IoT Hub is the Azure application for managing and communicating with IoT devices including edge gateways. The application is designed to scale and can be used to interface with the largest IoT deployments imaginable. The platform support bi-direction communication with endpoints leveraging three native protocols. They are HTTP, AMQP, and MQTT.
HMS edge gateways support all three of these protocols. HTTP/HTTPS based Restful APIs are easy to use and require very little advance planning as the contextualization information can be contained in the message traffic each time. In addition, HTTP/HTTPS ports are almost always open on corporate firewalls. One downside is HTTP is a transport layer protocol that requires security to be layered on top i.e. TLS/SSL. Pavan Podila put together a nice summary of HTTP for anyone new to the protocol.
AMQP is a mature protocol that requires a separate application layer protocol. AMQP provides a rich set of messaging scenarios. A smaller pool of developers have experience with this protocol. The default port 5672 associated with the protocol is not commonly open on corporate firewalls. The protocol is battlefield tested and has all of the necessary functions required for authentication and security. The OASIS industry standards group does maintain the AMQP protocol and the protocol is defined as a formal international standard ISO/IEC 19464 as of 2014. Abhishek Tiwari provides a brief overview of AMQP based systems.
MQTT is a lightweight client server publish/subscribe transport protocol. The protocol is thin enough to run on low cost micro controllers. The protocol is design to minimize transport overhead and protocol exchanges. Several tradeoff were made to create an ultra-thin protocol. The protocol is constrained to basic messaging topics in a single name space. This is coupled with a very simple security and user model. As a result, effort is required to create a secure verbose IoT solution leveraging this protocol. Similar to AMQP the default port for MQTT over SSL is port 1883 and MQTT over TLS is port 8883 which is typically not open by default on a corporate firewall. HiveMQ offers a quick overview of MQTT.
Regardless which protocol is chosen HMS gateways can support the connection. HMS OEM gateways are Microsoft Azure Certified for HTTP/HTTPS, AMQP and MQTT and verified with the Azure IoT Hub. eWON Flexy supports HTTP(s).