Evaluating MQTT Topic Granularity and Broker Placement for Shipboard Sensor Data Transmission

Shipboard gateways send many sensor channels to shore subscribers with different needs over narrow, intermittent links, yet how Message Queuing Telemetry Transport (MQTT) topic structure, broker placement, Quality of Service (QoS), publishing rate, and outage recovery should be combined has not been evaluated on real shipboard data. Traces from five liquefied natural gas carriers were replayed through four topic structures on a mock broker, on amqtt, Mosquitto, and EMQX, across two hosts, and over emulated satellite-like links, with repeated runs. Per-sensor topics cost fewer bytes per useful sample than vessel batches for subscribers needing under 44–48 % of the channels, and an analytical model predicts this break-even from the channel inventory within 0.8 %. Latency tails arose whenever the offered message rate exceeded the broker delivery rate and vanished at matched rates for every QoS level. Brokers differed in queuing or discarding under overload, so latency should be reported with the delivery ratio. Onboard bridge filtering cut forwarded bytes to 6–35 % and, on a 512 kbit/s link, 95th-percentile latency from 12 s to 0.9 s. Event-first recovery delivered events fastest only below a capacity-bounded token rate. The results give researchers a predictive cost model and operators computable gateway settings.