05How your phone talks to the network

The short version: A cell network divides an area into many small zones, each served by a low-power transmitter, reuses the same frequencies in zones that are far enough apart, and hands your call from one zone to the next as you move. That idea launched commercially in the US in Chicago on October 13, 1983, and every generation since has built on it.⁶³ Today the signal reaches you from big macro towers, from small cells on poles, and in places like subway tunnels from a distributed antenna system. US 5G runs on low, mid and very high (mmWave) bands, and the C-band slice of the mid band set off a long dispute with aviation over aircraft radio altimeters.⁵³⁶⁴

How does a cell network work?

What you see: Bars on your phone that rise and fall as you walk, and calls that keep going as you drive across town.

The founding trick of cellular is reuse. When Ameritech Mobile Communications opened the first US commercial cellular service in Chicago on October 13, 1983, it used low-power transmitters that each covered part of the city, reused the same frequencies in different areas, and had computers hand calls off from one area to the next as a caller moved.⁶³ Phones then cost about $2,200 to $3,500, with monthly charges of $150 to $200.⁶³

A cell site is the set of antennas, radios and backhaul links that serves one coverage area. A handoff (or handover) is the moment the network passes your connection from one site to another without dropping it. Because each site is low power, a frequency used in one neighborhood can be used again a few cells away, which is how a network serves millions of phones with a limited slice of spectrum.

Under the 3GPP standards published by ETSI, a phone that is switched on first selects a network (a PLMN, in 3GPP terms), then searches for a suitable cell on that network and "camps" on it, monitoring its control channel.⁶⁵ It may then register its presence in the tracking area of that cell, and if it later finds a more suitable cell under the reselection criteria, it moves there, using its own signal measurements to decide.⁶⁵ Those rules, from the 5G specification ETSI TS 138 304, cover a phone sitting idle; the handoff described above applies during a call or data session.⁶⁵

What do 1G, 2G, 3G, 4G and 5G mean?

What you see: A "5G," "LTE" or "4G" icon next to your signal bars, or an old phone that stopped working.

"G" stands for generation. Each generation changed how the radio link encodes voice and data, while keeping the reuse-and-handoff idea that the 1983 Chicago system used.⁶³ By 2024, US wireless connections numbered 579 million, about half of them 5G devices, according to the wireless trade group CTIA's annual survey.⁶⁶

Older generations do not last forever. Carriers switch them off to reuse the spectrum for newer service, which strands phones, alarm panels and car systems that only speak the old language (see "Why did my old phone stop working?" below).

In the US, Verizon switched on its LTE network on December 5, 2010, covering 38 metropolitan areas and more than 60 airports at launch.⁶⁷ Verizon launched mobile 5G on April 3, 2019, starting in parts of Chicago and Minneapolis.⁶⁸

Is that a cell tower or a small cell?

What you see: Either a tall lattice or monopole tower with rows of rectangular panel antennas near the top, or a slim canister or box on a light pole or building corner with a power meter and cabinet nearby.

A macro site is the classic cell tower or rooftop installation: high up, relatively powerful, covering a wide area. A small cell is a low-power site mounted on a pole or building, covering a much smaller area, added mainly to give a busy area more capacity rather than more reach. Small cells have spread fast: tower company Crown Castle alone counted about 115,000 small cells on air or under contract in 2022.⁶⁹

The two work together. The macro layer gives everyone a signal; small cells take the load in crowded blocks, so the macro site is not trying to serve a whole downtown at once. The street furniture that carries small cells, from poles to the cabinets bolted beside them, is covered in detail in the utility poles guide.

How many cell towers are there in the US?

What you see: A tower on a hill, a cluster of panels on an apartment roof, a church steeple that seems to have grown extra hardware.

Counts depend on what you call a site; towers explains how to tell the structures apart. CTIA counted 418,887 operational US cell sites at the end of 2021, a figure that does not include new 5G base stations added at existing sites.⁶⁹ CTIA's head cited more than 15,000 new cell towers activated in 2024.⁷⁰

The demand behind those numbers: Americans used about 132 trillion megabytes of mobile data in 2024, up from 100 trillion the year before.⁶⁶

What do low-band, mid-band and mmWave 5G mean?

What you see: Coverage maps with different shades for different kinds of 5G, and phones that are dramatically faster on some street corners than others.

US 5G runs on three tiers of spectrum. Lower frequencies travel farther and get through walls better; higher frequencies carry more data but fade quickly.

The FCC's "5G FAST" program auctioned the 24 and 28 GHz mmWave bands in 2019, and an auction of the upper 37, 39 and 47 GHz bands concluded in 2020, releasing 3,400 MHz in all.⁵³ For the mid band, the FCC auctioned 70 MHz of licensed 3.5 GHz (CBRS) spectrum in August 2020 and planned to clear the lower 300 MHz of the 3.7-4.2 GHz satellite band (the C-band) and auction 280 MHz of it, together creating a contiguous 530 MHz swath from 3.45 to 3.98 GHz.⁵³

The C-band sale, Auction 107, offered 280 MHz at 3.7-3.98 GHz in 5,684 blocks. Its clock phase ended in January 2021 at $80.9 billion gross, the highest-grossing US spectrum auction.⁷¹ For comparison, the forward auction for the 600 MHz band had raised $19.8 billion in 2017.⁴³

Why did 5G cause problems for airplanes?

