An apparatus or method for transmitting data blocks on a communications channel having a radio link between two stations including a user equipment comprises receiving first data blocks from the user equipment, and transmitting second data blocks to the user equipment. A polling interval is dynamically set for the transmission of polling messages to the user equipment after transmission of the second data blocks, the polling interval being set in accordance with at least one of: a size of one or more data blocks received by the apparatus from the user equipment, a size of one or more blocks transmitted from the apparatus to the user equipment, and a service to which the user equipment is subscribed. The apparatus may be used as a PCU in a cellular mobile telephone system.