How does a charger connect to the backend?

The charger opens a WebSocket to its backend, announces itself and then checks in at a set interval. Commands from the backend travel over the same connection.

Explained in

OCPP in depth · Stop 02How does a charger connect to its backend?WebSocket, boot announcement, heartbeat and status: the first seconds after a charger starts, and the connection it keeps open all day.

In OCPP over JSON, the charger opens the connection: an HTTP request that is upgraded to a WebSocket. That suits the real world, where chargers sit behind mobile networks and routers that would not let the backend call in. Once open, the connection carries messages in both directions, so the backend can still send commands at any time.

From “Why the charger calls”, stop 02

Terms

  1. OCPPOpen Charge Point Protocol: the open protocol between a charger and its operator's backend.
  2. WebSocketA connection that the charger opens to the backend and keeps open, so messages can travel in both directions at any time.
  1. Is a CSMS the same as a CPMS?Yes, both name the operator's backend. OCPP 1.6 calls it the Central System, OCPP 2.0.1 and 2.1 the charging station management system (CSMS); CPMS is the common industry term.
  2. What is the difference between OCPP 1.6 and 2.0.1?OCPP 2.0.1 is not backward compatible with 1.6: transactions became configurable and unified, and the device model was added. OCPP 2.1 builds on 2.0.1 and keeps it compatible.
  3. How secure is OCPP?OCPP has three security profiles. Profile 2 adds TLS and a backend certificate, profile 3 certificates on both sides; profile 1 is not recommended for the field.
  4. Which OCPP messages make up a charging session?In OCPP 1.6: Authorize, StartTransaction, MeterValues and StopTransaction. In 2.0.1, the transaction messages are events of one type, TransactionEvent.
  1. Open Charge Alliance: OCPP 1.6 Security Whitepaper, edition 4, February 2026. https://openchargealliance.org/ocpp-info-whitepapers/ocpp-1-6-security-whitepaper-4th-edition/. Checked 09 Oct 2026. Licence: CC BY-ND 4.0.