In a $2$ to $3$ page explanation $($without the assistance of AI text generators$),$ describe the following concepts in detail:

$1.$ The fundamental structure of NS$-2.$

$2.$ The role and usage of Object$-$oriented Tool Command Language $($OTcl$).$

$3.$ The concept of Time$-$Dependent Simulation.

$4.$ A comparison of Time$-$Driven Simulation and Event$-$Driven Simulation$($important$)$