The back haul data is the reverse channel for data, the latency is bidirectional, and as more users access a single tower, that site's latency increases with proportional loading.
Also, every phone has an overload access number assigned to it, the higher that number, the faster you get dumped in favor of a lower ordered number when loading begins to become a problem on specific tower sites.
An in-state carrier I contract for does this, and it is part of the commissioning phase of the build-out for specific sites, with fire and medical always acquiring the highest access overload numbers, and the rest of us peons get the lower-ordered access. Doing what I do, I am able to make changes to our phones so we are never dumped in favor of anyone, sort of selfish, but a perk I can live with. Since I have to answer calls for system outages at all hours, I also need to know if I need to call for assistance, I will not be dropped in favor of a facebook update.
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A picture of a VZW reverse channel tower signal, taken from my home monitoring system.