If you are talking about the 850/1900/2100 spec for Google Nexus One 3G, the 1900/2100 reference is NOT, repeat NOT a reference to one band, it is a reference to two bands. The official nomenclature and frequencies for these bands are:
Operaring Band: UMTS V
Frequency Band Name: 850
Common Name:CLR
Uplink Freq:824-849
Downlink Freq:869-894
Operaring Band: UMTS II
Frequency Band Name: 1900
Common Name
Uplink Freq:1850-1910
Downlink Freq:1930-1990
Operaring Band: UMTS I
Frequency Band Name: 2100
Common Name:IMT
Uplink Freq:1920-1980
Downlink Freq:2110-2170
AWS should NOT be referred to as 1700/2100, only as 1700. And, all bands are split for uplink/downlink and several overlap around the globe. AWS is not unique in this regard.
Operaring Band: UMTS IV
Frequency Band Name: 1700
Common Name:AWS
Uplink Freq:1710-1755
Downlink Freq:2110-2155
Any other usage is improper.