Spectrum. It sounds like a rainbow. I mean, it is kind of magical, right? It's what carries animated GIFs, YouTube videos and tweets to your phone. But like, what is it?
With carriers throwing out buzzwords based on competing technologies and mad marketing, deciphering cellphone speak basically requires a degree in engineering with a minor in bullshit. Hopefully, this'll help.
When we're talking about spectrum in the context of phones, we're talking about the radio part of the electromagnetic spectrum. So we're talking about radio waves—the same kind that carries music and the dulcet tones of talk radio blowhards to your car stereo, or teevee to your grandparents' set with bunny ears. And the rate at which these radio waves oscillate is known as frequency.
The reason you're hearing so much about spectrum lately is because each carrier's spectrum holdings determines their wireless communication capacity. Each carrier has a license from the FCC to transmit on certain frequencies and each frequency has a finite amount of users it can support. There's also a very finite amount of spectrum to go around. (To use a rough metaphor, think of spectrum like a highway, where frequencies are the lanes. Each lane can only hold so many cars.)
Potentially, the more spectrum a provider has, the better the service. (Like if you go from a one-lane highway to a three-lane more highway.) So, one of the reasons AT&T wants to purchase T-Mobile is to get their hands on the carrier's AWS spectrum in the 1700MHz range, which AT&T wants to use to build out its 4G network.
If you live in New York or San Francisco, you experience spectrum shortage first hand. Because only a finite amount of stuff can go across a range of spectrum—otherwise you get interference—networks get overloaded and start dropping calls. If a carrier has a narrow range of spectrum within a frequency, they can't offer the broadband speeds users expect. (Like if you're on a highway, with narrow lanes—I'm assuming you get that point by now.) As mobile broadband usage continues to climb, the problem will only get worse according to carriers. The wireless industry has been lobbying the FCC to allocate more spectrum for auction to help alleviate the issue. More spectrum means more devices can access the network at the same time and faster download speeds.
The speed of a carrier's network in a region is dependent on a few things. For one, what technologies they have deployed on their available frequencies, and just how much spectrum they have available for a particular frequency. AT&T's recent 4G LTE launch on the 700MHz spectrum in Chicago and Houston as chronicled by PC World is a good example of this.
In speed tests, Houston is pulling 24.64Mbps down on the AT&T 700MHz 4G LTE network, while Chicago gets a paltry 5.59Mbps down on the same AT&T 700Mhz 4G LTE network. The difference in the download speeds is explained by the amount of spectrum AT&T has in each region. In Houston, AT&T has 24MHz of the 700MHZ spectrum. That means the carrier can run paired 10Mhz channels in Texas. In Chicago the carrier only has 12MHZ of the 700MHz spectrum. So it has to run paired 5MHz channels.
Carriers have to take into account certain aspects inherent with wireless frequencies when planning their mobile strategy. A higher frequency is able to transmit more data—so it can be faster—but it becomes more susceptible to interference from objects, like buildings, trees, hills, etc. A lower frequency is the opposite—it transmits fewer bits per second, but it's more effective traveling through objects like walls and trees. (Do you have a dual-band router? Notice how the 2.4GHz band goes way further than the 5GHz, but the 5GHz is better for streaming HD video?) Hence the carriers' burning desire to use the 700MHz spectrum for 4G, because it's got an optimal balance of range and bandwidth. Comparatively, the 2500MHz spectrum used for WiMax could be screaming, but it doesn't penetrate buildings nearly as well, and it's way more susceptible to interference.
Each frequency band is used to deliver a certain network technology. For example, Sprint's iDEN network operates on the 800-900MHz frequency bands, while their CDMA technology operates at 800 and 1900MHz. This is true for all carriers. What carriers have done is allocate network technology to the spectrums they have available. One spectrum may get 3G technology while another gets 4G LTE. Here's a handy chart, showing who's using what. (Click to embiggen.)
Because your carrier is utilizing multiple frequencies, and you want your phone to still work while roaming and switching between towers, modern mobile phones have multi-band radios inside of them. This is especially important if you buy a 4G phone and have no 4G coverage in your area. If you can't get a 4G signal, the phone will fall back to your carrier's 3G frequency. This is why when you look at the tech specs of a phone you see something like this: UMTS/HSDPA/HSUPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz). Those are the frequencies a phone can use to access a network.
Obviously, this is just a tiny dip into the pool of frequencies and spectrum. Scientists with spiffy white lab coats spend years trying make sure you can call your friends to meet you for drinks after work. So tonight, have a drink for the scientists who make it possible to drunk text your ex at 2:30AM.