Ok, well there’ve been some good explanations so far and a few misleading ones. Since this is my business, I’ll try and clear a few things up… (Believe it or not, what follows is a simplified explanation, so please excuse if I’ve left some details out or over-generalized. I’ll also mostly talk about USA cellular with some other areas thrown in for comparison.)
In the olden days (pre-mid-80s) there was a mobile system for car phones that used the same technology, basically, as the “ship to shore” system. You had a car phone – actually a phone in the car and a big honking transmitter/receiver in the trunk and a big antenna on the top. In each geographical area, there were a half dozen or so frequencies allocated for these type of radio calls. (There weren’t many users, needless to say.) The phone had no dial, just a button that you pressed when you picked up the handset and which signaled the “mobile operator”. Actually, first you listened to make sure that no-one else was using the frequency or “channel” (sort of like a party line). When she (yes, usually she) answered, you gave her the phone number you wanted (“Hello mobile operator, I’d like to be connected to 555-4321.”) and she dialed it for you and connected you. These were not duplex systems, meaning that you had to “press to talk”, and if you were talking, you couldn’t hear (receive) what the other person was saying. There was usually a small number of (high power) antennas around town that carried these calls and connected them through the mobile operator to the rest of the world (called the PSTN or Public Switched Telephone Network). It was also quite expensive. I believe this type of land mobile service no longer exists (because I can’t imagine why anyone would still want to use it, given what other options now exist) but I’m not positive.
Right now there are a number of different types of commercial mobile telecommunications services referred to as “cellular” by the general public, and yes, they all operate by transmitting RF (radiofrequency) signals, but there are some differences which are best explained via a brief history lesson.
In the early-mid-80s, when the technology began advancing and people began realizing there was a market for this stuff, the FCC gave away (!!) the licenses to provide what was labeled “cellular” service in various geographic areas – first the major metros (MSAs or Metropolitan Statistical Areas, a classification used in a lot of different industries/uses) and then the RSAs (Rural Statistical Areas, mostly meaning the smaller cities and towns). Two cellular licenses were granted in each area, an “A” license and a “B” license. One (can’t remember if it was A or B) typically went to the local phone company (Bell Atlantic, GTE, PacBell, whomever) and for the other, the FCC held a lottery where pretty much anyone demonstrating a nominal level of expertise in wireless communications and the ability to raise financing could apply for a license. This is how many small companies, as well as some of the big guys (like Sprint and AT&T) got into the cellular biz. This kind of cellular operates in the 824-849 and 869-894 MHz range (USA). The system is composed of a number of “cells” with an antenna usually on a tall building or tower at the center that serves all the calls in its cell – usually a 5-10 mile radius circle. A caller within this cell is connected via the antenna (and associated computers, etc.) at the center of this cell to the cellular network and, through it, to the PSTN. That’s how you can just dial 555-4321 on your cell phone and the call goes through (or someone on another phone can call your cell phone). A caller with a cellular or, more accurately, “mobile” phone, as it travels along, is passed from one cell to the next, as described in the earlier post.
The words “cell(ular) phone” and “cell(ular) tower” come from this era.
Within the designation of “cellular”, there are a number of sub-categories, mostly having to do with how the signal is coded and carried on these frequencies. So GSM (Global System for Mobile), AMPS, NAMPS, etc. are all different types of cellular service. Also, the cellular networks were originally analog, because that was the only technology available at the time, but in recent years many have added/replaced them with digital systems. (Again this refers to how the signal is coded/carried “in the air”). The advantages of digital are 1) you can generally carry more simultaneous calls within a given frequency band (so the company can get more calls at once and make more money), and 2) the digital systems are designed to provide a lot of nifty features, like paging, voice messaging caller ID, etc. that weren’t designed into the analog system. (Theoretically you could do all these features with analog, too, but no one has bothered for business reasons.)
The big difference to the end user between analog and digital (besides the features) is how calls are degraded when the signal gets weak: for analog, you generally start getting a lot of static (but can typically still hear the other person, at least to a point), for digital you start getting “chop”, where parts of the conversation go missing completely, but what you do hear is clear – sort of like a CD skipping.
Most systems (i.e. most services) are one kind or another and unless you have a dual (or multi-) mode phone, you can only communicate on the type of network you phone is designed for. (Sort of like talking a different language.) So if you have an analog phone and you travel to an area where there is no analog service (either because it was never built or because it’s been phased out in favor of digital), your phone wont work. (I won’t even go in to roaming and roaming agreement unless someone specifically asks.) In the US, most analog systems are AMPS/NAMPS, digital are mostly TDMA/CDMA, although Omnipoint and VoiceStream are, I believe, GSM. GSM is also the standard in Europe and some other parts of the planet.
Starting in 1993, the FCC started awarding licenses in the 1800-1900 MHz ranges for “PCS” (Personal Communications Services). Only by now they had realized the value of these licenses and so they were auctioned off instead of just given away. (This brought $Billions into the federal coffers, BTW.) PCS is much like digital cellular, but since it operates at higher frequencies, its towers/antennas need to be closer together (radius of each cell is smaller). This is due to the propagation properties (how energy dissipates) at the higher frequency. Once again, phones designed to operate on a PCS network can’t make calls on a “cellular” network unless the phone is specifically designed for it.
(BTW, cellular is not being replaced by PCS. But analog cellular is being replaced by digital cellular in order to be more competitive, feature-wise, with PCS. Both digital cellular and PCS will continue to compete for the foreseeable future.)
There’s also something called ESMR (Enhanced Specialized Mobile Radio), which is another range of frequencies licensed by the FCC for commercial wireless services. This also operates in the 800 and 900 MHz range, and functions much like cellular, but is different – and requires a different, separate network – from the cellular one. Nextel is, I believe, an ESMR-based service.
So, in any given geographical area there are 2 licensed Cellular service providers, a couple of licensed ESMR providers, and 4-6 PCS providers (plus paging, satellite providers like GlobalStar, unlicensed radio like taxis use, etc.). This is part of the reason why there are so many “cell towers” going up. Each system/network needs its own antennas to serve its customers. It’s also why the cost of cell service has dropped so much in recent years – lots of competition.
To the end user, all this probably seems silly a/o confusing. Basically what’s happened (in the USA anyway) is that all these various services are commonly referred to as “cellular” (even if the FCC or the industry doesn’t see it that way) and the mobile handsets all as “cell phones”, which makes sense, b