## 5 IEEE 802.15.4 Technology
### Performance Trends of 802.15.4 Based WSNs  
In this section some examples of performance trends obtained from the study of IEEE 802.15.4 networks are shown. The aim is to provide some numerical results in terms of throughput and energy consumption and to show how the choice of the topology affects performance in WSNs. Results are achieved through experimental measurements, using Freescale devices IEEE 802.15.4-compliant working at 2.4 GHz [34], and through the mathematical models of non-beacon- and beacon-enabled 802.15.4 networks described in [35] and [36], respectively. In Figure 11 experimental results are shown. A point-to-point network, where a source node has to transmit data to a destination node, possibly through a number of routers, is considered. When one router between the source and the destination is present, a two-hop communication is performed; in case of two routers we have three hops, etc. Nodes work in beacon-enabled mode, and we set \(SO=BO=0\) for the one-hop case and \(SO=0\) and \(BO=2\) for the multi-hop cases. The figure shows the behavior of the throughput, that is the number of bits (of the MAC payload) per second successfully received by the final destination, as a function of the payload size. Even if the channel bit rate is 250 kbit/sec, the throughput is significantly smaller because of the protocol overhead, mainly due to the MAC layer. As we can see the throughput does not reach more than about 120 kbit/sec for point-to-point links. However, the throughput can be significantly lowered owing to the potentially interference among the separate hops disturbing each other. In Figure 12 a single-sink scenario where 802.15.4 nodes transmit data to the sink through one link (star topology), or possibly two-hops (3-level tree, rooted at the sink), is considered. A network composed of 30 nodes, working in beacon-enabled mode is accounted for. In the figure we show the throughput as a function of the size of the packets transmitted by nodes. The throughput here represents  
Figure 11: Throughput measured for a point-to-point 802.15.4 network when one, two three or four routers are present. the number of bits (of the payload) per second correctly received by the sink when all the 30 nodes try to access the channel and transmit their packets, assuming that nodes transmit packets of the same size. For a fair comparison in terms of delay, curves obtained by setting the same value of \(BO\) should be considered (meaning that the superframes have the same whole duration). As we can see, for low values of the packet size, stars outperform trees, whereas trees perform better when the packet size increases, since less nodes compete to access the channel at the same time (nodes are split in two levels). The best case is obtained in case of \"tree\" when \(SO=0\) and \(BO=2\). This is, in fact, for the network chosen (with 30 nodes and a mean number of 3 level one nodes) the best compromise between the duration of the active part of the sink superframe (where level one nodes transmit) and that of the inactive part (where level 1 superframes are located). The differences between results in Figure 11 for the case two-hop and those in Figure 12 for the tree case when \(SO=0\) and \(BO=2\) are due to the fact that in the first case only one node is transmitting toward the sink, whereas in the latter case 30 nodes are competing for the channel. The increase of the number of nodes on one hand increases the amount of data transmitted toward the sink per unit of time, but, on the other, the success probability, that is the probability that a node succeeds in transmitting correctly the packet, decreases [36]. Finally, in Figure 13 the mean energy spent by a single node having a packet of a given size to be transmitted, is shown. We assume that the network is composed of \(N\) 802.15.4 nodes working in non beacon-enabled mode and competing to transmit their packets to the sink through a direct link. The model developed in [35] is used.