# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 5 Discussion: Application, Challenges and Future Perspectives
### Routing Protocols for Energy Efficiency Enhancement  
Power consumption and energy conservation within sensor networks has been a critical issue for a number of years as sensor nodes are typically powered by various battery technologies which, as discussed in Section 4, can have limited lifespans and other constraints such as cost, installation complexities, maintenance and size. Therefore, energy-efficient design for sensor networks has been a focal point of research and development and has resulted in several energy-efficient routing (EER) protocol approaches being presented [124; 125; 126; 127]. These protocols can largely be categorized into three  
Figure 5: (a) Systematic approach presented by [PERSON] et al. [119] for optimal sensor placement within office buildings using clustering algorithms, data loss and the Pareto principle. (b) Best- and worst-case placements for four sensors (highlighted in green) in a 16-room building considering six different typically used building topologies [123]. main classifications, i.e., (i) Hierarchical (or Clustering-based), (ii) Data Centric, and (iii) Location-based (Geographical), although other classifications such as (iv) Data Relaying, (v) Mobility-based, and (vi) Heterogeneous protocols have also been proposed in the literature. 1. [label=()]
2. Hierarchical protocols--in these protocols the sensor network is broken down into clusters with multiple gateways where each sensor node within a cluster communicates with other nodes in a multi-hop manner, leading to more efficient energy consumption. Some of the prominent Hierarchical protocols include LEACH (low-energy adaptive clustering hierarchy) [128], PEGASIS (power efficient gathering in sensor information systems) [129], TEEN (threshold sensitive energy efficient network) [130] and APTEEN (adaptive threshold sensitive energy efficient network) [131]. 3. Data-Centric protocols--in these protocols, a set of sensor nodes can be selected based on a query-based model and data can also be acquired whilst being transmitted to the base station (i.e., the sink node where data is collected and processed). Some of the prominent Data-Centric protocols include SPIN (sensor protocols for information via negotiation) [132] and Directed Diffusion [133]. 4. Location-based (Geographical) protocols--in these protocols, the location information of sensor nodes is required in order to calculate the distance between two particular nodes and draw the most energy-efficient path between them. Some of the prominent Location-based protocols include GAF (geographic adaptive fidelity) [134], GEAR (geographic and energy-aware routing) [135] and LEAR (location-based energy-aware routing) [136]. In Table 7, we present a comparison of some of the aforementioned routing protocols in terms of key performance characteristics such as scalability, power management, mobility, resource awareness, data aggregation capabilities and network lifetime. The performance of various routing protocols is directly dependent on the design of the sensor network, and in this regard, there are several challenges at present, such as scalability, mobility, energy consumption, data aggregation, network lifetime, delay times and quality-of-service (QOS), which must be overcome for optimum results. To address these challenges, we can expect future research to focus on vital areas such as application-specific routing protocol development, more rigorous mathematical modelling and simulations, and hardware experimentation on real test beds to address any shortcomings within mathematical models and simulations [126].