What happens after you tap your card?
The tap. This time, follow the data instead of the people.
The card reader reads the card, the charger sends a message through its mobile connection, and a server answers within a second.
The questionWhich route does that message take, and why does it depend on whose card it is?
Guess firstThe charger's operator has no contract with this driver. Is the session refused?RevealHide
No. The operator recognises the card through its partner, the company that issued it. Either the operator already holds that partner's list of valid cards, or it asks the partner directly. This exchange between companies is called roaming.
Route A: the operator knows this card and authorises it itself.
The charger speaks OCPP with its operator's backend.
Route B: the card belongs to another company's customer.
The first step is always the same: OCPP to the operator's backend.
Switch the route, then select each step.
Two protocols with two jobs
- connects the charger with its own operator's backend: status, meter values, authorisations, start and stop.
- connects companies: operators and card providers exchange card lists, tariffs, locations and session records.
A simple way to keep them apart: OCPP stays with the charge point, OCPI works between companies. OCPI is not the only protocol. The Hubject platform, for example, uses its own protocol, .
Vehicle and charger establish a safe electrical connection first. With a card, no contract is checked at this point.
Four systems and two protocol layers, in this session in less than two seconds.
Why roaming exists
Roaming lets one card work in many networks. Without it, drivers would need a separate contract for every operator. With it, the card provider settles with each operator in the background.
Operators can keep the card lists of their partners and check a tap against them. Card providers can also ask to approve every session in real time.
Connecting every company directly with every other would take thousands of separate links. reduce that: one connection reaches many partners, and companies can add direct links to their most important ones. The same connections later carry the session record that becomes the invoice at stop 5.
Check your understanding
4 questions. Answer all of them to complete this stop. Each answer explains itself.
Why was OCPI needed in this charging session?
OCPI connected the operator with the card issuer, through a shared card list or a real-time request.
Which protocol did the charger itself speak in both routes?
The station's world ends at its CPMS. Everything beyond is handled by roaming protocols such as OCPI.
Why do roaming hubs exist?
Each company connects once instead of to every partner. That is what hubs are for.
An unknown card is tapped at a charger, and no roaming agreement covers it. What happens?
Roaming needs a contract path. Where none exists, there is no one to confirm the card, and the session is refused.
- OCPP Open Charge Point Protocol: the open protocol between a charger and its operator's backend. Glossary
- OCPI Open Charge Point Interface: an open protocol for roaming between operators and e-mobility service providers. Glossary
- OICP Open InterCharge Protocol: the roaming protocol of the Hubject platform. Glossary
- Roaming hub A platform that connects many operators and service providers, each through one connection. Glossary
- Roaming Charging in another company's network with your own card or app. Glossary
- Open Charge Alliance: Open Charge Point Protocol (OCPP). https://openchargealliance.org/protocols/open-charge-point-protocol/. Checked 07 Oct 2026.
- Open Charge Alliance: Open Charge Point Protocol (OCPP). https://openchargealliance.org/protocols/open-charge-point-protocol/. Checked 07 Oct 2026.
- EVRoaming Foundation: Open Charge Point Interface (OCPI). https://evroaming.org/ocpi-background/. Checked 07 Oct 2026.
- Hubject: Open InterCharge Protocol (OICP), specification on GitHub. https://github.com/hubject/oicp. Checked 07 Oct 2026.
- EVRoaming Foundation: OCPI 2.2.1, Tokens module: cached token lists and real-time authorisation. https://raw.githubusercontent.com/ocpi/ocpi/2.2.1/mod_tokens.asciidoc. Checked 07 Oct 2026.
- EVRoaming Foundation: Open Charge Point Interface (OCPI). https://evroaming.org/ocpi-background/. Checked 07 Oct 2026.