## 4 Main Features in Wireless Sensor Networks Design
### Example of Wireless Sensor Network Design  
Owing to the plethora of features, WSNs design involves a wide range of aspects and considerations and often imposes that several issues, like connectivity, access to the channel, signal processing techniques, etc., must be accounted for together.  
As an example, in [24] a self-organizing single-sink WSN, enabling environmental monitoring through the estimate of a scalar field over a bi-dimensional scenario, is considered. Nodes are assumed to be distributed according to a Poisson point process (PPP) over the area and are organized in a cluster-based topology. Connectivity issues, randomness of the channel, MAC issues and the role of distributed digital signal processing (DDSP) techniques are jointly accounted for, in a mathematical framework developed in the paper. Owing to the requirement of low device complexity together with low energy consumption (i.e., long network lifetime), a proper balance between communication and signal processing capabilities must be found. The adoption of DDSP techniques aims at reducing the amount of transmitted data over the wireless medium; on the other hand, the complexity of the signal processing performed at a single node has to be kept under control [26; 27; 28]. In [24] the possibility that nodes perform DDSP is studied through a distributed compression technique based on signal re-sampling. The DDSP impact on network energy efficiency is compared through a novel mathematical approach to the case where the processing is performed entirely by the sink.  
The model developed allows the analysis of the network under two different perspectives: the estimation of the process and the energy consumption. The trade-off between energy conservation and estimation error is discussed and a design criterion proposed.  
As an example result, the required node density is found as a trade-off between estimation quality and network lifetime for different system parameters and scalar field characteristics. It is shown that both the DDSP technique and the MAC protocol choice has a relevant impact on the performance of a WSN.  
The main goal of [24] is neither to design specific communication protocols, nor DDSP techniques; rather, the joint consideration of all aspects mentioned, under realistic but simple working conditions, aims at stressing their interdependencies in a formalized framework.  
Therefore, being the goal of [24], the proposal of a new approach for designing WSNs suffers the following limits: (i) a single-sink scenario and not the more general multi-sink scenario, is accounted for; (ii) the MAC protocol is very simple (slotted ALOHA), and no reference to any specific standard air interface is provided.  
In [29; 30; 31] a multi-sink WSN, collecting data from the environment through the sampling of some physical entities and sending them to some external user, through multiple sinks, is considered. Through a simple polling model, sinks periodically issue queries, causing all sensors perform sensing and communicating their measurement results back to the sinks they are associated with. IEEE 802.15.4-compliant devices are accounted for and nodes access the channel through the CSMA-basedMAC protocol described in Section 5. This paper introduces the concept of _area throughput_, that is, the amount of data per second successfully transmitted to the sinks from a given area. This performance metric is strictly related to both connectivity and MAC issues: it depends, in fact, on the probability that a given sensor node is not isolated and that it succeeds in transmitting its packet (i.e., the packet does not collide).