A Bluetooth master radio frequency unit addresses a slave radio frequency unit, to enable the slave to synchronize to the master, by sending poll packets and optionally null packets over an active link, the master being arranged so that receipt of a response from the slave unit to a poll packet is sufficient to maintain the active link. The slave unit does not have to respond to all of the poll packets. This approach can allow the slave to preserve more (transmit) power by going into a deep sleep mode in which a low power oscillator may be used while still allowing the master unit to detect whether the slave has resynchronized or not (and thus to update a Link Supervision Timer, for example).