WirelessMoves

How Large Is A URA?

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.

Release 8 Fast Dormancy Now In 3 UMTS Network In Germany

Two and a half years ago I wrote a lengthy post about power consumption problems of smartphones and one remedy for it, referred to as 3GPP Release 8 Fast Dormancy. This feature enables the mobile device to inform the network that it thinks that no further data will be transferred for the moment and that it likes the radio link to be downgraded to a more energy efficient state. This way, the timeout period during which power consumption is high can be significantly be reduced. This is very beneficial in cases when only background traffic such as keep-alive pings and email push/pull services communicating with a server produce short bursts of traffic while the mobile is not actively used. Also, another benefit is that the connection is put into a semi-dormant state (Cell-PCH / URA-PCH, see the post linked above) from which it can be reactivated much more quickly than from a fully idle state. Shortly after that post one German network operator actually switched on the feature.

So when I recently made a check of the state of the networks in Germany I was very positively surprised that three out of four networks have the feature implemented and activated by now. Two of them switch the connection to the Cell-PCH state while one uses URA-PCH. Only one laggard remains, incidentally the least performing one in recent network comparisons.

So what's the difference between Cell-PCH and URA-PCH? In Cell-PCH state, the mobile needs to send a cell update message whenever it changes from one cell to the other so the network can send a paging message for incoming voice calls, SMS messages or IP packets via the right cell. When users are moving or are located just in between two cells this results in increased cell update signaling. URA-PCH on the other hand groups several cells into a common registration area (URA = UTRAN Registration Area) thus reducing the cell-update signaling. If this is better than Cell-PCH depends, of course, on how many cells are in an URA.

How Antennas Change Over Time

Antennas and base station sites, they can be seen everywhere these days but it's pretty difficult to see how they change over time. When I recently came home and looked out the window I could at first not  exactly say what it was but I had the impression that something had changed at the antenna site on the building at the opposite side of the street. But I couldn't quite figure out what was different. Then I remembered that I took a picture two years go so it was easy to compare.

And here's the result: The left part of the image (click to enlarge) shows how the antenna construction looks today and the right part shows how it looked like two years ago. Before the configuration was changed there were three antennas covering each sector. One antenna was installed on top and two antennas were mounted below closely side by side. Today, there's only a single antenna casing with at least two antennas inside as can be deducted by the number of cables at the bottom of the antenna casing. Furthermore, a second microwave antenna has been installed on the main mast below the one already used two years ago.

Quite a significant change and I can only speculate why it was done!? I am pretty sure the top antenna belonged to a different network operator than the lower antenna. So does this absence mean that this operator no longer uses the tower? It's likely as I am not aware of any antenna sharing deals between network operators. And how could the lower antennas have been changed at the same time as the upper antenna of presumably a different company was removed? Coincidence? Cooperation?

Questions over questions. But one thing is for sure: I don't remember my surroundings in as much detail as I always thought as otherwise I would have immediately noticed the missing top antenna instead of having to compare today's state with that of two years ago. That is interesting as well.

P.S.: Note that the sky is grey on both pictures. I'll let you draw your own conclusions...