The cloud gateway ingests device events at the cloud boundary, using a reliable, low latency messaging system.
Devices might send events directly to the cloud gateway, or through a field gateway. A field gateway is a specialized device or software, usually colocated with the devices, that receives events and forwards them to the cloud gateway. The field gateway might also preprocess the raw device events, performing functions such as filtering, aggregation, or protocol transformation.
https://learn.microsoft.com/en-us/azure/architecture/guide/architecture-styles/big-data