What you see: News stories in late 2021 and early 2022 about canceled flights and "5G buffer zones" around airports.

A radio altimeter measures an aircraft's height above the ground by bouncing radio signals off it, and it matters most during low-visibility landings. These instruments work in a frequency band close to the top of the C-band, which is how the two came into conflict. The aviation section covers the rest of a plane's radios.

The FAA's December 2021 airworthiness directive states that commercial aviation radio altimeters operate in the 4.2-4.4 GHz band, while US 5G C-band service operates at 3.7-3.98 GHz, 220 MHz below.⁷²

In December 2021 the FAA found that 5G C-band interference could stop radio altimeters from working properly and issued an airworthiness directive.⁶⁴ In January 2022 Verizon and AT&T voluntarily added mitigations around airports.⁶⁴ In March 2023, Verizon, AT&T, T-Mobile and UScellular agreed in writing to keep those mitigations until January 1, 2028.⁶⁴

The FAA then set deadlines for the aircraft side. From June 30, 2023, planes without approved altimeters were barred from certain low-visibility operations, and from February 1, 2024, US airliners flying under Part 121 had to have approved altimeters.⁶⁴ About 180 planes needed new altimeters and 820 needed filters, at an estimated cost of $35.2 million.⁶⁴

The fix, in short, came from both ends: carriers kept mitigations in place around airports for a period of years, and airlines replaced or filtered the older altimeters that were most exposed.

What is a DAS, and why is there signal in some tunnels?

What you see: Small white antennas or flat panels on tunnel walls or station ceilings, and coaxial cable running along the tunnel.

A distributed antenna system (DAS) is a network of small antennas fed from a central hub, called a head end, used where signals from outside towers cannot reach: subway tunnels, stadiums, big buildings. Each carrier connects its radios at the head end, and the DAS carries the signal out along cable to antennas spread through the space. One shared system can then serve several carriers, instead of each company wiring the same tunnel on its own.

In Boston

What you see: Bars that hold steady as a train runs underground between stations on the T.

The MBTA signed a 15-year contract with InSite Wireless in 2005 to wire its tunnels for cellular service.⁷³ Coverage arrived in pieces: Verizon served four downtown stations from 2007, AT&T and T-Mobile had the Orange Line and part of the Red Line from 2010, and T-Mobile added the Blue Line and most of the Green Line in March 2012, with full wiring targeted for the end of 2012.⁷³

Later, American Tower worked with the MBTA to replace the original 2005 DAS equipment and add 5G.⁷⁴

The sources describe the tunnel network differently. The Boston Globe in 2012 put it at 19 miles of tunnel and 35 underground stations;⁷³ the Wireless Infrastructure Association's 2026 case study says more than 20 miles of tunnels linking more than 40 stations.⁷⁴ The difference may reflect what each counts as a tunnel or a station, or later expansion; neither source explains it.

An October 2016 article by InSite Wireless's DAS technology chief said five companies were connected to the T's tunnel DAS - AT&T, Sprint, T-Mobile and Verizon, plus Comcast for Wi-Fi on station platforms - after a build in five phases between 2007 and 2015.⁷⁵ The WIA case study gives no completion date or carrier list for the 5G upgrade.⁷⁴

Why did my old phone stop working?

What you see: An older phone, smartwatch, home alarm or car system that lost service in 2022.

US carriers scheduled their 3G shutdowns for 2022 and early 2023. As of February 2022, AT&T planned to switch off 3G on February 22, 2022, with about 2.7 million customers possibly affected.⁷⁶ T-Mobile set March 31, 2022 for its 3G CDMA network (inherited from Sprint) and July 1, 2022 for its own 3G UMTS network.⁷⁶ Verizon, which had originally targeted the end of 2019, set January 1, 2023.⁷⁶

A device that only speaks 3G has nothing to connect to once that happens, no matter how strong the bars look on a newer phone beside it.

Elsewhere in the world

What you see: Different 5G band numbers on a phone's spec sheet, or a UK phone that lost 3G between 2023 and 2025.

UK 5G spectrum. Ofcom, the UK regulator, consulted on its 700 MHz and 3.6-3.8 GHz auction in December 2018 and published principal-stage results on March 17, 2021.⁷⁷ Final results on April 27, 2021 showed £1,379,400,000 raised in total. EE won 3680-3720 MHz plus 700 MHz blocks; Telefonica (O2) won 3760-3800 MHz plus 700 MHz; Vodafone won 3720-3760 MHz; and Three won paired 700 MHz spectrum.⁷⁸ The UK's assigned 3680-3800 MHz overlaps the bottom of the US C-band (3.7-3.98 GHz).⁷¹

UK 3G and 2G switch-off. As of February 2023, Ofcom reported that Vodafone's 3G switch-off was starting early that year in Plymouth and Basingstoke, EE would start in early 2024, Three would finish by the end of 2024, and all operators had committed not to offer 2G or 3G service beyond 2033.⁷⁹ Virgin Media O2 was the last, with Durham the first city to lose its 3G, on April 2, and the full switch-off due by the end of 2025, by which time Vodafone, EE and Three had finished.⁸⁰ In other words, UK operators planned for 2G to outlive 3G, with 2033 as the outer limit.⁷⁹

In a post dated December 18, 2025, Virgin Media O2's chief technology officer said the company would complete its 3G switch-off "in the coming weeks," and that some customers might still see a 3G signal for a short period in early 2026.⁸¹

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