After my recent discovery that three mobile network operators in Germany now use Release 8 Fast Dormancy to reduce signaling overhead and power consumption of UMTS devices and also to improve responsiveness to user input from a more power efficient state I wanted to dig a bit deeper to see how each one makes use of the feature. This post is about the network that uses URA-PCH instead of Cell-PCH like most networks do today as the more energy efficient state.
The difference between Cell-PCH and URA-PCH is that the mobile does not have to send a Cell-Update message whenever it moves from one cell to another. Instead, an update is only necessary when the mobile reselects to a cell that is in a different URA (UTRAN Registration Area, 3GPP TS 25.401). This saves signaling overhead and power when a mobile is located just between two cells and thus frequently switches between them. It is equally beneficial for fast moving users in cars or trains where mobiles select a new cell quite frequently, often several times a minute.
The big question for me was how large a URA actually is. Just one cell, several cells or even much bigger? As I am a frequent public transportation commuter, the answer was not too difficult to come by and actually quite surprising. On a 30 km trip on a train, all cells, and I estimate there are around 25-35 cells on that route (based on Cell Logger traces) are in the same UTRAN Registration area as I didn't observe a single update message being sent to the network. So an URA is quite large in that network, perhaps as large as a whole Location Area which usually includes a major city such as Cologne or Bonn and it's surroundings.
The downside, of course, is that for incoming data packets the mobile has to be paged not only in a single cell but in all cells of the URA. For devices not using the fast dormancy trigger mechanism on their side such as UMTS/LTE sticks, for example, this is not a big problem as the network timer to go to URA-PCH state (from Cell-FACH) is set to 30 seconds. For mobiles that are using the Release 8 Fast Dormancy Functionality (Signaling Connection Release Indication) things could be different. Triggering it too early could result in a state ping pong and frequent paging in all cells of the URA until all data has been exchanged. From my practical experience with the feature, however, that seldom happens.
To summarize: Using URA-PCH instead of Cell-PCH can be quite advantageous for network operators as no signaling required when the user moves. For the user URA-PCH has the advantage over Cell-PCH that less power is used for non user-data related signaling while they are in trains and cars. Let's see, perhaps it will be a growing trend.