You can select which network modes (3G and/or GSM) your mobile phone can use. Your choice determines the data speeds you can achieve.
Tap the menu icon.
Tap Mobile network.
Tap Network mode.
Tap WCDMA only, GSM only or GSM / WCDMA auto.
3G enables you to achieve higher data speeds than when using GSM. If you select WCDMA only, your mobile phone can only connect to 3G networks. If you select GSM only, you can't achieve high data speeds. If you select GSM / WCDMA auto, your mobile phone automatically switches between the two network modes provided you're within range of a 3G network.