Sprint, to put it kindly, doesn’t have the best mobile network. Although it’s winning “most improved” stickers faster than anyone else, its network is still dead last compared to its nationwide competitors, according to every single comparison.

While its average download speed is getting much better — and now on par with AT&T, according to some tests — Sprint’s download speed still lags. An explanation from OpenSignal analyst Kevin Fitchard hypothesizes why that might be, and how Sprint is trading faster downloads for significantly slower uploads.

According to OpenSignal’s data, which relies on millions of crowdsourced speed tests, Sprint’s average upload speed is 2.5Mbps, several times slower than industry leader T-Mobile, and nearly half the speed of third-placed AT&T. The results don’t initially make sense when you consider Sprint’s download speed is nearly on par with AT&T — normally, upload and download speeds are correlated, so an improvement in one comes at the same time as an improvement with another.

Fitchard explains how Sprint may have been sacrificing upload speed for faster downloads:

The answer most likely lies in the nature of Sprint’s LTE technology. A large portion of Sprint’s 4G network uses a technology called time division duplexing, or TDD, to transmit data to and from devices. Unlike most cellular networks, which use separate dedicated frequency bands for the uplink and downlink, TDD networks use the same spectrum for uplink and downlink. The network essentially splits every two-way transmission into time intervals. One time interval is used to send data from the tower to the device (download) while another time interval is used to send data from the device to the tower (upload). The neat trick about TDD networks is operators can adjust the proportion of time intervals devoted to upload and download as they see fit.

Sprint’s faster download speeds but stagnating uploads are reflected by the data:

As customer whims and habits shift, Sprint can configure its LTE network to match them. In May 2017, it revealed its most recent LTE tweak, favoring download much more heavily than upload to align with the data usage patterns on its networks (customers were downloading 10 to 12 times more data than they were uploading). We’re likely seeing that new configuration reflected in our metrics.

The analysis is an interesting peak behind the curtain at how networks can manage their limited resources to get the best network experience. The days when all traffic was treated equally and networks didn’t have congestion issues are long gone. The popularity of unlimited plans and shift towards streaming video have placed a measurable strain on the carriers’ networks, and as time goes on, we’re likely to see more and more optimization and network tricks that will benefit the many, but at the expense of a few.