A communication system comprises a plurality of cells, each cell being associated with a respective serving area of TUPF. A base station controls a communication session of a communication device via the base station and a TUPF. Mobility of the communication device between a source cell and a target cell is performed as follows: when the communication session uses a nomadic IP address and the target cell is associated with the same TUPF serving area as the source cell but the base station is unable to determine the TUPF serving area of the target cell, the base station initiates a redirection procedure rather than a handover procedure. When performing the redirection procedure, the communication connection for the communication session via the source cell is broken before the target cell has been prepared to receive communication for the communication session of the communication device.