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