Abstract: Performance of a communications device with a processor, a memory, and a SIM in a wireless communication network is tested to provide a range of QoS KPIs. A test server sends a payload to the device modem, the size of which has been computed to avoid adversely affecting real time performance. The modem uses the payload to fill a buffer of restricted size in the memory, the buffer being smaller than the payload. The modem processor signals the SIM to indicate that data is available in the buffer, and the SIM instructs the processor to purge the buffer. There are repeated cycles until all of the payload has been written to the buffer and purged. The buffer signals the test server to indicate that all of the payload has been written to the buffer and purged, and the server determines a time duration from initial download of the payload to final purging as a downlink performance indicator.