An industry standard to connect charging stations to a software back-end. It has been permanently refined for over 10 years.
In the early stages OCPP used to use SOAP – which is unusual in the world of IoT due to the verboseness of the protocol. Nowadays the developments favors JSON via websockets.
Thanks to the reduction of data size due to websockets and JSON, the usage of SIM-cards for data-transmission has become much more viable.
A modern Charge Point Management System (CPMS) like Gridware supports all versions of the OCPP, since devices of all generations and manufacturers are to be supported.
For more informations see https://www.openchargealliance.org/