When troubleshooting a network performance problem today, I discovered some strange behavior when using an internet connection over a cellular network (GSM in my case):

When the line was idle for half a minute or so, the connection seems to hang a few seconds before working again. I verified this with our mobile provider Proximus and got the following interesting response: (liberally translated by me)

When no traffic is exchanged over the radio link for a while, the radio resources are reallocated to the pool. The IP connection remains open.

Technology Timeout Setup delay
2G (GPRS, EDGE) 44 sec 0.5 sec
3G (UMTS, HSDPA, HSUPA, HSxPA) 10-20 sec 3-5 sec

These are the averaged values for re-establishing the radio resource. (I have to admit that we have more delay on 3G.)

However, we are working on this issue. The project would not close down the radio resources completely, but leave them “logically” open. This feature is called CELL_PCH. This should enhance the setup delay by about 1 – 1.5 seconds (with the exception of the initial setup phase).