Method and device for controlling congestion in mobile communication system

A method by a terminal in a mobile communication system, the method comprising:

• transmitting a service request message for circuit switched (CS) fall-back to a first entity;
• deriving a CS security key based on a key associated with a packet switched (PS) domain;
• and
• performing a CS call establishment procedure based on the CS security key.

A first entity in a mobile communication system, the first entity comprising:

• a transceiver;
• and
• a controller configured to:
• control the transceiver to receive a service request message for circuit switched (CS) fall-back from a terminal,
• derive a CS security key based on a key associated with a packet switched (PS) domain, and
• control the transceiver to transmit a mobility management (MM) context including the CS security key to a second entity, the CS security key being used between the terminal and the second entity.

A terminal in a mobile communication system, the terminal comprising:

• a transceiver;
• and
• a controller configured to:
• control the transceiver to transmit a service request message for circuit switched (CS) fall-back to a first entity;
• derive a CS security key based on a key associated with a packet switched (PS) domain;
• and
• perform a CS call establishment procedure based on the CS security key.