What is a frequency band?
Cell phones use radio waves to transmit your conversations. These radio waves can be at different frequencies, just the same as regular radio stations are at different frequencies.
For example, if you're listening to FM radio stations, they are in the FM radio band (of course!) which is between 88-108 MHz. If you're listening to AM radio stations, they are in the AM radio band, between 0.55 and 1.6 MHz.
GSM cell phones use frequencies within four different frequency bands :
850 MHz (824.2 - 848.8 MHz Tx; 869.2 - 893.8 MHz Rx)
900 MHz (880-2 - 914.8 MHz Tx; 925.2 - 959.8 MHz Rx)
1800 MHz (1710.2 - 1784.8 MHz Tx; 1805.2 - 1879.8 MHz Rx)
1900 MHz (1850.2 - 1909.8 MHz Tx; 1930.2 - 1989.8 MHz Rx)
Although 850 and 900, and 1800 and 1900 are very close together, a phone that works in one frequency band unfortunately can not also work in the frequency band next to it unless added as a specific extra frequency band. For comparison, when you have your FM radio tuned to a radio station at 98.1 MHz, there's no way you'll hear what is happening on another radio station at 98.3 MHz unless you retune your radio.
Which frequencies are used in the US?
Originally, the US used only 1900 MHz for its GSM cell phone service. In the last year or so, there has been a growing amount of GSM service on the 850 MHz band. This type of service will usually be seen in rural areas, because the 850 MHz band has better range than the 1900 MHz band. It can sometimes also found in city areas, particularly if the cell phone company has spare frequencies unused in the 850 MHz band, but no remaining frequencies to use in the 1900 MHz band.
Most of the 850 MHz service belongs to AT&T, and some to Cingular (these two companies are in the process of merging). Although T-Mobile does not (as of July 04) have any of its own 850 MHz service, because it has roaming agreements with both AT&T and Cingular, even a T-mobile user might sometimes find themselves in an area where the only signal available is on 850 MHz.
What about 800 MHz? Is this a fifth band?
Some people refer to the 850 MHz band as being the 800 MHz band. This is incorrect. The actual frequencies in the band are closer to 850 MHz and the standardized naming convention as promulgated by the GSM Association is to refer to this band as '850 MHz'.
If you see someone referring to a phone with 800 MHz service, they probably are simply mistaken and mean to refer to the 850 MHz band.
Do you need both frequencies in the US?
This really depends on the areas in which you use your cell phone. If you're in a major metropolitan area, you probably won't need the 850 MHz band, but if you travel to secondary areas regularly, you will find the extra coverage of the 850 MHz band to be valuable.
Looking into the future, it is probable we'll see increased use of 850 MHz to expand GSM's overall coverage into more of the country.
And then, looking further into the future, it is possible we'll see 1900 MHz coverage duplicating the 850 MHz coverage.
Bottom line : If you travel out of the main cities, you'll definitely benefit from a phone that supports both 850 MHz and 1900 MHz.
Which frequencies are used internationally?
GSM was originally developed in Europe, and only came to the US recently.
Initially, all countries with GSM service used the 900 MHz band. In the past few years, service providers have increasingly been adding 1800 MHz coverage, due to congestion in the 900 MHz band.
When the US started to use GSM, a few other countries with very close links to the US chose to copy the US and use the same frequencies that the US used - first 1900 MHz, and in a few cases, 850 MHz also.
Almost without exception, all international countries that use the non-US international frequency bands have 900 MHz service, and many have some 1800 MHz service as well.
All international countries that have the US frequency bands have 1900 MHz service. A very few might also have some 850 MHz service.
Which frequencies do you need when traveling internationally?
That depends on the countries you plan to visit.
Refer to the table below to get a feeling for which countries use which frequency bands. For a more expanded set of information, complete with network coverage maps, refer to the official GSM Association's website.
As the table suggests, 900 MHz is the most common band used internationally. 1800 MHz will give you expanded coverage in countries that also have 900 MHz. And some countries only have 1900 MHz rather than 900 or 1800 MHz.
Note that countries with both 900 and 1800 MHz service generally provide better coverage in the 900 MHz band than in the 1800 MHz band.
Frequency Bands by Country