When people first hear about PEPPOL, they often think of it as a file format or a piece of software. It is neither. PEPPOL is a network — a set of agreements and technical standards that let businesses in different countries exchange documents with each other without needing a direct point-to-point connection.
The way that network is organised is described by the 4-corner model. Understanding it makes the whole PEPPOL architecture click into place, and it explains why sending a PEPPOL invoice involves more moving parts than emailing a PDF.
The four corners
The model gets its name from four parties involved in every PEPPOL document exchange:
Corner 1: the sender. The business issuing the document — in most cases, a supplier sending an invoice. The sender produces the document in a PEPPOL-compatible format (UBL 2.1 or UN/CEFACT CII) and hands it to their access point.
Corner 2: the sender's access point. A certified service provider that connects to the PEPPOL network on behalf of the sender. The access point receives the document from the sender, looks up where to deliver it, and transmits it securely over the network. Access points are certified by OpenPEPPOL and must meet technical and security requirements to maintain their certification.
Corner 3: the receiver's access point. A different certified service provider that connects to the network on behalf of the receiver. It accepts the incoming document from Corner 2 and delivers it to the receiver. In some cases, Corner 2 and Corner 3 can be the same provider — if both sender and receiver use the same access point — but they still play distinct roles in the model.
Corner 4: the receiver. The business receiving the document — the buyer, in an invoicing context. The receiver gets the document from their access point and processes it in their ERP or finance system.
The key point is that the sender and receiver never communicate directly. Their access points handle the transmission between them. This is what makes PEPPOL a network rather than a direct integration: you connect once to an access point, and through that connection you can reach any other PEPPOL participant, anywhere in the network.
How a document actually travels: a Swiss example
Here is what happens when Müller AG, a Swiss supplier in Zurich, sends a PEPPOL invoice to the Swiss Federal Chancellery in Bern.
Step 1: Müller AG generates the invoice. Their ERP produces a UBL 2.1 XML file in the SwissDIGIN profile, which extends PEPPOL BIS Billing 3.0 with Swiss-specific fields. The invoice includes the Federal Chancellery's PEPPOL participant ID as the recipient address.
Step 2: The document goes to Müller's access point (Corner 2). Müller AG has contracted with a PEPPOL access point provider. Their ERP sends the XML file to that provider, typically over an AS4 transport connection or via an API.
Step 3: The access point looks up the receiver. Before transmitting, Corner 2 queries the PEPPOL SMP/SML infrastructure — a distributed directory of PEPPOL participants — to find out which access point is registered to deliver documents to the Federal Chancellery's participant ID. This lookup happens automatically and takes milliseconds.
Step 4: Corner 2 transmits to Corner 3. Müller's access point sends the document to the Federal Chancellery's access point over the PEPPOL network using the AS4 messaging standard. The transmission is encrypted and the access point on the receiving end sends back an acknowledgement.
Step 5: Corner 3 delivers to the Federal Chancellery (Corner 4). The Federal Chancellery's access point receives the document and makes it available to their finance system — either by pushing it into their ERP directly or by making it available for retrieval via API.
The whole process typically completes in seconds.
Why two access points instead of one?
The 4-corner model deliberately separates sender-side and receiver-side responsibilities. Neither party has to care how the other has set up their infrastructure. Müller AG does not need to know which access point the Federal Chancellery uses, or what their ERP expects. The Federal Chancellery does not need to know anything about Müller's billing system. Each party just needs a connection to their own access point.
This is the same principle that makes email work. When you send an email, you do not think about which mail server the recipient uses. Your server handles the delivery. PEPPOL is the same idea, applied to business documents with stronger security and delivery guarantees.
It also means that switching access point providers does not require any changes to your trading partners. Your PEPPOL participant ID stays the same; only the underlying service provider changes. This prevents lock-in and keeps the market competitive.
The participant ID and how receivers are found
Every PEPPOL participant has a participant ID — a unique identifier used to address documents on the network. For Swiss businesses, this is typically based on the Swiss UID (Unternehmens-Identifikationsnummer) in the format 0230:CHE-123.456.789.
The discovery process — finding which access point handles a given participant ID — uses two components: the SML (Service Metadata Locator) and the SMP (Service Metadata Publisher). The SML is a central DNS-based lookup that points to the SMP responsible for a given participant. The SMP holds the actual capability information: which document types the participant can receive, and which access point delivers to them.
This is all handled automatically by access points. You do not need to query the SML or SMP manually. But understanding that this directory exists explains why registering as a PEPPOL participant involves publishing your capabilities to an SMP — you are putting your entry into the directory so senders can find you.
What the 4-corner model does not cover
The 4-corner model describes the transport layer. It does not specify the content of the documents being exchanged. PEPPOL BIS Billing 3.0 defines the invoice format. SwissDIGIN adds Swiss extensions. The EN 16931 semantic model defines the underlying data requirements. These are separate layers that sit on top of the 4-corner transport infrastructure.
There is also a 5-corner model used in some contexts — notably in e-procurement and in scenarios involving a central platform or government portal as an intermediary. For most Swiss B2B and B2G invoicing, the 4-corner model is what matters.
Choosing an access point
For Swiss businesses, there are several certified PEPPOL access point providers operating in Switzerland. They vary in pricing model (per-transaction vs subscription), ERP integration options, and the Swiss-specific support they offer. The top PEPPOL access point providers in Switzerland post covers the main options and what to look for when comparing them.