You can select which network modes your mobile phone can use, eg. 3G or GSM. Your choice determines the data speeds you can achieve.
Slide your finger down the display starting from the top edge of your mobile phone. Tap the settings icon.
Tap Tethering & networks.
Tap Mobile networks.
Tap Network mode.
Tap GSM / WCDMA auto, WCDMA only or GSM only.
3G (UMTS) enables you to achieve higher data speeds than when using 2G (GSM). 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. 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.