initiating a communication sequence between an implantable medical device (IMD) and an external device, the communication sequence comprising a sequence of real time data units transmitted by the IMD to the external device, the real time data units transmitted by the IMD during the communication sequence being temporally separated by respective synchronization signals, wherein during the communication sequence the synchronization signals are received in an uninterrupted ongoing sequence by the IMD from the external device such that an interruption in the sequence of synchronization signals ends the communication sequence;

detecting, at the external device during the same communication sequence, an error in a received data unit;

in response to detecting the error, including an echo code in a synchronization signal during the same communication sequence, and communicating the synchronization signal with the included echo code to the IMD during same communication sequence, wherein the echo code corresponds to a request for a number of duplicate data units to be sent, and wherein a duplicate data unit corresponds to a real time data unit previously transmitted by the IMD during the same communication sequence; and

in response to receiving at the IMD the synchronization signal with the included echo code, transmitting, during the same communication sequence, a number of duplicate data units from the IMD to the external device, wherein the number of duplicate data units is determined using the echo code.