Recently we saw Open5Gs’s Update Location Answer response putting the Subscribed-Periodic-RAU-TAU-Timer AVP in the top level and not in the AVP Code 1400 (APN Configuration) Diameter payload from the HSS to the MME.
But what exactly does the Subscribed-Periodic-RAU-TAU-Timer AVP in the Update Location Answer response do?
Folks familiar with EUTRAN might recognise TAU as Tracking Area Update, while RAU is Routing Area Update in GERAN/UTRAN (UMTS).
Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received.
Section 5.3.5 of 24301-9b0 (3GPP TS 24.301 V9.11.0)
So the Periodic Tracking Area Update timer simply defines how often the UE should send a Tracking Area Update when stationary (not moving between cells / tracking area lists).
Let’s look at how the Tracking Area Updates work from the point of view of the network.
Let’s take an example of a UE which has been sent the Tracking Area List TA0 and TA1, which is currently in ECM_IDLE state served by eNBs in Tracking Area 1.
The UE is moving towards another eNB in Tracking Area 2. As the UE listens on the Broadcast Channel the power of the new eNB overtakes that of the previous eNB, but the UE notes the Tracking Area of the new eNB, which is not on the UE’s Tracking Area List.
So the UE must make a Tracking Area Update to inform the network.
The first thing to do is to establish a radio connection.
Once the radio connection is setup a S1-AP connection is setup, upon which an NAS message – EMM Tracking Area Update Request is sent which contains the GUIT and old Tracking Area ID, which is sent to the MME.
The MME then sends back a new Tracking Area List for the UE and new TMSI to update the GUTI of the subscriber.
The UE updates it’s GUTI, updates it’s Tracking Area List, sends an EMM TRACKING AREA UPDATE COMPLETE and the UE returns to ECM_IDLE state.
As we saw before larger Tracking Areas minimize the number of UEs between terminals to update their location.
The problem is the cells/eNBs on the edge of the Tracking Area have to handle almost all of the Tracking Area Update requests, to inform the network the UE has moved to a new TA.
There’s an obvious imbalance between edge cells that handle almost all of Tracking Area Updates and the central cells inside a Tracking Area that handle very few many Tracking Area Update messages.
As we know we only have one radio interface, and sending Tracking Area Updates eats into our valuable radio resources that can’t be used to carry user data. Because of this users can experience a lower bit rate on edge cells.
To get around this we group Tracking Areas together into Tracking Area Lists.
A Tracking Area List is provided by the network to the UE, and contains a list of Tracking Areas, so long as the UE stays within the list of Tracking Areas, there is no need for it to send a Tracking Area Update.
You might think this just makes our problem worse, as now at the edges of the cells in the Tracking Area List we have even more signaling traffic, the clever part comes from the fact the network gives out different Tracking Area Lists to different UEs.
In the example below we can see UE2 has a different Tracking Area List to UE1.
This means the cell edges are different for UE1 and UE2, which spreads the signaling load across Tracking Areas, so while UE2 will send a Tracking Area Update when it reaches the border from TA1 to TA4, UE1 will send a Tracking Area Update when it passes from TA6 to TA9.
The other limitation of this is now to reach a UE paging must be sent on all cells in the Tracking Area List.
As we saw with the Network Triggered Service Request, the network needs to know which eNB / cell the UE is currently being served by.
The UE knows which cell it should use as it’s always listening on the broadcast channel to know the received power levels of the nearby eNBs.
If our UE is in ECM IDLE state and the network needs to contact the UE, the eNB sends sends a Paging Request on the Beacon (Broadcast) Channel with the UE’s RNTI.
The UE is always listening on the Beacon Channel for it’s own RNTI, and when it hears it’s own RNTI it follows the process to come back from ECM_IDLE state to ECM_CONNECTED state.
For this to work the network needs to know which eNB to send the Paging request to.
For this to work our UE would need to inform the network each time it changes eNB, but, as we’ve touched upon several times, minimizing power consumption is a constant architecture constraint in LTE.
So if the UE has to transmit each time a UE moves to a different eNB / Cell, the UE power consumption would be high and the battery life of the UE would be low.
If we imagine driving along a freeway at speed, with each eNB serving an area of 1km, at 60kph, our UE would change cells every minute, and if the UE needs to transmit to let the network know it’s changing location, we’d be transmitting data every 60 seconds even if the UE is sitting in our pocket, all these transmissions would lead to lower battery life on the UE.
To work around the power wastage of each UE transmitting data to the network to let it know each time it changes eNB, 3GPP designers decided to group eNBs in the same geographic area into Tracking Areas or TAs.
This means instead of the network knowing exactly which eNB a UE is located in, it has it’s location down to a tracking area made up of several eNBs. (Tens to hundreds of cells per TA)
To go back to our freeway example, we might group all the eNBs along a freeway into one Tracking Area, all of which broadcast the ID of each eNB and the Tracking Area of each eNB.
As the UE moves from one eNB to another eNB in the same Tracking Area, there’s no need for the UE to send a Tracking Area Update message as it’s reamining in the same Tracking Area.
Tracking Area Update messages only need to be sent when the UE moves to an eNB in a different Tracking Area.
Paging a Tracking Area
As the network knows the location of our UE down to a tracking area, when it comes time to Page a UE a Paging Request is simply sent from the MME to all eNBs in the Tracking Area that the UE is in.
This means the RNTI of the UE is broadcast out of all eNBs in that tracking Area, and the UE establishes connectivity once again with it’s nearest eNB.
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