SIB

Before a phone can do anything useful in a cell, it needs the cell's ground rules — what frequencies to use, how to access the network, which neighbours exist. The base station broadcasts all of that in System Information Blocks, sent on the downlink so every device in the cell can read them without an individual connection.

The structure is layered. The MIB (Master Information Block) comes first with the bare essentials to get started. Then SIB1 carries cell-access information and, importantly, the scheduling for all the other SIBs. From there each SIB has a job: cell-reselection parameters, intra- and inter-frequency neighbour lists, parameters for other radio technologies, ETWS/CMAS emergency alerts, and so on. The numbering differs between LTE and NR, so it's worth checking which standard you're reading. The practical reason this matters to a field engineer: misconfigured system information is a common cause of cells that look healthy but that devices won't camp on or reselect to correctly — if the broadcast parameters are wrong, the UE behaves wrong, no matter how good the RF is.