8 hours sitting at home on a Saturday or 8 hours on a roadtrip. If in both scenarios you don’t have to use your phone. Which one will drain your phone battery the most ??
You say, ” Oh so simple Azar. It will drain more on the road. ”
You say , ” Ah, Hmm, Ah …. because I am travelling ”
Close but not quiet precise. If you want to know the answer in two sentences or less. Go all the way down
at the end of this post.
If want to relish all the details, continue reading. Can you tell me how tracking area is associated with your
phone battery consumption ? What happens to tracking area in this case ?
The concept of tracking area (TA) is based on the simple fact, that a group of eNodeBs are combined together based on a particular value of tracking area code (TAC) . These group of eNodeBs combined are termed as TA.
Tracking area is used by the MME in Idle mode to keep track of user terminals (UEs) mobility. In this way , no matter whatever your area and wherever your phone is. You keep on getting phone calls , text messages, web information, mobile internet etc. through paging.
Tracking areas have a huge role to play in paging and keeping track of UEs mobility in Idle Mode.
How many eNodeBs to group in one tracking area over another depends upon a number of factors such as market type, users behavior in the area, rural or urban area , signalling load etc.
What is the reason to have Tracking Areas ?
As said earlier, Tracking areas make it convenient for the core network (MME) in this case to keep track of user terminals mobility even in case of idle mode. MME keeps track of users’ mobility in Idle mode at tracking area level.
When UE gets register to the network , it is given a tracking area list. Based on that tracking area list. UE will keep on moving both in idle and connected mode in those tracking areas included in the Tracking Area list.
If a UE happens to pass through a tracking area which is NOT included in the tracking area. The UE will get registered with a new tracking area and the new TA will be added in the TA list.
Keep in mind, UE keeps on entering , registering and leaving new and old tracking areas over time,as if the user remains in mobility. To keep track of this user mobility , UE’s position is tracked by the MME at tracking area level in MME.
Don’t get it ?? Let’s understand by an example.
Lets say that a UE is registered with a network. During registration UE will receive a tracking area list. Assuming there are three TAs included in the list for UE
Let’s say UE was in tracking area TA 1 when it entered into Idle mode. Whenever a UE enters into Idle mode. A timer starts running. This timer is called T3412 timer. Whenever this timer expires UE will send a Tracking Area Update (TAU) message to the MME. Indicating to MME that it is a certain TA 1.
Assuming that UE is in TA 1 when it entered Idle mode. It will camp onto any cell in TA 1. MME will keep track of UE’s position among tracking areas as a list of PAST and PRESENT. So if at the moment when UE entered in Idle mode. If it was in TA 1 . MME will keep information as , that UE is in TA 1 at present and in the past as well it was in TA 1.
Let’s say UE moves from TA 1 into TA 3 in Idle mode. UE will camp on any cell in TA 3. While camping on a cell in TA3 . UE will read TAC information from the cell to be camped on by reading SIB 1. TAC value arrives in SIB 1. After reading SIB 1 , UE will find out that the TAC value for TA3 is the among the same list which was given to the UE when it registered to the network. If UE remains camping onto the cell on TA3 and the timer T3412 expires. At the expiry of this timer, UE will send a TAU to the MME, indicating that now the UE is in TA3.
After receiving the TAU. MME will update the PAST and PRESENT list. The old information is removed. In this case MME will change the PAST as TA 1 , while the PRESENT is changed to TA 3 as shown in figure below.
If the user is continuously moving and enters into TA 2. Similar procedure will take place like the previous step. Before camping onto a cell in TA 2. UE will check the TAC value from TA2 and verify it with its tracking area list. If its allowed. It will camp onto it.
Once T3412 expires , UE will send a TAU to MME. Upon receiving the TAU. MME will update the PAST and PRESENT list. In this case, PAST will be changed to TA3 and PRESENT will be changed to TA2 as shown in figure below.
What if a Paging Message Arrives during TAU ?
Unfortunately if a paging message arrives for the UE during the time UE is sending a TAU. On account of TAU procedure, paging message will be ignored. UE cannot respond to paging during TAU.
