User equipment, base station, and connection establishment method

1. A user equipment of a mobile communication system that supports a function for establishing a connection by reusing a user equipment side context information and a base station side context information retained in the user equipment and in a base station, respectively, the user equipment comprising:

a transmitter that transmits, to the base station, a first message indicating that the user equipment retains the user equipment side context information;

a receiver that receives, from the base station, a second message indicating that the base station retains the base station side context information associated with the user equipment;

a processor that establishes, by using the user equipment side context information, a connection with the base station after receiving the second messages; and

a memory,

wherein the first message includes authentication information of the user equipment,

wherein the receiver receives, from the base station, a connection release message for causing the user equipment to transition from a connected state to an idle state, and, wherein upon detecting information in the connection release message indicating that the user equipment side context information is to be retained, the receiver retains the user equipment side context information in the memory during the idle state, and

wherein the processor, upon detecting that a cell reselected by the user equipment is another RAT, releases the user equipment side context information retained in the memory.

2. The user equipment according to claim 1 ,

wherein the processor, upon detecting that a predetermined time has elapsed in the idle state from a time point at which the user equipment transitions from the connected state to the idle state, invalidates the user equipment side context information retained in the memory.

3. A base station of a mobile communication system that supports a function for establishing a connection by reusing a user equipment side context information and a base station side context information retained in a user equipment and in the base station, respectively, the base station comprising:

a receiver that receives, from the user equipment, a first message indicating that the user equipment retains the user equipment side context information;

a transmitter that transmits, to the user equipment, a second message indicating that the base station retains the base station side context information associated with the user equipment, in response to receiving the first message; and

a processor that establishes, after transmitting the second message, a connection with the user equipment using the base station side context information,

wherein:

the first message includes authentication information of the user equipment,

the processor obtains an identifier of the user equipment from the first message,

the processor retrieves the base station side context information corresponding to the identifier from a plurality of retained base station side context information,

the transmitter transmits a connection release message to the user equipment, the connection release message comprises information indicating that the user equipment side context information is to be retained and the connection release message causes the user equipment to transition from a connected state to an idle state.

4. A connection establishment method executed by user equipment in a mobile communication system that supports a function for establishing a connection by reusing a user equipment side context information and a base station side context information retained in the user equipment and in a base station, respectively, the connection establishment method comprising:

transmitting, to the base station, a first message indicating that the user equipment retains the user equipment side context information;

receiving, from the base station, a second message indicating that the base station retains the base station side context information associated with the user equipment; and

establishing, after receiving the second message, a connection with the base station using the user equipment side context information,

wherein the first message includes authentication information of the user equipment,

receiving, from the base station, a connection release message for causing the user equipment to transition from a connected state to an idle state, and,

upon detecting information in the connection release message indicating that the user equipment side context information is to be retained, retaining the user equipment side context information in memory during the idle state, and

releasing the user equipment side context information upon detecting that a cell reselected by the user equipment is another RAT.

5. A connection establishment method executed by a base station of a mobile communication system that supports a function for establishing a connection by reusing a user equipment side context information and a base station side context information retained in a user equipment and in the base station, respectively, the connection establishment method comprising:

receiving, from the user equipment, a first message indicating that the user equipment retains the user equipment side context information;

transmitting, to the user equipment, a second message indicating that the base station retains the base station side context information associated with the user equipment, in response to receiving the first message; and

establishing, after transmitting the second message, a connection with the user equipment using the base station side context information,

wherein the first message includes authentication information of the user equipment,

the method further comprising:

obtaining an identifier of the user equipment from the first message,

retrieving the base station side context information corresponding to the identifier from a plurality of retained base station side context information, and

transmitting a connection release message to the user equipment, the connection release message comprising information for indicating that the user equipment side context information is to be retained and the connection release message causing the user equipment to transition from a connected state to an idle state.