When I get on a new mobile network I usually give it a thorough check for my research and to discuss results with interested operators. Most already do application testing on their own such as how many videos are viewed without interruption or how fast web pages load, etc. However, some do overlook some other important parameters.
One of the first things I noticed in Vodafone’s 3G network here in Spain at the Mobile World Congress is that sometimes when I log into the network with my mobile phone and remain connected, the battery runs flat within only 2 hours. A bit of research revealed that the reason for this is that they are assigning public IP addresses for transparent Internet connections (via the APN airtelnet.es).
So you might wonder what the battery drain has to do with a public IP address assignment!? Well in theory, not much, but in practice, quite a lot. When getting assinged a public IP address, all IP packets to this address get routed to the mobile device no matter whether they are wanted or not. Since IP addresses are dynamically assigned the new holder of an IP address potentially gets the ‘leftover’ from a previous user and all the junk emitted from viruses and other malware on the Internet. While the mobile device doesn’t really care and discards those incoming packets, the radio link is constantly active instead of falling back to a power conserving state while no user data is exchanged.
In my case I noticed that in some cases when I attached to the network the air interface link was constantly kept up as every couple of seconds an unsolicited IP packet was received. Most operators use network address translation (NAT), which assings a private IP address to the mobile and thus fixes this issue.
A somewhat crude fix for this problem from the user side short of hooking up the mobile to a power source once an hour is to fall back on the 2G GPRS network. The unwanted IP packets still come in but the power consumption in 2G mode is significantly less as the air interface link is kept for a much shorter time than in 3G mode after the last IP packet has been received and also requires less power.
So Vodafone, while I appreciate a public IP address for my 3G notebook dongle I think it’s a particularly bad idea for battery driven devices and user statisfaction. Also from a network point of view this is far from ideal as it wastes significant ressources on the radio link that would better be used for real traffic.
PS: I’ve noticed that there is also an APN with private IP addresses but that seems to be only good for web surfing as all my other applications are blocked.