One train of thought I followed with my easy smartphone Wi-Fi tracing setup I wrote about recently is how often a typical smartphone contacts the network per hour even if it is not used and just lies on the table and what impact that has on the cellular network in a larger context. Even though I monitored the devices behavior over Wi-Fi the result can be applied to cellular as well as it is likely that most applications do not make a difference anymore between Wi-Fi and cellular connectivity. The result is quite interesting:
Even without user interaction my smartphone contacts the network 10 times per hour. Out of these, 4 times is for checking email. Another 4 times Android calls home to Google, mainly using a Google Talk domain, even though I've disabled the app. Less frequently a DNS query and subsequent traffic can be observed to a number of additional Google domains. I feel quite observed by such unwanted behavior but there's something that can be done about it with a rooted phone as I've described here in the past. Further connections are made for various other purposes. My calendar and address book are synchronized with my Owncloud server at home every four hours, the NTP server is queried to keep the clock in synch, crash reports are sent to crashlytics.com (have I consented to this?), the weather app requests updates, the GPS requests ephemeris data periodically, etc.
So what does this mean on a larger scale? Let's say a network operators has 15.000 3G base stations (extrapolated from here) and 10 million smartphones. If those smartphones were evenly distributed across all base stations there would be around 660 smartphones per base station or around 220 smartphones per sector. If each smartphone connected to the network 10 times an hour that's 2200 requests an hour per sector. If the connection is held for 10 seconds on average, that's 2200 requests / (60 minutes * 60 seconds / 10) = 6 concurrent connections just for the background traffic of the devices.
Some cells are obviously busier than others so some cells probably see two or three times this number, i.e. 15-20 concurrent connections just for background traffic. As the number of concurrent users in 3G cells is likely to be less than a three digit figure that's quite a sizable percentage. And the 10 connections per hour is perhaps even a conservative number as many subscribers use instant messengers that need to send frequent TCP keep-alive packets so they don't loose connectivity to the server. On the other side, many smartphones are used over Wi-Fi, especially when people are at home, which is likely to significantly reduce background traffic over cellular in residential areas. Not so in business areas, however.
So where do we go from here? One good thing is that LTE networks are mostly in place now and many new smartphones, especially those of heavy users are LTE capable by now. That significantly reduces the load on 3G networks. And from what I hear the number of simultaneous users in an LTE cell can be much higher than in a 3G cell. The right technology at the right time.