How to test and debug network based applications? Which tools are useful?

I'm currently learning something about network development and wondering what the development process usually looks like and what kind of errors one commonly faces in this process. Especially in the case of more complex networks, with a mostly wireless communication structure, as with Smart Home Systems, where e. g. Mesh Networks can play a role and with many clients of different functionality, which are interdependent in the matter of data exchange.

With this kind of networks how to:

• apply the TDD paradigm to (wireless) Networks? How to test individual components or the system as a whole?
• debug errors in such networks or prevent errors from occurring in the first place? Especially with regard to malfunctioning data transmission, where I suppose it can be quite difficult to determine the source of errors in a complex and perhaps non-linear process chain.

Which tools can be or are used to address these problems? Are there any best practices? Do you use any tools to visualize the network topology or isn't this helpful?

Since I've never worked with larger networks before, I think that a visualization could be quite useful to grasp the whole network structure and get a better understanding of it and the ongoing processes.

Does anybody have experience with these topics or can provide interesting sources?

Many thanks in advance and stay healthy