C. Data connection
From a user perspective, it makes more sense to be all the time connected to the GSM network and switch to 3G only if data connection is needed. In fact the energy saved while using SMS or voice services can be used for other services offered by the 3G phones, such as Internet connection, VoIP, media and entertainment. On-demand use of 2G and 3G networks depending on the requested service, requires including this cost in the energy analysis. The results are shown in Table III and are obtained by averaging the values of 10 handoffs. Figure 10 shows the plot of the mean values of energy per bit spent for downloading 500 KB of data for different data rates. When the phone is connected to the GSM network, the technology used for downloading is GPRS. In contrast, when connected to the UMTS network, HSDPA was used for downloading data. The plot shows clearly that the energy per bit decreases with the increase of the data rate for both data connections.