If I opt to get the OnePlus International plan, there is unlimited 4G LTE mobile hotspot. Will I be throttled down if I use a lot of data via tethering? Or is that throttling only for people on the 3G mobile hotspot data plan?
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Great question. The answer is that "It depends".
It actually depends upon the state of the T-Mobile network
where you are receiving your service, If the network is temporarily congested, everything will slow down for everybody somewhat. T-Mobile then starts "prioritizing " who will be slowed down more than others. If a person uses the majority of his unlimited data, to tether laptops and other devices instead of using it for unlimited web and media viewing on the phone, he might be given a lower priority than people who mostly just use their unlimited data for phone web browsing.
The rational is that mobile data should be used for mobile uses,
not a substitute for a land line Internet connection. It is sort of like bus and carpool lanes which bestow priority on travelers who contribute less to traffic congestion. Once rush hours are over, everybody can use the whole bandwidth of the highway, since it is no longer relevant which individual may be contributing the most to congestion. So if and when there is congestion you won't actually be "throttled down". You might just not be able to use the bus and carpool lane. You will still be able to drive at the same rate of speed as the other cars who must also use the congested lanes. Having said that. I don't remember ever reading about people who were actually deprioritized for an excessive tethering ratio.
The greater problem would be if you exceed the 3% data usage threshold for all "on network data" used whether tethered or not, and there is congestion. You absolutely could be literally stuck in traffic until the congestion subsides. The current 3% threshold is 32GB of total on network data. Multiply that number by the number of devices you use as hot spots and you have what I would consider a pretty safe estimation of the minimum amount of high speed data you can expect to have available per month.