  As the demand for mobile connectivity continues to grow, there is a strong
need to evaluate the performance of Mobile Broadband (MBB) networks. In the
last years, mobile "speed", quantified most commonly by data rate, gained
popularity as the widely accepted metric to describe their performance.
However, there is a lack of consensus on how mobile speed should be measured.
In this paper, we design and implement MONROE-Nettest to dissect mobile speed
measurements, and investigate the effect of different factors on speed
measurements in the complex mobile ecosystem. MONROE-Nettest is built as an
Experiment as a Service (EaaS) on top of the MONROE platform, an open dedicated
platform for experimentation in operational MBB networks. Using MONROE-Nettest,
we conduct a large scale measurement campaign and quantify the effects of
measurement duration, number of TCP flows, and server location on measured
downlink data rate in 6 operational MBB networks in Europe. Our results
indicate that differences in parameter configuration can significantly affect
the measurement results. We provide the complete MONROE-Nettest toolset as open
source and our measurements as open data.
