# IoT: A Long-Reach Fully Passive Low-Rate Upstream PHY for IoT over Fiber
## 2 Materials and Methods
### The Role of Optical Connectivity in IoT  
An alternative to tackle IoT connectivity issues is to move, at least, a part of the IoT devices to wired links. In this case, optical fiber is a natural candidate, especially for IoT covering wide geographical areas. Fiber optics offer a huge bandwidth with very low attenuation, typically 0.2 dB/km for standard single mode fiber (SMF), allowing throughput and reach figures that are orders of magnitude superior to what wireless systems offer to IoT. The optical equivalent to frequency division multiplexing, used in wireless systems to create several channels, is the wavelength division multiplexing (WDM) in optical communications systems. This dramatically increases capacity (data rate) of a single fiber by using different wavelengths for different carriers [PERSON] et al. (2019).  
Optical fibers are usually thought of as big pipes for transporting aggregated IoT traffic beyond IoT gateways [PERSON] et al. (2019). The discussion here focuses on providing optical connectivity to IoT devices themselves. We call this modality the IoTFo to express this direct physical connectivity (PHY) and to differentiate the other use of optical fibers in IoT. Fibers directly at IoT devices may initially seem an overkill solution, but this would be viable and applicable when availability, reliability, security and other limiting factors are handicapping physical connectivity solutions based on wireless and other wired technologies. Optical cables are easily found in metropolitan areas, industrial plants as well as alongside highways and remote rural roads, power transmission lines, and railways. Therefore, dark fibers availability (i.e., spare fibers in already deployed optical cables) may be key for IoT; not only for its unexploited broadband capability but also for providing very basic connectivity as proposed here by the IoTFo concept.  
For instance, IoTFo can find niches in monitoring urban areas with crowded and/or polluted spectrum, power lines structure and environment monitoring, collecting data in gas and oil industry, and IoT wildfire monitoring in extremely remote locations, where dense forests in hilly regions may create very unfavorable wireless propagation environments. IoTFo may be thought as an alternative to reduce both cost and latency in replacing the upstream side of satellite-based Internet of remote things (IoRT) [PERSON] et al. (2019).  
Before exploiting optical fibers in this new context, there are fundamental mismatches between IoT requirements and the enablers factors from optical fiber communication systems to be addressed. In order to make IoTFo viable, an alternative approach for optical communications should be sought. Note that a similar challenge has been faced before in access networks employing optical communications. Fiber strands were only able to physically reach the premises of individual domestic customers after the concept of passive optical networks (PON) has been developed to ease capital and operational expenses (CAPEX and OPEX, respectively) of optical solutions [PERSON] et al. (2019).