Code Division Multiple Access is an alternative technology to GSM for 2G and 3G networks. In CDMA, voice signals are digitised, and the frequency of the transmitted signal is varied according to a predefined code. This signal can only be intercepted by a receiver whose frequency response is programmed to vary with the same code. Since there are trillions of frequency-shifting sequences, CDMA signals offer considerable privacy. These signals use the 800 MHz and 1.9 GHz bands. CDMA signals utilise more bandwidth compared to GSM signals because of the variations in frequency. CDMA handsets are typically locked by carriers, which means you cannot switch your carrier without changing your handset.