# Design, Deployment and Evolution of Heterogeneous Smart Public Lighting Systems
## 5 Network Architectures and Transmission Technologies
### Wired Networks  
The most straightforward way to integrate all public light fixtures into a wired communication network is to exploit the existing power lines to convey also data signals. This is the Power Line Communication (PLC) technology, which turns the conventional electrical grid into a communication infrastructure. As reported in [83; 84], PLC systems can be classified according to their bandwidth:  
* broadband PLC (1.8-100 MHz), which yields data rate in the order of Mbps with a coverage of few hundreds meters,
* narrowband PLC (3-500 kHz), which provides data rate in the order of few hundred of kbps over several kilometers,
* ultra-narrowband PLC (125-3000 Hz), which conveys very low data rates (roughly 100 bps) at tens or even one hundred kilometers.  
Broadband PLC and ultra-narrowband PLC are not usually adopted in smart lighting scenarios, owing to their low coverage range and data rate, respectively [85; 86]. Narrowband PLC is, instead, commonly adopted for this application. As shown in Figure 2, which depicts the typical PLC-based smart lighting infrastructure, communications take place over the low-voltage power cables (segments) between the luminaires and the electrical cabinet (segment controller). On its turn, the cabinet is connected to the Control Center by means of a wired (e.g., optic fiber) or wireless (e.g., cellular) wide area network (WAN).  
This architecture is widely adopted in smart lighting infrastructures currently operating worldwide. PLC is, in fact, a mature technology, with the consequence that many manufacturers have been providing off-the-shelf solutions since many years. In perspective, however, it is questionable whether PLC is the best solution for the smart lighting infrastructures of future smart cities in the IoT era, which are expected to serve as backbone for plenty of location-based wireless services. In fact, future streetlights will be certainly equipped with wireless communication technologies, aimed at gathering/exchanging data from/with IoT devices (sensors, smart bikes, cars, parking lots, waste bins,...) located in their surroundings. In view of this, there is no reason not to use the same wireless network to support both future IoT services and the smart lighting service, with no need of any PLC system dedicated to the latter.  
Indeed, the awareness that IoT wireless services and smart lighting will be tightly intertwined in future smart city scenarios, is forcing several PLC manufacturers to upgrade their products in order to accommodate also IoT wireless technologies; PLC is mainly used for data transmission between the electrical cabinet and the luminaires, whereas the supplementary RF network, established by means of radio devices installed in each streetlight, is used to support communications with nearby IoT devices as well as (if necessary) between the luminaires.  
In the authors' opinion, this solution, although in the right direction and fully understandable in a commercial perspective, does not have solid technical motivations, as the wireless network alone could easily uphold also (among the others) the smart lighting service, with obvious cost reductions. In addition, a properly designed wireless network is characterized by an intrinsic redundancy which is absent in common PLC architectures, which are more vulnerable to failures. Adding redundancy to PLC networks is certainly possible, but it comes at cost, as duplicated devices must be installed.  
In the following section, an overview of the wireless communication technologies that can support smart lighting applications is provided, along with a thorough discussion of their strengths and weaknesses in such scenario.