How is it possible for multiple cellular carriers to broadcast from one tower?
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The “tower” isn’t a singular thing. Any given tower has a many different receivers/transmitters on it. Multiple providers can have equipment on the tower at any given time.
Now, if you are asking why those signals don’t conflict with one another, it is because the signals are _slightly_ different from each other – and that difference is enough for your phone to know the difference and the signals to not interfere with one another _too_ much.
It is like WiFi. We say that WiFi is at 2.4ghz, but that isn’t exactly true. Wifi has channels, and each channel is a slightly different frequency: Channel 1 operates at a center frequency of 2.412 GHz.
Channel 11 operates at 2.462 GHz. Those differences are enough to separate the transmissions and let each phone and tower operate on different networks, even when they are close by.
A tower is just a high place to put transmitting/receiving equipment, it can have a TON of hardware up there for different communications bands, directional signals, etc….
To the more important part: Your carrier does not necessarily have to own the hardware your cell phone is using. It can use anything that allows its traffic as long as the hardware is communicating in a compatible frequency band and protocol. Part of what it communicates is who you are and who your carrier is, that’s the most critical thing the SIM card provides. If it uses hardware that doesn’t belong to your carrier (assuming that’s allowed by the network you’re communicating with), then the network owner bills that back to your carrier, who in turn assigns that usage to your account and bills you accordingly. Not every carrier automatically has these agreements, but most of them do. That’s how small/budget cell providers work. While and AT&T or a Verizon may have a lot of their own network, Cricket and whatever are just using the big companies networks and paying for that access.
That radio frequency can be used for many millions upon millions of what amounts to radio jamming in the extreme because as each digitally encoded radio burst from said tower is uniquely coded. Sent directly to every phone in its bubble, only yours knows it must now ring because part of that coded radio signal is your unique sim id.
A tower has a range of frequencies, just like a wifi system. So these frequencies go on an auction and companies which want more bandwidth buy more and vice versa. The tower broadcasts a range of frequencies, and some of the frequencies belong to some companies.
For a more ELI5 answer, think of a radio being able to connect to different channels coming from different locations/signals. The different radio stations are your cellular companies and the radio is your cell tower. Instead of throwing out signals in terms of audio, the cell tower just throws the signal in its range.
In addition to all of the other correct answers, you can see all of the equipment that’s on top of a tower on cellmapper dot net.
Super cool website. Also a great way to check out cell of say… a house you’re looking at buying/renting.
if you select a carrier, then click on a tower, you’ll see all of the antennas on that tower.
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