There are two scenarios when network initiates paging is initiated at network.
In first case when a downlink packet arrives at SGW for an UE for S1-U tunnel ids are active. In this case SGW sends downlink data notification message to MME and MME prepares S1-Paging message for UE and sends to all eNodeBs serving the tracking areas present in UE context. In this case network sends UE identity index value, UE paging identity and list of tracking area identities.
In second case when eNodeB want to notify all UEs about the modified system information or etws information, it prepares and sends paging message which is applicable for all UEs. In this in case, paging message is applicable for all UEs which are id idle mode.