What is the difference between the 800 and 850 bands?
Nothing. The 800 cellular band has always been called 800 until some GSM marketeer decided to call it GSM-850. 800 and 850 are the same exact band! Talk about confusing! Most people seem to be calling it 850 these days.
What is the difference between the 850 and 1900 bands?
They are the two different wireless bands available to North America. 850 was the original cellular band, and it was split into two, the "A" band and the "B" band. The "B" band was for the wireline phone company and the "A" band was for a non-wireline provider. The 850 band has been around for 15+ years, and the systems are very well built out. The FCC mandated that a certain amount of land be covered by a signal. The 1900 band was placed in operation several years after 850. The 1900 band is also known as PCS, and the two terms are used interchangeably, which can be confusing when trying to follow a conversation. There are 6 bands A through F, and some of those can be split into others. The requirements for the 1900 were not as strict as 850. Only a certain percentage of the population needed coverage (67% IIRC). Than means building the urban areas pretty much met the entire FCC buildout requirement for a given area. From what I can gather, there rules have been relaxed even further. The Nextel iDEN system also uses 800 MHz, but its a different band (originally used exclusively for two-way radio), not used by any other provider. So we'll still call it 800. The only requirement about which technology can run in a particular band is that AMPS (analog) must run in the 850 band through 2008. AMPS is only available on TDMA and CDMA tri-mode phones. TDMA is being phased out probably in the same 2008 timeframe as AMPS. CDMA phones are moving to all digital transceivers. GSM phones never supported analog (there are two exceptions ... the Nokia analog sleeve and GAIT phones ... but neither are currently being manufactured).The FCC is allowing analog to be turned off starting 2/18/2008.
What are the 900 and 1800 bands?
Many other parts of the world (other than North America) use two different bands, 900 and 1800. These are not compatible with the North American bands of 850 and 1900. There are some quad band GSM phones (most notably Motorola) that cover all four GSM bands, but usually it's one or the other. If you buy a phone not intended for the North American market, 1. make sure that it will work here, 2. make sure that it is unlocked and 3. make sure it will work on the cellular provider you wish to use.
Which is better, 850 or 1900?
In general, you are going to get more performance out of 850 than you are going to get out of 1900 for several reasons. 1. As mentioned earlier, back when the 850 licenses were issued, they had to cover a certain amount of land cover. This required deploying their system throughout many rural areas (not ALL though). 1900 licenses only need to cover up to 67% of the population, and in many cases they don't even have to meet that. 2. The higher the frequency, the shorter the usable range. You need approximately twice as many 1900 MHz towers to cover a given area than 850 MHz towers. Most 1900 MHz towers are in urban and suburban areas. A properly built 1900 system will work as well as a properly built 850 system, but it will likely cost more to deploy and operate. Sometimes 1900 will work better in a city because 1900 MHz signals tend to work better in the middle of the city with large buildings as the shorter wavelength allows the signal to go around corners easier. Also, due to network loading, 850 towers have to be "turned down" in urban areas so as to not overload, so the playing field is leveled. 3. Leaving the technical details aside, it seems that 850 MHz signals penetrate most modern buildings better than 1900 MHz signals. There are many factors involved such as the material of the walls, the proximity of the local cell towers and various other factors. The fact that 850 MHz carriers have been in operation longer and have optimized their coverage is an important factor to consider. If there is a window nearby, chances are that either system will work, assuming that there is some sort of signal available at the window! The bottom line is this: when you try out a service, make sure you bring your phone to all the areas you'll be using to make sure it works where you need it.
Why don't all countries use the same frequencies?
The various governments around the world have licensed different bands for different uses over the years. In North America the 900 and 1800 bands are allocated for other uses and are not available for public wireless communications. All the cool GSM phones come out for Europe first. They are tri-mode 900/1800/1900 models. What will I give up by not having 850? Good question. As shown by the maps below, you give up LOTS of coverage. However, if you live in an urban area and rarely leave, other than to travel the interstates and visit other large cities, it might not matter. The first map is Tracfone T-Mobile GSM-1900 regular plan coverage. Most major markets are covered, but coverage is limited to the metros and the connecting highways. Roaming is allowed on most other GSM systems EXCEPT AT&T. You see what that gives up! T-Mobile coverage has improved with new roaming agreements, especially with GSM-850. (note that these maps are old but you get the idea)
This is the Tracfone AT&T GSM-850 & GSM-1900 regular plan coverage. You can see many areas are covered more completely. Although you can't roam on most T-Mobile systems, that does not hurt coverage.