1. A communication system comprising:
a user equipment;
a base station configured to perform radio communication with the user equipment;
a mobility management device configured to manage a mobility of the user equipment; and
a gateway device configured to perform data transmission/reception with the base station;
wherein the data transmission/reception between the base station and the gateway device is performed using an interface between the base station and the gateway device or via the mobility management device depending on a connection state between the base station and the gateway device.
2. The communication system as claimed in claim 1, wherein the data transmission/reception between the base station and the gateway device is performed using the interface between the base station and the gateway device when the base station and the gateway device are connected to each other.
3. The communication system as claimed in claim 1, wherein the data transmission/reception between the base station and the gateway device is performed via the mobility management device when the base station and the gateway device are not connected to each other.
4. A user equipment used in a communication system comprising a user equipment, a
base station configured to perform radio communication with the user equipment, a mobility
management device configured to manage a mobility of the user equipment, and a gateway
device configured to perform data transmission/reception with the base station,

wherein the data transmission/reception between the base station and the gateway device is performed using an interface between the base station and the gateway device or via the mobility management device depending on a connection state between the base station and the gateway device.
5. A base station used in a communication system comprising a user equipment, a base
station configured to perform radio communication with the user equipment, a mobility
management device configured to manage a mobility of the user equipment, and a gateway
device configured to perform data transmission/reception with the base station,
wherein the base station is configured to perform the data transmission/reception with the gateway device using an interface between the base station and the gateway device or via the mobility management device depending on a connection state between the base station and the gateway device.
6. A mobility management device used in a communication system comprising a user
equipment, a base station configured to perform radio communication with the user equipment,
a mobility management device configured to manage a mobility of the user equipment, and a
gateway device configured to perform data transmission/reception with the base station,
wherein the mobility management device is configured to relay the data transmission/reception between the base station and the gateway device depending on a connection state between the base station and the gateway device.
7. A gateway device used in a communication system comprising a user equipment, a
base station configured to perform radio communication with the user equipment, a mobility
management device configured to manage a mobility of the user equipment, and a gateway
device configured to perform data transmission/reception with the base station,
wherein the gateway device is configured to perform the data transmission/reception

between the base station and the gateway device using an interface between the base station and the gateway device or via the mobility management device depending on a connection state between the base station and the gateway device.