Method of controlling uplink synchronization state at a user equipment in a mobile communication system

1. A method of controlling an uplink synchronization state of a user equipment in a mobile communication system by performing random access procedures, the method comprising:

receiving, by the user equipment, a random access response including a time alignment command from a network during a first random access procedure;

starting, by the user equipment, a time alignment timer for uplink time alignment when the time alignment command is received from the network;

receiving, by the user equipment, a contention resolution message from the network to determine whether the first random access procedure is successful or not, wherein the user equipment considers itself in an asynchronous state and stops the time alignment timer when the user equipment determines that the first random access procedure is not successful based on the contention resolution message; and

performing, by the user equipment, a second random access procedure when the user equipment obtains uplink data to be transmitted to the network after the time alignment timer has expired.

2. The method of claim 1 , wherein the user equipment considers itself as in a synchronous state if the user equipment determines that the first random access procedure is successful based on the contention resolution message.

3. The method of claim 1 , further comprising periodically performing a random access procedure with the network.

4. The method of claim 1 , further comprising:

receiving a message allocating uplink radio resources from the network; and

transmitting uplink data to the network through the allocated uplink radio resources.

5. The method of claim 4 , wherein the uplink data includes information indicating that the data transmission is to maintain the uplink synchronization state.

6. The method of claim 1 , wherein the user equipment releases uplink or downlink channel resources allocated by the network.

7. A user equipment (UE) for controlling an uplink synchronization state by performing random access procedures in a mobile communication system, the user equipment configured to:

receive a random access response including a time alignment command from a network during a first random access procedure;

start a time alignment timer for uplink time alignment when the time alignment command is received from the network; and

receive a contention resolution message from the network to determine whether the first random access procedure is successful or not, wherein the user equipment considers itself as in an asynchronous state and stops the time alignment timer when the user equipment determines that the first random access procedure is not successful based on the contention resolution message; and

perform a second random access procedure when the user equipment obtains uplink data to be transmitted to the network after the time alignment timer has expired.

8. The user equipment of claim 7 , wherein the user equipment considers itself as in a synchronous state if the user equipment determines that the first random access procedure is successful based on the contention resolution message.

9. The user equipment of claim 7 , wherein the user equipment is further configured to:

receive a message allocating uplink radio resources from the network; and

transmit uplink data to the network through the allocated uplink radio resources.

10. The user equipment of claim 9 , wherein the uplink data includes information indicating that the data transmission is to maintain the uplink synchronization state.

11. The user equipment of claim 7 , wherein the user equipment releases uplink or downlink channel resources allocated by the network.