Topics are buses used by nodes to transmit data. Topics can be transmitted without a direct connection between nodes, which means that the production and consumption of data is decoupled. A topic can have various subscribers and can also have various publishers, but you should be careful when publishing the same topic with different nodes as it can create conflicts.

Each topic is strongly typed by the ROS message type used to publish it, and nodes can only receive messages from a matching type. A node can subscribe to a topic only if it has the same message type.

The topics in ROS can be transmitted using TCP/IP and UDP. The TCP/IP-based transport is known as TCPROS and uses the persistent TCP/IP connection. This is the default transport used in ROS.

The UDP-based transport is known as UDPROS and is a low-latency, lossy transport, so it is best suited to tasks such as teleoperation.

ROS has a tool to work with topics called rostopic. It is a command-line tool that gives us information about the topic or publishes data directly on the network.
This tool has the following parameters:

• rostopic bw /topic: This displays the bandwidth used by the topic.
• rostopic echo /topic: This prints messages to the screen.
• rostopic find message_type: This finds topics by their type.
• rostopic hz /topic: This displays the publishing rate of the topic.
• rostopic info /topic: This prints information about the topic, such as its message type, publishers, and subscribers.
• rostopic list: This prints information about active topics.
• rostopic pub /topic type args: This publishes data to the topic. It allows us to create and publish data in whatever topic we want, directly from the command line.
• rostopic type /topic: This prints the topic type, that is, the type of message it publishes.

We will learn how to use this command-line tool in upcoming sections.