Greater Boston Suburbs
How Cell Phones Work
How Cell Phones Work
This page is a bit off topic, but so many people have asked me to explain cell phone service to them, that I figure it doesn't hurt to publish a little explanation here for those who really want to understand it.
The first point that you need to understand is that just like radio and television, mobile phone service uses radio waves to transmit information between your phone and a cell site. Just like changing the shape of a written character conveys a different message to the reader, changing something about the radio wave changes the message (ie - the sound) that you will hear. Different systems change the wave in different ways (how big it is, how strong it is, or how often it recurs) and this is partially why there are different telephone systems. The best analogy for this is the comparison between AM and FM. While we perceive that one is BETTER than the other, the truth is that they are just DIFFERENT. Since the AM system is OLDER, the sound quality that it was designed to create isn't as good as that of FM, but a different use of AM could produce just as good sound. So, it is with cellular phone systems: There is not necessarily a better or worse one. They are just slightly different in how they convert the radio waves to sound, and thus some tend to travel a little farther, to break up a little less or to fit more calls into a given area. The two most common systems in use around the world are CDMA and GSM. CDMA stands for Code Division Multiple Access and GSM stands for Global System for Mobile. CDMA is widely in use in the US but is otherwise somewhat obscure, cropping up in places around Canada, Mexico and China. GSM is widely in use throughout almost the entire world. For this reason, GSM has the advantage of being more universal.
Cellular phone service differs from the original mobile phone system which used a central tower (like a radio station) to cover a particular city. Outside of that coverage area (typically 50 miles or so) you lost the connection, just as you would a radio station. The concept of cellular telephone service is to create 'cells' meaning small regions of only a few miles square which are covered by a 'cell site', meaning a cell tower.
Each cell site uses low power so that its coverage does not extend much beyond the intended radius and is connected to the telephone network either via a standard telephone cable, or through a microwave dish. This technology is (has been?) gradually been replaced by connections to the internet which then connect to the phone network. The point is that the cell site uses a small radio transmitter to allow your phone to speak to the telephone network in some way. What makes the system radically different though is that as you move from one 'cell' to the next, the system automatically transfers your call to a neighboring site. This is called a 'hand off'. In this way, you do not need a powerful cell tower nor a large and powerful phone with a big antenna. The radio portion of your call only needs to travel a few miles, to the nearest site. Calls are usually dropped when either you reach the edge of the site that you are communicating through and there is no neighboring site, or the system to transfer your call has failed to do so (which can occur for many reasons). Although the system isn't perfect, it has substantial advantages over the original concept of having one large tower to cover a whole city. In densely populated areas, carries can lower the power of the cell sites and increase the quantity, reducing the cell size itself but allowing more calls at one time. In temporarily densely populated areas such as during major sporting events like the Superbowl, or the Olympics, carriers can add mobile cell sites, which are actually cell towers located on vehicles. In big cities, cellular providers actually put cell sites in buildings, further reducing the cell size and many subway systems have cell sites in the tunnels which is why you do not lose service. The flexibility of cellular technology has really change the availability of communications in our world.
When you travel or 'roam' your phone connects to the nearest cell site and sends a message, asking if that cell site is connected to a network (ie - a mobile provider like AT&T or Verizon) which has a roaming agreement with your system. This basically means that the provider who operates the cell site, has the ability to bill your home system for the call. If it does, then you will be able to connect to and use that cell system as if it were your own. The quality of the call will be exactly the same. Generally speaking, all mobile phone providers have roaming agreements with all other mobile phone providers when it is technologically possible. This means that almost all GSM providers, will have roaming agreements with other GSM providers. However, it is not technically possible for a GSM user to connect to a CDMA technology and therefore you can never use a CDMA phone on a GSM network. In recent years, CDMA providers (notably Verizon) have wanted to offer the ability to roam to other countries where CDMA doesn't exist (ie - most of the world outside of N. America) so they have asked manufacturers to create multi-band phones (tri-band, quad band, etc). These phones basically have a GSM and CDMA phone built into one package, so that you can use CDMA when on your home network, or switch over to GSM when travelling). Mobile providers actively pursue roaming agreements because it is where they make alot of money. For instance, a Sprint user who cannot find service in parts of the US will usually be able to find a Verizon site because of Verizon's extensive network. Verizon charges Sprint way more than it charges its own users for its customers to access the Verizon network. Consequently, Verizon benefits by offering use of its network at a very high premium. Verizon can often make more money off Sprint users than off Verizon users, but of course it would never be able to remain in business if it didn't have the dependable income from its own users. Because most of the world has at least one GSM network in their locale, world travelers tend to prefer GSM service because it greatly improves their chance of finding a compatible network that they can roam on. In the US only AT&T and T-mobile use GSM. Sprint and Verizon both use CDMA. But beware of one major issue with respect to GSM: Not all carries use the same frequencies. Just like your radio dial only ranges from 87mhz to 108mhz, the GSM carriers usually use one of three different bands: 850mhz, 1800mhz or 1900mhz. The latter is typically used in North America. The others are spread out throughout the world. It is much easier to find a GSM phone though that has all three bands (a tri band phone) than a CDMA phone that also works on GSM. Travelers from outside the US to the US must have a phone which will use the 1900mhz band.
One of the other differences to GSM is that all GSM phones contain a SIM card. This is just a computer chip, similar to an SD card that you find in digital cameras, which plays the role of carrying all of the information regarding your mobile account.
For this reason, you can move the SIM card from one phone to another and keep the same service. GSM users in Europe can move their SIM card to a tri-band or 1900mhz phone and thus use their phone in the US. No such system exists for CDMA. Your phone itself is programmed to handle your account and if you lose your phone or want it to be replaced, a new phone must be programmed with your information. Thus GSM technology separates the purchase of a phone from the provider and the agreement that you have. Since the US cellular phone industry depends on you buying phones as part of your cellular package (and also depends on you wanting to upgrade your phone every few years), the GSM providers in the US generally LOCK their phones. This means that they will only work with the SIM card that they provide you. Phones can sometimes be UNLOCKED using a secret code, however using the wrong code can cause your phone to BRICK, meaning that it locks up and will not work, period. So, the US cellular providers have muddied the waters of GSM flexibility. Nevertheless, if you just keep your same phone that came with your plan, you can travel and ROAM almost anywhere in the world.
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