Google’s (s goog) Nexus One phone is a heap of fun to play with, but so far in the U.S. the only 3G network you can access it on is that of T-Mobile. There are two issues at play here: the underlying technology a cell-phone network runs on, such as HSPA for 3G or EDGE for 2G, and the frequency the radios in the device use to communicate. In the case of the Nexus One, it uses the 900MHz, 1700MHz and 2100MHz frequency bands for 3G data and the GSM 850MHz, 900Mhz, 1800MHz and 1900MHz bands for 2G. Because the T-Mobile 3G network in the U.S. is listening for 3G data in the 1700MHz and 2100MHz bands, it’s the only U.S. network with which the Nexus One phone can communicate.
AT&T’s 3G network listens for phones transmitting 3G traffic in the 850 MHz and 1900 MHz bands, which the Nexus One cannot do. All it can do is send 2G traffic in those bands, so it does, making a slower data experience for those using the Nexus One in Ma Bell’s mobile network. As the world moves to services like Long Term Evolution, which carriers will roll out this year and will be widespread in 2012, it will be easier to share devices across networks, but frequency bands will still be an issue.