An implanted medical device (e.g. infusion pump) and an external device communicate
with one another via telemetry messages that are receivable only during windows
or listening periods. Each listening period is open for a prescribed period of
time and is spaced from successive listening periods by an interval. The prescribed
period of time is typically kept small to minimize power consumption. To increase
likelihood of successful communication, the window may be forced to an open state,
by use of an attention signal, in anticipation of an incoming message. To further
minimize power consumption, it is desirable to minimize use of extended attention
signals, which is accomplished by the transmitter maintaining an estimate of listening
period start times and attempting to send messages only during listening periods.
In the communication device, the estimate is updated as a result of information
obtained with the reception of each message from the medical device.