## 1 Introduction  
Measuring household consumption (e.g., water, gas, etc.) is currently based on two types of devices. The first type consists of various traditional mechanical meters (TMs). TMs are dial meters and do not provide any smart metering or telemetry functions. Employing this type of metering device requires a lot of expensive manual labor for manual readings, often disturbing privacy. On the other hand, smart meters (SM) utilize recent technologies to measure consumption more efficiently. They usually feature wireless readings and low-power operation necessary for battery-powered devices. To ensure stable wireless connectivity for metering devices placed in challenging environments such as basements, shafts, holes, and closed objects, many power utilities turn their attention to Narrowband IoT (NB-IoT) technology as a suitable low-cost solution. NB-IoT is an addition to the 4G LTE standard that allows up to 50,000 low-power devices (e.g., SMs) to connect to a single base station. NB-IoT will enable massive cellular IoT and is a game-changer for the mobile industry and operators, who aim to use NB-IoT to provide seamless connectivity for billions of new devices. The key features in favor of the utilization of NB-IoT in the SM scenario are as follows [1]:  
* Ultra low-power operation--When a small payload is sent a couple of times daily, power saving mode (PSM) allows for a battery lifetime of up to 10 years. * Deep indoor penetration--Coverage extension modes allow for up to 20 dB gain compared to the GSM employing repeated transmissions. * High-Density Connectivity Solution--The ability to connect a large number of devices without network congestion. * Efficient Low-Data Transfer--Technology is optimized for small, infrequent data transmissions. The primary advantages of utilizing NB-IoT cellular technology stem from its reliance on existing base station infrastructure, significantly reducing the labor demands associated with system setup and data acquisition. Additionally, NB-IoT's PSM is pivotal for extending battery lifespan, allowing devices deployed in remote or hard-to-reach locations to function for prolonged periods. This capability minimizes the need for maintenance interventions, particularly regarding battery replacement, thereby enhancing operational efficiency and reducing long-term upkeep costs. Additionally, state-of-the-art modules provide several fallback options in case the NB-IoT technology is not supported in the corresponding region, including GSM and long-term evolution machine type communication (LTE-M). Consequently, these modules will consistently remain operational whenever a mobile network signal is accessible, which is the most widely available connectivity option. This paper presents an innovative new solution that \"smartifies\" the TMs in a retrofitting manner [2], as an alternative to the existing energy-inefficient (e.g. GSM/GPRS) products or non-telemetry-based products. Our energy-efficient device using wireless NB-IoT technology transforms the TM into an SM by applying an innovative, low-cost upgrade of the existing TM (no need to replace TM). It consists of an embedded low-cost camera and a smart number recognition system. The SM architecture is depicted in Figure 1.  
The system architecture (Figure 2), which is described in more detail in Section 3, has been designed to provide end-to-end connectivity between SMs and cloud infrastructure using NB-IoT provided by a mobile network operator (MNO) and cloud services. The end-users can retrieve the data using RESTful-based interfaces and other application-layer protocols such as MQTT or CoAP. Figure 1: Proposed SM architecture used for old TM retrofitting. Figure 2: The system architecture consists of a collection of deployed SMs that communicate via MNO with the cloud. The \"smartification\" of the TMs is based on the addition of a machine learning (ML) algorithm. Based on deployment, the ML models can be divided into three groups: cloud-based, mobile-based, and tiny.