如何正确测量NSInputStream数据速率

I'm trying to measure actual transfer speed during ftp download, download itself is working, streams are hooked up in run loop. Measurment is done in NSStreamEventHasBytesAvailable using CFTimeGetCurrent on event start and at the end, after data is written to file elapsed time is computed with (double)previousTimestamp-CFAbsoluteTimeGetCurrent, but the time I get is absolutely unreasonable. Tested on simulator and device, can anyone enlighten me?I'm trying to measure actual transfer speed dur