There is provided a network system including one or more first MMEs (30), and a second MME (40) separated from the first MMEs (30). In one of operation cases, the first MME (30) pushes, to the second MME (40), security context for a UE (10) that attaches to the first MME (30). The second MME (40) stores the security context. The first MME (30) further pushes the latest security context to the second MME (40), during a switch-off procedure for the first MME (30). The second MME (40) updates the stored security context with the latest security context. The first MME (30) pulls the security context from the second MME (40), when the UE (10) re-attaches to the first MME (30) or is handovered from different one of the first MMEs (30).