In order to make sure the SIBs are received by the UE, an SI-window is defined to enable multiple transmissions of the SI message within the window. The length of the SI-window can be set to 1, 2, 5 10, 15, 20 or 40 ms. Within an SI-window, only one SI message can be sent, and may be repeated multiple times. When a UE tries to acquire an SI message, it listens from the starting of the SI-window until the SI message is received.
The following figure shows an example of the configuration of the periodicities of SIBs 2, 3, 6, and 7. Here, we use two SI messages, SI1 and SI2. SI1 contains SIB2 and SIB3 with a periodicity of 16 radio frames, while SI2 contains SIB6 and SIB7 with a periodicity of 64 radio frames. Assume that the SI-window length is 10 ms (1 radio frame).