However, if the previous scenario does not happen. In other words, UE is not sending a TAU. In this case when a paging message arrives for the UE. MME will send paging to the last known eNodeB first, which was used to send the tracking area update. If UE does not respond to the paging request then it will send the paging message to all eNodeBs in the TA list included in PRESENT list. If still no response MME send paging to eNodeBs in the PAST list as well (depending). All of this depends, what Adaptive paging strategy has been adopted and deployed by the operator. So what will happen in the actual network depends upon operator policies.
What if a UE does not find the TAC value in the Tracking Area List ?
The fun begins, if the UE enters into a Tracking Area that is not included in the Tracking Area list given to the UE during registration. UE will verify after reading TAC value of new tracking area and comparing it with the Tracking area list given to the UE. If UE does not find the new TAC value in the Tracking Area list. This will be an indication that UE is in a new tracking area. In this case UE is going to send an explicit Tracking area update message back to MME right away indicating the new TAC value UE has read.
After receiving the TAU message MME will do two things.
Timer T3412 and Special Scenario:
Another important question to ask ourselves is. What is the best and sure way for UE to send a Tracking Area Update message back to the MME ? Is it the expiry of Timer T3412 or when UE finds itself in a new Tracking Area which is not in the Tracking Area List . Does it sends Tracking Area Update right away ??
The answer is YES and YES. Whichever among the two happens first. UE will send Tracking Area Update to the MME.
What I mean by this is. Either timer T3412 expires first or UE enters into a new Tracking Area which is not in the list. UE will send TAU right away whatever scenario comes first. When TAU is sent at the expiry of T3412. It is termed as Periodic TAU
The scenario is explained in the diagrams below.
Does T3412 Timer Keeps running all the time ?
Timer T3412 only starts running whenever a UE enters into Idle Mode. If UE goes back to connected mode from Idle Mode. T3412 stops running right away. It is depicted clearly in the picture below.
Planning/Dimensioning Tracking Areas
When planning tracking areas it mainly depends on the market consideration, traffic type, signalling load etc.
However, if your question is should you design a large tracking area or smaller tracking area ? Both approaches have their pros and cons.
Large Tracking Area (Increasing eNBs in TA list):
The main benefit of large TA is a reduced requirement for TA updates resulting from mobility. In other words less signalling.
The main drawback associated with large TA is an increased paging load.
Important Note 1: In general TA should be planned to be relatively large rather than relatively small. Their size can be subsequently reduced if the paging load becomes high.
Important Note 2: TA boundaries should not traverse dense population or subscriber areas. In addition TA boundaries should not run close and parallel to major roads or railways.
Small Tracking Area ( Decreasing eNBs in TA list)
Pro: The main benefit of small tracking areas is less signalling (theoretically)
TA Planning with 2G and 3G Networks: Operators with existing 2G or 3G networks can plan their TA boundaries to coincide with their routing area or location area boundaries.
How TAI is constructed ?
A: The TAI is constructed from a concatenation of the Mobile Country Code (MCC) , Mobile Network Code (MNC) and TAC.
Q: How many TAC can a cell broadcast ?
A: A cell can broadcast a maximum of 1 TAC but up to 6 PLMN identities (MNC + MCC) , so can have a maximum of 6 TAI.
Q: What is the range of TAC ?
A: The TAC has a range from 0 to 65535 (16 bits).
Q: Can a TA span multiple MME pools ?
A: TA cannot span multiple MME pools. An MME pool area includes one or more complete TA.
For better understanding take a look at the pictures below.
Another example is given below
Q: How many values are reserved to mark TAC as deleted ?
A: Two values are reserved to mark the TAC as deleted. These two TAC values are 0000 and FFFE.
After having said all that. Let’s come back to why you will have more battery drainage on the road. Even if you are not using your phone. It still is sending tracking area updates, whenever UE moves from one tracking area to another, in addition to timer T3412.
When you are at home all day, not using your phone. You are not changing your tracking area. So the only Tracking Area update UE will send is periodic TAU which is sent after expiry of T3412.
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