I will correct you.
The DL TBF is used to carry the downloaded data, it will last as long as the file you required takes to go from the web to your phone.
During this download, your phone will need to regularly send uplink ACK. Let’s say once every 2 seconds, for example.
So every two seconds, the MS will ask to setup an ul tbf. It will use it to send the ACK. And then the ul tbf will be closed.
You then have 1 DL TBF + 1 UL TBF every 2 seconds.
Thatt’s just an example. The ACK rate in TCP is not 2 seconds : it varies depending on bandwidth and delay.