Communication system

1. A method operating in a secondary communication apparatus configured to communicate with a terminal which is configured to be connected to a master communication apparatus and to the secondary communication apparatus simultaneously, the method comprising:

transmitting, to the master communication apparatus, a first message related to security key update when a Packet Data Convergence Protocol (PDCP) count is about to wrap around;

receiving, from the master communication apparatus, a second message to deliver a security key of the secondary communication apparatus derived from a security key of the master communication apparatus;

deriving, from the derived security key of the secondary communication apparatus, a user plane security key; and

using the derived user plane security key for protecting user plane signalling between the terminal and the secondary communication apparatus.

2. The method according to claim 1 , wherein the first message is operable to be transmitted over an X2 interface.

3. A method operating in a master communication apparatus configured to communicate with a terminal which is configured to be connected to the master communication apparatus and to a secondary communication apparatus simultaneously, the method comprising:

deriving, from a security key of the master communication apparatus, at least one control plane security key for a protection of Radio Resource Control (RRC) traffic between the terminal and the master communication apparatus;

receiving, from the secondary communication apparatus, a first message related to security key update when a Packet Data Convergence Protocol (PDCP) count is about to wrap around;

deriving, from the security key of the master communication apparatus, a security key of the secondary communication apparatus in response to the receiving the first message; and

transmitting, to the secondary communication apparatus, a second message to deliver the derived security key of the secondary communication apparatus that is used to derive a user plane security key for protecting user plane signalling between the terminal and the secondary communication apparatus.

4. The method according to claim 3 , wherein the first message is operable to be received over an X2 interface.

5. The method according to claim 3 , further comprising:

transmitting, to the terminal, a first RRC message that is integrity protected using the at least one control plane security key.

6. The method according to claim 5 , further comprising:

receiving, from the terminal, a second RRC message in response to the first RRC message.

7. A secondary communication apparatus configured to communicate with a terminal which is configured to be connected to a master communication apparatus and to the secondary communication apparatus simultaneously, the secondary communication apparatus comprising:

at least one processor; and

at least one memory operatively coupled with the at least one processor, wherein the at least one processor is configured to:

transmit, to the master communication apparatus, a first message related to security key update when a Packet Data Convergence Protocol (PDCP) count is about to wrap around;

receive, from the master communication apparatus, a second message to deliver a security key of the secondary communication apparatus derived from a security key of the master communication apparatus;

derive, from the derived security key of the secondary communication apparatus, a user plane security key; and

use the derived user plane security key for protecting user plane signalling between the terminal and the secondary communication apparatus.

8. The secondary communication apparatus according to claim 7 , wherein the first message is operable to be transmitted over an X2 interface.

9. A master communication apparatus configured to communicate with a terminal which is configured to be connected to the master communication apparatus and to a secondary communication apparatus simultaneously, the master communication apparatus comprising:

at least one processor; and

at least one memory operatively coupled with the at least one processor, wherein the at least one processor is configured to:

derive, from a security key of the master communication apparatus, at least one control plane security key for a protection of Radio Resource Control (RRC) traffic between the terminal and the master communication apparatus;

receive, from the secondary communication apparatus, a first message related to security key update when a Packet Data Convergence Protocol (PDCP) count is about to wrap around;

derive, from the security key of the master communication apparatus, a security key of the secondary communication apparatus in response to the reception of the first message; and

transmit, to the secondary communication apparatus, a second message to deliver the derived security key of the secondary communication apparatus that is used to derive a user plane security key for protecting user plane signalling between the terminal and the secondary communication apparatus.

10. The master communication apparatus according to claim 9 , wherein the first message is operable to be received over an X2 interface.

11. The master communication apparatus according to claim 9 , wherein the at least one processor is further configured to:

transmit, to the terminal, a first RRC message that is integrity protected using the at least one control plane security key.

12. The master communication apparatus according to claim 11 , wherein the at least one processor is further configured to:

receive, from the terminal, a second RRC message in response to the first RRC message.