[ns] Mac & IFq communication query

[Andrew Tang <atang@cse.unsw.EDU.AU>]

Hi,

I'm having some trouble understanding how the IFq communicates with MAC,
this line in particular

ll.cc
	// let mac decide when to take a new packet from the queue.
		s.schedule(downtarget_, p, delay_);

From what I understand this leaves it up to the Mac layer to retrieve the
packet waiting in the queue. The scheduler inserts the event into the
event stack for the specific line latency time. 

How does MAC request packets from the queue? From what I understand the
recv() function is called, on at the mac interface, but who makes this
call and what determines the call? Is it when "pktTx_" is 0 in mac.cc?

Also I'm unsure as to how the packets are pushed onto the queue? I'm
fairly certain it has someting to do with the line above, is the
downtarget_ infact is a queue rather that MAC?

Thanks in advance
Andrew