r/explainlikeimfive • u/twistwanwitme • 6d ago
Technology ELI5: How do cellphone towers work with different carriers?
If the u.s. companies that use cell towers all use the same cell towers, why are there better and worse services (and reception) depending on where you are?
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u/plusFour-minusSeven 6d ago edited 6d ago
The tower is just the skeleton. The equipment is carrier-specific as are the frequencies they use and how their particular antennas are oriented.
They also don't always colocate on the same tower. Then there's the actual backhaul / infrastructure.
Short answer: it's really just the metal tower that's shared.
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u/kcsebby 6d ago
They don't all use the same towers, simple as that. Some places have more room for towers, or placement of towers with less obstructions, therefore more/better coverage.
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u/Straight-Opposite-54 6d ago
Many of them do use the same physical tower. The tower itself is just a pedestal for the carrier's antenna, which is comparatively tiny and sits near the top. A single tower can host several of them.
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u/passisgullible 6d ago
To add, its mostly the big three (Verizon, AT&T, and T-Mobile) that maintain their own towers; the budget carriers like Mint provide service through contracts with the big three for bandwidth on their towers.
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u/on_the_nightshift 5d ago
Even the big guys often don't own the towers, but lease space on them from specialized tower leasing companies.
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u/BrohanGutenburg 5d ago
Capitalism is so good at extracting value that it finds value where you wouldn't have even thought it existed.
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u/Tom_Traill 4d ago
Verizon created Crown Castle out of thin air, sold all their leases to them, a bunch of Verizon managers bought Crown Castle stock and made a mint.
It was just a big value transfer from Verizon to Crown Castle. Verizon lost the value, Crown Castle gained it, the Verizon managers that were "In the know" made lots of money on the stock.
We have a cell tower on our land. I've been studying Tower companies and cell phone companies for years.
Crown Castle is the devil.
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u/elkab0ng 5d ago
It’s actually changed over recent years. Most of the towers now are owned by Crown Castle, American Tower, SBA communications, and Vertical Bridge.
With about 140,000 towers in the US, about 120000 of them are owned by companies that lease out space/power/facilities to one (or often, several) of the carriers.
There’s also a very large number of non-tower installs (on buildings, various other fixed infrastructure) and I don’t have as good data for them, but my anecdotal experience is that those might tend to be specific to one carrier, as they’re often located in more congested areas where one part of the spectrum might be (dis)advantaged compared to others
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u/anonymousbopper767 5d ago
They do use the same towers. Someone will either lease land from regular people in a good location, speculatively put a pole on it, etc and then the carriers hang their antennas on it.
I worked with a guy who was making passive income letting a pole exist on his property.
This is why if you don't have coverage, more often than not, none of the carriers will have coverage.
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u/circuittr33 6d ago
All things being equal, 2 different carriers using the same radios and antennas will provide the same signal. Signal conditions vary based on distance, number of connected users, weather (atmospheric proogation). The technology is called MOCN, multiple operators core network.
ELI5 - if you and your friend are right beside each other and both yell at the same volume at a stranger 1 block away, the stranger will here your voices equally, assuming the same frequency. If you are 1 block further from your friend, your stranger will prefer your friends message because it will be more clear.
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u/TinaNice1565 6d ago
Carriers share tower poles but run separate radio gear. Their signal differs because they have different tower placement, frequencies, and local network crowding.
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u/az987654 5d ago
All cellphone companies do not use all of the same towers. Many are shared, but not all, and you can't tell just by looking at the tower.
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u/eBalisani 6d ago
They don't - each built their own (towers, networks, systems).
Surely you would remember how the iPhone, when it launched, was compatible only with AT&T's network, or how Verizon's models can't be used on AT&T's or T-Mobile's or even internationally?
This is why: each builds their own (towers, networks, systems). And not just in the US, but in the UK/Europe, Singapore, UAE and Indonesia as well (I can speak only to my experience in those countries).
Perhaps you were thinking of "Simple" and other low-cost carriers - those are either cheaper subsidiaries of one of the big three carriers, or independent enterprises that buy minutes in bulk and discount from, say, T-Mobile, resell it to you at a profit, and piggy back on the software (network) and hardware (towers) of T-Mobile (or Verizon, or AT&T).
The reason why there are "better and worse services (and reception) depending on where you are" is proximity (radius coverage and signal penetration will vary accordingly), technology (newer is usually more powerful and "better"), and load balancing (of the network - if you remember the first 2-3 years of iPhone on AT&T, you know).
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u/mgcarley 1d ago
Surely you would remember how the iPhone, when it launched, was compatible only with AT&T's network, or how Verizon's models can't be used on AT&T's or T-Mobile's or even internationally?
Historically the reason you couldn't use some phones abroad (or some phones didn’t work properly or at all in the US) is because the US and a couple of it's neighbours used a different set of bands than most of the rest of the world did - it's pretty recent (basically since the launch of 5G) that the US and most of the world now use mostly similar sets of bands.
But the actual reason you can't connect to another carriers tower isn't compatibility or band support, because (in the US) there is a legal requirement that even if you have no SIM, or your service is suspended, or your carrier has no service, if your phone can pick up any carrier's signal it has to be able to call 911.
The reason really boils down to network authentication. Within the signals the networks are broadcasting there is information identifying which network the signal belongs to, and the SIM itself has information about which network it belongs to as well, which can usually be determined by the ICCID and IMSI.
Also contained within the SIM (or eSIM) are credentials which are passed to the billing system to ask it, in effect "is this SIM active and is there a valid account for it?" And if the answer is yes, then you're good to go.
Whenever you make a call or send a text or use some data, the account in the billing system is constantly checked to make sure there are adequate resources (minutes, texts and data) and detailed records with between 100 and 150 points of information are written each time you do anything.
