Dual connectivity mode of operation of a user equipment in a wireless communication network

A secondary base station capable of performing a change of a serving cell group (SCG) in a wireless communication system supporting a dual connectivity of a user equipment, the secondary base station comprising:

• a communication interface configured to communicate with other network entity;
• and
• a controller configured to control transmitting a message including information indicating the change of the SCG to a master base station for the dual connectivity, to request the change of the SCG, and receiving a security key associated with the secondary base station, the security key being refreshed by the change of the SCG,
• wherein the information indicating the change of the SCG comprises information representing wraparound of a packet data convergence protocol (PDCP) count at the secondary base station.