# Join multiple channels (/en/realtime-media/rtc/build/join-and-manage-channels/join-multiple-channels/windows)

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Agora RTC SDK enables you to simultaneously join multiple channels. This capability allows you to receive and publish audio and video streams across multiple channels concurrently.

      
  
      
  
      
  
      
## Understand the tech

RTC SDK's multi-channel functionality is based on two key components:

* `RtcConnection`

  The `RtcConnection` object identifies a connection. It contains the following information:

  * Channel name
  * User ID of the local user

  You create multiple `RtcConnection` objects, each with a different channel name and user ID. Each `RtcConnection` instance can independently publish multiple audio streams and a single video stream.

* `IRtcEngineEx`

  The class contains methods tailored for interacting with a designated `RtcConnection` object.

  To join multiple channels, you call `joinChannelEx` method in the `IRtcEngineEx` class multiple times, using a different `RtcConnection` instance each time.

When joining multiple channels:

* Ensure that the user ID for each `RtcConnection` object is unique and nonzero.

* Configure publishing and subscribing options for the `RtcConnection` object in `joinChannelEx`.

* Pass the `IRtcEngineEventHandler` object to the `eventHandler` parameter when calling the `joinChannelEx` method to receive multiple channel-related event notifications.

## Prerequisites

Ensure that you have implemented the [SDK quickstart](/en/realtime-media/rtc/get-started-sdk) in your project.

## Implementation

This section explains how to join a second channel as a host after you have already joined the first channel.

1. Define a connection and join the channel using a random user ID.

   ```cpp
   agora::rtc::ChannelMediaOptions options2;
   options2.autoSubscribeAudio = false;
   options2.autoSubscribeVideo = false;
   options2.publishAudioTrack = false;
   options2.publishCameraTrack = false;
   options2.publishSecondaryCameraTrack = true;
   options2.clientRoleType = CLIENT_ROLE_BROADCASTER;
   // Define the connection
   connection.localUid = generateUid();
   connection.channelId = szChannelId.data();
   // Join the channel
   int ret = m_rtcEngine->joinChannelEx(APP_TOKEN, connection, options2, &m_camera2EventHandler);
   ```

2. Listen for the `onUserJoined` callback and set up the remote video.

   ```cpp
   // Listen for the onUserJoined callback
   void CAGEngineEventHandler::onUserJoined(uid_t uid, int elapsed) {
     LPAGE_USER_JOINED lpData = new AGE_USER_JOINED;

     lpData->uid = uid;
     lpData->elapsed = elapsed;

     if (m_hMainWnd != NULL)
       ::PostMessage(m_hMainWnd, WM_MSGID(EID_USER_JOINED), (WPARAM)lpData, 0);
   }
   ```

3. Set up the remote video

   ```cpp
   VideoCanvas canvas;
   canvas.renderMode = RENDER_MODE_FIT;
   canvas.uid = 123;
   canvas.view = videoWnd.GetSafeHwnd();

   connection.localUid = 123;
   connection.channelId = "channel123";

   // Set up the remote video
   rtcEngine->setupRemoteVideoEx(canvas, connection);
   ```

## Reference

This section contains content that completes the information on this page, or points you to documentation that explains other aspects to this product.

### Sample project

Agora provides the [MultiChannels](https://github.com/AgoraIO/API-Examples/tree/main/windows/APIExample/APIExample/Advanced/MultiChannel) open-source sample project for your reference. Download the project or view the source code for a more detailed example.

### API reference

* [`RtcEngineEx`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengineex.html#class_irtcengineex)
* [`RtcConnection`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_rtcconnection.html)
* [`joinChannelEx`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengineex.html#api_irtcengineex_joinchannelex)
* [`setupRemoteVideoEx`](https://api-ref.agora.io/en/video-sdk/cpp/4.x/API/class_irtcengineex.html#api_irtcengineex_setupremotevideoex)