QUEUE OF AVAILABLE ELEMENTS           QUEUE FOR A DEVICE HANDLER

         +--------+                      LQE: Q2
I.QHDR:  |   Q1   |--+                   CQE: Q2
         +--------+  |
                     |
         +--------+  |
Q1:  +---|   Q3   |<-+
     |   +--------+
     |   |        |
     |   |        |
     |   +--------+
     |
     |                                  +--------+
     |                           Q2:    |    0   |
     |                                  +--------+
     |                                  |        |
     |                                  |        |
     |                                  +--------+
     |
     |   +--------+
Q3:  +-->|    0   |
         +--------+
         |        |
         |        |
         +--------+