Hybrid Transmission of MQTT and CoAP for Real-Time Communications in NAT-Based IoT Environments

Communication using NAT(Network Address Translation) requires outbound communication (internal node → external node) before performing inbound communication (external node → internal node). After a certain period of time, if the session of the NAT mapping table expires, the external node may not discover the internal node. To address this problem, LwM2M(Lightweight M2M) ensures that an internal node first sends a request to an external node during the initialization process, and that the requests generated in the off-line status are automatically stored in a queue so as to process those requests when they get into the on-line status. However, this approach may not provide real-time communication effectively, and large cumulative overhead may be incurred so as to maintain many NAT bindings for real-time communication. In this paper, to solve this problem, we propose the M-CoAP scheme that combines the two well-known protocols: MQTT(Message Queuing Telemetry Transport) and CoAP(Constrained Application Protocol). From testbed experimentation, we confirmed that the proposed M-CoAP scheme can provide real-time communication. By performance comparison with the existing LwM2M scheme, we also see that the proposed can improve the transmission throughput by 17%.