+------------------------------------------------+
        |                                                |
        | qos-level                                      |
        |   A                                            |
        |   |                                            |
        |  4|                                            |
        |   |                                            |
        |  3|             +------+                       |
        |   |             |      |                       |
        |  2|        +----+      +----+     +---         |
        |   |        |                |     |            |
        |  1|   +----+                +-----+            |
        |   |   |                                        |
        |  0+---+---------------------------------> time |
        |                                                |
        +------------------------------------------------+