WCDMA 2100Mhz is the European, Mid-East, and Asian 3G band, where 1900Mhz is the North/South American 3G band. Here's a Wiki that will help you understand.
Frequency bands deployment
* Band I (W-CDMA 2100) in Europe, Africa, Asia, Australia, New Zealand (ITU Region 1) and Brazil (part of ITU Region 2)
* Band II (W-CDMA 1900) in North America and South America (ITU Region 2)
* Band IV (W-CDMA 1700 or Advanced Wireless Services) in the United States (T-Mobile USA) and soon will be offered in Canada
* Band V (W-CDMA 850) in Australia, New Zealand, Brazil, Canada, the USA, Venezuela, other parts of South America, Israel, parts of Asia (ITU Region 2 and ITU Region 3)
* Band VIII (W-CDMA 900) in Europe, Asia, Australia, New Zealand (ITU Region 1 and ITU Region 3), and Venezuela (CorporaciÃ³n Digitel, C.A.)