On my commute to work I make good use of the excellent network coverage along the railway track. LTE coverage is almost perfect, but just almost as there are a few locations where my data session is handed over to the 3G network. Once on 3G the only way back to LTE, at least for now, is for the network to set the device into Cell-PCH or Idle state so it can search for LTE and return autonomously. That unfortunately doesn't happen in my case as my OpenVPN Server sends a UDP keep-alive packets every 10 seconds, thus preventing my smartphone I use for tethering to return to LTE. It's not that big of a deal as 3G is quite fast as well so I hardly notice the difference. But I'm a perfectionist... So I had a closer look at the OpenVPN sever configuration (in /etc/openvpn/server.conf) and noticed an option for keepalive timers:
keepalive 10 120
The "10" suspiciously looked like the 10 seconds interval that keeps my 3G connection in Cell-DCH state. After changing the line to
keepalive 30 120
the UDP keepalive packets are now spaced 30 seconds apart. That's more than enough time for the network now to set my device to Cell-PCH or Idle state, which in my case, happens after around 12 seconds of inactivity. Shortly after, my tethering smartphone then changes back to LTE.
Perfect! And on top of all this I might even save some battery power as fewer packets are sent and received now.