# WSI: Pub/Sub

PortSIP PBX provides a Publish/Subscribe (Pub/Sub) mechanism over WebSocket, known as PortSIP WSI. This mechanism allows applications written in any programming language to subscribe to PBX events.

When a subscribed event occurs, PortSIP PBX automatically pushes event notifications to the client in JSON format, enabling real-time, event-driven integrations without polling.

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### Supported Versions

* PortSIP PBX v16.0 or later

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### Service Port

PortSIP PBX (including UCaaS deployments) exposes the WSI service using **secure WebSocket (WSS)**:

* **Port:** `8887`
* **Protocol:** WSS (WebSocket over TLS)
* **Firewall Requirement:**\
  The server firewall must allow **TCP traffic on port 8887**.

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### Server URL

The WSI server URL depends on the PortSIP PBX version:

* **v22.0 or later**

  ```
  wss://pbx.portsip.com:8887/wsi
  ```
* **v16.x**

  ```
  wss://pbx.portsip.com:8885
  ```

> **Note:** Replace `pbx.portsip.com` with your **actual PBX domain or IP address** when configuring your WebSocket client.