si_Periodicity tells about the period in terms of radio frames which helps the UE to read the required SIB. The value of this parameter is used for calculating the Radio frame where SI will be received.
Refer this link: http://lteinwireless.blogspot.in/2011/06/all-about-sibs-in-lte.html
You will get good idea of System information mapping.
Please correct me ,If I am wrong.