Prime Use For UMTS Fast Dormancy Today: Fast Reselection to LTE

Just a quick thought today about a use case change of a feature over which hard battles were fought about in the past: UMTS Fast Dormancy.

One upon a time, not so long ago, back in 2010, mobile device manufacturers recognized that many UMTS networks were not configured in a power efficient way, leaving the radio connection in place for very long times when no data was transmitted. The problem for them was that there was little they could do about this as connection management is controlled by the network. But then, someone clever discovered a loophole in the specification, the “Signaling Channel Release Indication” message. Defined for an error scenario, devices started to use the message not for this purpose but to tell the network that they would go into RRC idle state on their own because they don’t expect further data transfers in the near future whether the network liked it or not. Network operators were not exactly thrilled about this loss of control and came up with a standardized way for the mobile to request to take down the radio channel if they thought no more data was transmitted for some time that still leaves the network in control of the proceedings. 3GPP Release 8 Fast Dormancy was born and back in 2010 I wrote about the details here. In 2012, the feature was used in three of the four UMTS networks in Germany.

Fast forward to 2016 and the situation is somewhat different from 2012. In many countries in Europe, LTE coverage now significantly exceeds UMTS coverage. Still, mobile devices fall back from LTE to UMTS for a variety of reasons. As a handover from UMTS to LTE is in the specifications but not widely used so far, a device might get stuck in UMTS for quite a long time if it can’t go back into idle state quickly and reselect on its own. So as Release 8 Fast Dormancy helps to go to RRC Idle more quickly it in effect helps to return to LTE more quickly. In other words, in many cases the benefit of the feature moves away from power efficiency to inter-RAT reselection efficiency in networks that are not releasing the radio connection quickly on their own. An interesting change.