[ns] Question about the LL

[黄伟]

Hi All,
In void LL::sendDown(Packet* p), the code is:
 if (tx == 0) {
  Scheduler& s = Scheduler::instance();
 // let mac decide when to take a new packet from the queue.
  s.schedule(downtarget_, p, delay_);
 }
I do not konw how this **let mac decide when to take a new packet from the 
queue**
It seems that delay_ is a constant value which can be set before simulation. 
Can anybody tell me why?
 
Regards,
HuangWei