Hi,
The wireless protocol among the vehicles will depend on the type of communication you want to evaluate and it is not something we will tell you. It is what you want to simulate.
For example, the same 20 vehicles you have created could communicate either with IEEE 802.11x (e.g WAVE), 3GPP LTE and other.
Read the documentation and examples and you can find your answer.
Similar questions have been asked in the past in this list. Please search before you ask.
Just a clarification, the file you have created (mobility.tcl) just represents the mobility of those 20 nodes (vehicles). It is nothing more. You will use this as input to the scenario (there are examples on that in the ns-3 code).