The communication protocol used for real-time data transmission between devices is the WebSocket protocol. WebSockets provide a persistent, full-duplex communication channel over a single, long-lived connection, allowing data to be sent and received in real-time. This is especially useful for applications requiring instant updates, such as chat applications, online gaming, and live sports updates. By enabling continuous interaction without the overhead of multiple HTTP requests, WebSockets improve efficiency and responsiveness, enhancing the user experience.