The FastDDS Discovery Server allows for one device on the network to act as the discovery server, while the rest of the devices become discovery clients. This is akin to the ROS Master in ROS 1. Check out the Discovery Server documentation for more details.
With this configuration, the TurtleBot 4 is able to fully function without the Create® 3 being connected to Wi-Fi.
Each configuration has its own advantages and disadvantages. Simple Discovery makes it easy to interact with the robot from any device on the network. It is the default discovery protocol in ROS 2, so there is no additional setup required. Also, it supports both CycloneDDS and FastDDS. The main disadvantage is that this protocol uses multicasting which can be problematic with some Wi-Fi networks, such as university and corporate. Additionally, the Create® 3 must also be connected to Wi-Fi for a remote PC to be able to interact with the TurtleBot 4, so the network must bridge the 2.4 and 5 GHz bands.
Using a Discovery Server bypasses multicasting issues and allows for the TurtleBot 4 to function off of just the Raspberry Pi Wi-Fi. This means that if configured as an Access Point, the TurtleBot 4 does not require an external network. The disadvantages of this configuration is that extra setup is required on the Create® 3, Raspberry Pi, and User PC. Additionally, the Raspberry Pi must route traffic from the User PC to the Create® 3, and vice versa. Discovery Server also requires FastDDS as the middleware.
Discovery Server currently does not support communicating with multiple TurtleBot 4’s simultaneously from one computer.