## 1 Introduction
Current communication protocols, such as Dedicated Short-Range Communication (DSRC), 4G LTE, and the more recent 5G technology, are commonly used in connected vehicle (CV) applications. These protocols  
Figure 1: The evolution of mobile networks. enable real-time data sharing between vehicles and other transportation agents, allowing for safer and more efficient traffic management [13; 14; 15; 16; 17; 18]. While 5G technology offers faster speeds and higher bandwidth, its implementation is expensive, particularly in developing countries [19; 20; 21]. The upcoming 6G networks promise even greater performance but face similar cost barriers. As a result, many regions struggle to deploy the necessary infrastructure for V2X communication. In Mexico, for example, the cellular infrastructure for voice channels is well-developed and provides reliable coverage across most regions, but data channel coverage remains inconsistent and less reliable. This disparity presents a challenge for the deployment of advanced V2X systems, which rely heavily on stable data channels for effective communication. To address these limitations, this work introduces the use of dual-tone multifrequency (DTMF) codes as part of a real-time communication system for transmitting diagnostic trouble codes (DTC) generated by the vehicle's electronic control unit (ECU). The proposed system leverages the extensive voice channel infrastructure already in place, allowing for reliable transmission of critical data without the need for costly new infrastructure. By focusing on a low-cost hardware solution, this approach provides similar data transmission rates to commercial V2X systems but at a fraction of the cost, making it a viable alternative for regions with limited data channel coverage. Table 1 presents a comparison of methods or systems that use voice-based vehicular communication, which is compared with the system proposed in this work, considering that it is viable and has an advantage over other systems mainly in the cost of implementation. In some European countries, shutdowns of 2G and 3G networks have been announced, due to the transition to providing capacity to 4G and 5G technologies. In the case of Latin American countries, 2G and 3G technologies are still in operation, in which shutdowns are very slow and the infrastructure for these networks are still functioning. Mainly in Mexico, 2G (see Figure 2) and 3G technologies are still running in some territories, only some main states of the country have adopted 4G and 5G, but it is not the entire Mexican national territory, and there is currently no date for the closure or shutdowns of the aforementioned technologies. Due to this, this developed hardware system consists of the implementation of a low-cost communication protocol prototype that can be executed in real time through voice channels using 2G cellular technology. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Module Type**} & \multirow{2}{*}{**Cost**} & \multicolumn{2}{c}{**Communication**} & **Range** & **Real-Time** \\  & & **Technology** & **Technology** & **Requirement** \\ \hline VLC, CAN Module [22] & High & Bluetooth, Ethernet & Short & No \\ IoT, ESP8266 [23] & High & WiFi & Medium & No \\ UAV [24] & High & WiFi & Medium & Yes \\ Google Assistant, Arduino [25] & Low & Serial & Medium & No \\ UAV, MAV [26] & High & WiFi, Simulation & Medium & Yes \\ Embedded System, SIM900 shields & Low & Cellular & High & Yes \\ (Proposed System) & & Technology & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of different voice-based vehicular communications systems with the proposed system: Visible light communications (VLC), Internet of Tings (IoT), Unmanned aerial vehicles (UAV), Manned Aerial Vehicles (MAV). Key contributions of this work include the design and implementation of a cost-effective, real-time V2X communication system that transmits DTCs via DTMF codes.