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