# On the Feasibility of Wireless Multimedia Sensor Networks over IEEE 802.15.5 Mesh Topologies
## 6 Discussion  
The previous section has shown the performance of IEEE 802.15.5 mesh networks transmitting with a packet generation rate of 53 kbps and dispatching one packet per _wakeup interval_, since transmitting more than one packet per _wakeup interval_ was observed to lead to congestion and therefore worsen the performance of the network. In particular, the effect of using different WO-AO configurations for several network sizes has been studied. In spite of considering that just a single node is transmitting video, the previous metrics show that there is some drop of performance because different packets of the same stream may be routed by different paths and compete for transmission. In this regard, the performance drop observed is due to repetitive failures to gain access to the medium while it is busy (CSMA-CA failure), aggravated by the exposed and hidden nodes problems. To cope with these problems, the reservation-based method of the SES mode proposes the use of a relatively short _active duration_ period, where all contention for reserving transmission slots takes place, and the _inactive duration_, where nodes have slots where transmission is guaranteed. In data-intensive applications, though, this inactive period can also be affected by the hidden node problem to some degree, as mentioned. During the active period, nodes have the opportunity to reserve slots for subsequent transmissions in the inactive period. Since the active duration of all neighbor nodes is synchronized, it is likely that several nodes will try to perform simultaneous reservations. In order to avoid collisions, the CSMA-CA scheme allows that, if some reservation message cannot be sent because the channel is busy, several subsequent retransmission attempts (up to a configurable maximum number in the IEEE 802.15.4 standard, _macMaxCSMABackoff_, 4 by default) can be performed. However, according to the simulations, AO values of 1 and 2 result in active durations not long enough to perform all CSMA-CA retries. For example, the maximum delay (in a worst case scenario) due to the CSMA-CA backoff is 36.8 ms (calculated as \(\sum_{backoff=0}^{backoff=4}(2^{BE}-1)*\)UnitBackoffPeriod \(*\) symbol_time where \(BE=\min(\text{macMinBE}+\text{backoff, macMaxBE})\)) using the IEEE 802.15.4 default values at 2.4 GHz, whereas for AO = 1 the active duration is only 10 ms long. This implies that several active durations may be necessary in order to reserve one transmission slot. However, during that time, new packets would also require to be transmitted, increasing the contention for medium access (if the packets are received by neighbor nodes) or the queue size of the node, and therefore the performance of the network would get worse as time goes on. Given all the factors that affect negatively the transmissions during the _active duration_, it is clear that it is very important to select an AO value that allows transmission with an appropriate QoS. It has been observed for all the previous QoS metrics that for AO values equal to or greater than 3, the performance has been quite satisfactory. Taking into account that the AO parameter is detrimental to power consumption and network lifetime, a value of 3 can be used for the scenarios studied without affecting latency, jitter, throughput and message delivery ratio. Regarding the selection of the _wakeup interval_ length, and consequently the WO parameter, it is important to find a balance between power consumption (which improves with high WO values) and throughput (which improves with low WO values). In addition, it has also been observed that the size of the network may affect the quality of the received video due to the hidden terminal problem, and more particularly secondary collisions. Other negative factors such as contention for medium access, the exposed node problem or primary collisions due to the hidden node problem are avoided by the use of transmission in reserved slots of the inactive period.