大部分Sytem Info, UE并不会“周期性"去读的， 如果系统消息有改变，而是UTRAN通过Paging通知Value Tag变化：
25.331
8.1.1.7.1 Modification of system information blocks using a value tag
Upon modifications of system information blocks using value tags, UTRAN should notify the new value tag for the master information block in the IE "BCCH modification info", transmitted in the following way:
1> to reach UEs in idle mode, CELL_PCH state and URA_PCH state, the IE "BCCH modification info" is contained in a PAGING TYPE 1 message transmitted on the PCCH in all paging occasions in the cell;
1> to reach UEs in CELL_FACH state or TDD UEs in CELL_DCH with S-CCPCH assigned, the IE "BCCH modification info" is contained in a SYSTEM INFORMATION CHANGE INDICATION message transmitted on the BCCH mapped on at least one FACH on every Secondary CCPCH in the cell.
Upon reception of a PAGING TYPE 1 message or a SYSTEM INFORMATION CHANGE INDICATION message containing the IE "BCCH modification info" containing the IE "MIB value tag" but not containing the IE "BCCH modification time", the UE shall perform actions as specified in subclause 8.1.1.7.3.
If the IE "BCCH modification time" is included the UE shall perform actions as specified in subclause 8.1.1.7.2.
但是System Info Block Type 7需要“周期性“读。
具体可以看看25.331 Table 8.1.1: Specification of system information block characteristics
如果系统消息发生变化，会通过paging type 1或者system information change indication通知UE。如果小区系统消息一直没有发生变化，我认为应该6小时后接收一次。因为协议上说，6小时后把value tag置为无效。此外由于要做LAU或RAU，注册完成后从释放连接，又会搜网解一次系统消息的。