QUEUE OF AVAILABLE ELEMENTS           QUEUE FOR A DEVICE HANDLER

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