...BBBBBBBBB | AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA | BBBBBBBBB...
             |<======================================>|
             |                   L                    |
...=========>|<==================><==================>|<==========...
             |         L/2                L/2         |
             |<====>|                          |<====>|
                d   |                          |   d
                    |<========================>|
                    available counting interval