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how do mobile phones work?

by Stephanie Coates

I’ve always been curious of how when I talk on the phone, how my voice gets transmitted tens, hundreds, or even thousands of miles to someone else’s device, and played nearly instantly. How does it actually work?

When you talk to your friend on the phone, your voice is picked up by the phone’s microphone and translated into a digital signal in form of binary. The phone’s (nowadays, likely internal) antenna sends out the digital signal in the form of electromagnetic waves. The binary 0’s and 1s are differentiated by the amplitude and frequency of the waves - low frequencies are used for 0s, and high frequencies are used for 1s.

At a high level, the way a digital call works is to transmit these electromagnetic waves from your cell phone all the way to your friend’s cell phone.

However, electromagnetic waves can’t travel long distances. They lose their strength due to physical buildings, electrical equipment, and environmental factors. Even without these barriers, the waves wouldn’t carry on forever due to the earth’s curved structure. This is where cell towers come into play.

Cell service providers divided geographic areas into hexagonal cells, each cell having its own tower and frequency slots. These cell towers are connected via underground optical fibre cables and under the ocean to provide international connectivity. Check out this video on the critical importance of undersea submarine cables for global internet. SO COOL.

When you make a call, the electromagnetic waves travel from your phone to your nearest cell tower, are converted into light pulses and transmitted to your friend’s closest cell phone tower via underground cables, which then converted back into electromagnetic waves and emitted to your friend’s cell phone. His/her cell phone picks up the waves, converts them into binary and then into voice, and he/she hears your voice. And this all happens so quick!

How does your tower know which tower to send the signals to in order to emit them to your friend?

Via mobile switching centers, or MSCs, which manage the cell towers in a single coverage area. Wherever you buy a SIM card, that sim card is connected to its nearest MSC, which will become your home MSC. It stores info like your phone plan, current location, and activity status. When you move away from your home MSC, the new MSC (known as the foreign MSC) will communicate with your home MSC so it knows to forward requests to the foreign MSC to emit waves from a cell tower near it to connect to your phone.

You home MSC always knows where you’re located and essentially acts as a router to forward requests to whichever foreign MSC you’re near so it can pass on the signal near you. Foreign MSC data is updated on a time-based schedule, whenever you cross MSC borders, and whenever you turn your phone on.

Say you’re in Colorado and you want to call your friend Sam who lives in Illinois, but is currently on vacation in Maine. You dial Sam’s number, and the electromagnetic waves are transmitted from your cell phone to a cell tower connected to your home MSC in Colorado. It then routes to a cell tower near Sam’s home MSC in Illinois. The MSC in Illinois knows Sam’s current location is in Maine, so it routes the signal to a cell tower in Maine, closest to Sam’s location. Once it arrives at the tower, the electromagnetic waves are emitted and hit Sam’s phone, causing her phone to ring and the call will be connected.

How does SMS differ from voice calls?

SMS (short message service) was invented before internet became a thing on mobile phones. In addition to sending voice traffic over the telephone network via electromagnetic waves, SMS was added to the set of signaling commands to carry short messages to and from mobile phones, just like voice calls were. MMS (multimedia messaging service) uses same technology as SMS. These texts only require a wireless plan from a cell carrier to work, not an internet connection.

How does SMS differ from sending/receiving messages over wifi?

iMessage, WhatsApp, WeChat, and FB messenger, known as OTT (over the top) applications, don’t require a cellular network connection and instead use IP (internet protocol). In order for these to work, your device needs to be connected to the internet - specifically, to a wireless router, found in your home or at your local coffee shop. The router plugs into a wall and connects to an Internet Service Provider, or ISP, either over a wifi of mobile internet connection. There are pros and cons to both - SMS is universal, while specific messaging apps don’t overlap, i.e. you can’t call someone on WeChat using FB messenger. However, OTT tech is growing in popularity due to of less file size limitations, and because they’re free.

Whats the difference between wifi data and cellular data/mobile internet connection?

These two technologies do the same thing - allow you to connect to the internet wirelessly. Some devices (computers) are built to use wifi only, while others, like 4G smartphones, have Wifi, 3G, and 4G LTE access. Connecting to wifi is as simple as looking up wifi networks near you and entering a password if required.

If you’re using data through a cellular plan/mobile internet connection, you don’t need a router or wifi. Your phone connects to a cell tower the same way it would for a voice call, assigns the phone an IP address and the cell tower connects to the internet.

1G to 5G

Today I learned (!) G stands for generation. Each G represents a significant milestone in the development of mobile communications.

1G - 1980

  • Users could carry a phone without a cable attached to it
  • Wireless transmission used analog radio signals, resulting in poor quality and security
  • No such thing as text messages

2G - 1990

  • Providers began using digital data in phone conversations instead of analog data
  • Reduced the intricacies of data transmission
  • Introduced SMS (short message service)
  • Introduced internet browsing at a speed of up to 50 Kbps

3G - 2001

  • Set the standards for most of the wireless networks we know today
  • First Blackberry came out in 2003
  • 3G still in use today when superior 4G signal fails
  • Higher data transfer speed: 2MBps with more data capacity
  • Enabled GPS, voice and video calls, web browsing, TV/video streaming, online games, sharing/downloading multimedia
  • These features paved the way to smart phones
  • 3.75G introduced MIMO (multiple input multiple output) technology, where multiple antennas are used by both the transmitter and receiver. This upped data transmission speeds up to 168mbps download speeds and 22mbps upload speeds

4G (LTE) - 2009

  • Five times faster than the 3G network
  • In theory, can provide speeds up to 100 Mbps
  • Better latency, higher voice quality, easy access to instant messaging services and social media, quality streaming and faster downloads

5G - 2019 and on

  • Released in 2019, currently being rolled out throughout 2020
  • Improved speed and capacity will open doors for new IoT trends: connected cars, smart cities, smart manufacturing, and AI applications