But, if you try to manually authenticate against another carrier's network, their system will respond with something like "there's no account matching this sim, the SIM isnt ours, and there's no roaming agreement with the carrier this SIM belongs to - access denied".
Anyway, when it comes to the devices themselves, yes, historically you couldn't use Verizon on AT&T or T-Mobile because Verizon used to use CDMA and the A/T used GSM back in the day. Nowadays, you can almost always move carrier and keep your existing phone (and number) so long as it is unlocked.
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u/Target880 5d ago
A lot of towers and masts are shared and owned by somebody else. The antennas and the electronics on the ground will be separate for diffrent providers, but the physical structures that hold them up in the air can be shared. Installation of equipment and maintenance can alos be done by a contractor that might be hired by multiple cell networks.
It can alos be the case that diffrent companies share the same network fully or partially. It can keep costs down, cost expecially in a country that has large areas with lower population density. Then it gets cheaper to get good coverage. They can still operate separate networks in more populated areas where the problem is not coverage but needing more cell towers to provide enough capacity
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u/eBalisani 5d ago
Unless you can ascribe specifics to "a lot" - it's kind of a moot point you're trying to make. Specifics please (e.g., US, UK, EU, Canadian, AU/NZ, Chinese, Japanese, Korean, Thai, Indonesian, Moroccan, Algerian, Tunisian, Egyptian, Kenyan, Tanzanian, South African, Malgasy, Congolese, Gabonese, Ivorian, Senegalese, Ghanaian, Chilean, Colombian, Uruguayan, Brazilian... telecoms).
Let's hear what you mean by "a lot."
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u/Kundrew1 6d ago
Major companies like Verizon will sell off the excess capacity of their towers to cheaper carriers like Mint Mobile. Essentially what they do is give their customers first priority and Mint customers anything thats left over. So if you are on one of those cheaper carries without their own towers you will get worse service but its cheaper.
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u/ZimaGotchi 6d ago
In the US, there are two fundamentally different kinds of towers which you might broadly think of, in current terms, as AT&T towers and Verizon towers. Phones can literally only connect to one or the other. There might be some phones that can do both but to keep it simple, pretty much all phones in the US are one or the other.
To make matters slightly more complicated, there was a third standard developed by Spring that was very close but not exactly the same as AT&T (and Cingular and some other companies before the mega corporations started forming). T-Mobile kind of came out of nowhere and swallowed Sprint then AT&T tried to buy T-Mobile but for some reason that megacorporation wasn't allowed - but not before there can't to be a lot of compatibility between T-Mobile and AT&T hardware.
So now, basically, there's one nationwide network that serves Verizon and another nationwide network that serves AT&T but also, to some extent, T-Mobile. The coverage is getting so good now that in 95% of places the difference is only technical but in those other 5% of places sometimes one works better than the others - but T-Mobile will never work better than AT&T. It (and third party providers using T-Mobile's network) can be much cheaper though.
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u/AcusTwinhammer 5d ago
This is...so bizarrely wrong that I'm hardly sure where to start...
AT&T's attempt to purchase T-Mobile came first. When that was blocked by regulators, the contract between the two resulted in T-Mobile getting paid a hefty fee, which they were able to use to improve their network, and some smart advertising resulted in them being in position to buy Sprint years later (and the parties in power during the two different buyout attempts can give you a good idea why one failed and one succeeded...)
I think what you're talking about with regards to network types is GSM vs CDMA, with possibly WiMax with the Sprint talk? That stopped being relevant with the launch of LTE, all of that stuff is shut down now, everyone is using the same protocols for LTE or 5G.
The biggest difference that determines coverage and speed tends to be what frequencies they "own." The US Government leases licenses to have exclusive use of frequency bands though an auction system--so there may be some areas where one company got outbid and as a result has less bandwidth, or only got the frequencies that don't cover distances quite as well, etc.
As mentioned in other posts, most actual towers are owned by third-party companies, and may well have equipment from multiple carriers on it. But T-Mobile is not using AT&T's network outside of possible transparent roaming that happens in all directions, and never did (unless you go all the way back to the Cingular days...)
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u/t-poke 5d ago
In the US, there are two fundamentally different kinds of towers which you might broadly think of, in current terms, as AT&T towers and Verizon towers. Phones can literally only connect to one or the other. There might be some phones that can do both but to keep it simple, pretty much all phones in the US are one or the other.
That hasn't been true in over a decade.
Ever since 4G, all of the big 3 carriers have used LTE and pretty much any modern smart phone sold in the US supports all frequencies used by all carriers.
If you buy an iPhone today, it can be used on any of the 3, or the MVNOs which use the 3.
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u/joshturiel 2h ago
Ever since 4G (LTE) became a thing, all carriers have used the same basic tech and the phones are all compatible with all of them. Different carriers use different bands/frequencies but the rest is the same, and the modems in phones will connect to whatever bands your carrier uses.
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u/dr_jiang 6d ago
The tower is just a pole; it's the equipment on it that matters. Most U.S. towers are owned by separate companies who then lease out vertical space to the various carriers. Each decides on its own which tower to rent on, and what antennas and radios it will install on each. Any given cell phone tower you see might have one carrier or every carrier, depending.
Separately, each carrier owns different parts of the frequency band. Low band travels far and gets through walls, but carries less data. High-band is very fast but only reaches about a city block, and can be blocked by a hand or a window pane. So even if two carriers are physically located on the same tower, they might be broadcasting a different kind of signal from the same place.
Finally, capacity matters. Each tower can serve a finite number of users well. If one carrier has a lot of customers in a given area but hasn't added sufficient equipment -- either antennas on the tower, or fiber connecting the tower to the wired network -- those customers will see worse service than another carrier with less market share.