can't read “ns_”: no such variable – what causes this ns-2 error?

In fact, the Answer is simple, you didn’t declare the variable “ns_” in the global memory space.

you only declared a variable named “ns”.

# ==================================================== =================
# Main Program
# ======================================================================
set ns [new Simulator]

So, if you are referring an undeclared variable “ns_” from the global memory space, then you should receive the error

can’t read “ns_”: no such variable
while executing
“global $ns_”
(file “wireless.tcl” line 24)

Since there is no such variable declared in global scope.