty cycle WSN performance. The following findings are a subject for further work. First, while the proposed methods to reduce the hidden node problem in ZigBee are shown as valid, analytic evaluation on the trade-offs and optimal parameters on different duty cycles, traffic loads, and packet error rates is required. Second, our ZigBee evaluation did not utilize the contention-free channel access, as its performance depends on the reservation policy that is not explicitly specified in the standard. The effect of different reservation methods to the performance should be evaluated. Last, the reliable data forwarding algorithm should be compared against other end-to-end reliability schemes, such as WSN transport protocols or TCP modifications for wireless multihop networks. Also, the impact of utilizing alternative links to the reliability and energy-efficiency should be examined more carefully.