If cellAccessRelatedInfo IE within SIB1 has csg-Indication="TRUE" then csg-Identity shall be present and cell consider as "closed subscribers group" cell. When a cell marks a CSG cell then it means only CSG subscribers can access this cell. Once MME receives attach request from UE, it fetches UE's subscription profile from HSS and HSS responds with CSG Information(CSG Id, Subscription timer) . MME proceeds further based on CSG matching. If CSG-Id in response message from HSS and csg-Id of cell selected by UE is same then MME shall proceed with attach procedure and sends attach accept else attach reject is sent by MME with EMM cause "Not authorized for this CSG".
