Combined Attach in LTE/4G

Some time ago I'v covere the Attach Procedure, from then many times I saw questions about the Combined Attach.

So here it is, the Combined Attach himself.

First it has to be addressed what is the Combined Attach and why it's so different from Attach procedure already covered.

What Combined Attach is?

It's attach for both EPS and non-EPS services, or both EPS services and "SMS only".

The combined attach procedure is also used by a UE in CS/PS to attach for EPS services if it is already IMSI attached for non-EPS services.

How Combined Attach is triggered?

When the UE initiates a combined attach procedure, the UE shall indicate "combined EPS/IMSI attach" in the EPS attach type IE.

If the UE is in EMM state EMM-DEREGISTERED, the UE initiates the combined attach procedure by sending an ATTACH REQUEST message to the network, starting timer T3410 (More about EMM timers? Check the EMM timers or ESM timers article) and entering state EMM-REGISTERED-INITIATED.

If timer T3402 is currently running, the UE shall stop timer T3402. If timer T3411 is currently running, the UE shall stop timer T3411.

Extended Service Request procedure

The Extended Service Request and Service Request are same procedure just used for different purposes.
To make it simple, the below are color-coded. The cases when the Extended Service Request are triggered are marked light blue, the cases in black trigger Service Request procedure.

 Service Request triggers

a)    the UE in EMM-IDLE mode receives a paging request with CN domain indicator set to "PS" from the network;
b)    the UE, in EMM-IDLE mode, has pending user data to be sent;
c)    the UE, in EMM-IDLE mode, has uplink signalling pending;
d)    the UE in EMM-IDLE or EMM-CONNECTED mode is configured to use CS fallback and has a mobile originating CS fallback request from the upper layer;
e)    the UE in EMM-IDLE mode is configured to use CS fallback and receives a paging request with CN domain indicator set to "CS", or the UE in EMM-CONNECTED mode is configured to use CS fallback and receives a CS SERVICE NOTIFICATION message;
f)    the UE in EMM-IDLE or EMM-CONNECTED mode is configured to use 1xCS fallback and has a mobile originating 1xCS fallback request from the upper layer;
g)    the UE in EMM-CONNECTED mode is configured to use 1xCS fallback and accepts cdma2000® signalling messages containing a 1xCS paging request received over E-UTRAN;
h)    the UE, in EMM-IDLE mode, has uplink cdma2000® signalling pending to be transmitted over E-UTRAN;
i)    the UE, in EMM-IDLE or EMM-CONNECTED mode, is configured to use 1xCS fallback, accepts cdma2000® signalling messages containing a 1xCS paging request received over cdma2000®  1xRTT, and the network supports dual Rx CSFB or provide CS fallback registration parameters; or
j)    the UE, in EMM-IDLE or EMM-CONNECTED mode, has uplink cdma2000® signalling pending to be transmitted over cdma2000® 1xRTT, and the network supports dual Rx CSFB or provide CS fallback registration parameters.

Periodic Tracking Area Update (TAU) - T3412 timer

Today I was asked about the Periodic Tracking Area Update. Question was what is happening when the UE is in one place cell or TA in general.

So here is what the Spec Rel 9. is saying about it.

Periodic TAU

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.

No Periodic TAU case

If the timer T3412 received by the UE in an ATTACH ACCEPT or TRACKING AREA UPDATE ACCEPT contains an indication that the timer is deactivated or the timer value is zero, then the timer T3412 is deactivated and the UE shall not perform periodic tracking area updating procedure.

This part is a small change in Spec comparing to Rel 8. I feel this is because Machine to Machine (M2M) communication will take more part in overall. Because what else could be a reason to resign from (Periodic) TAUs?

Timer's behavior

The timer T3412 is reset and started with its initial value, when the UE goes from EMM-CONNECTED to EMM-IDLE mode. The timer T3412 is stopped when the UE enters EMM-CONNECTED mode or EMM-DEREGISTERED state.

S1 interface based handover

 Last two articles (you can find them here1 and here2) were about handover in scenario where there is direct connectivity between eNB. But what happen when there is no X2 connection old and new eNodeB?
Answer to that is S1 based handover procedure which you can find described below.

All of this information you can find by reading specific section of 3GPP TS 23.401 document.

As it is now like a little tradition, we will start with high level of abstract image.

Fig. 1. UE is moving from old to new RAN coverage provided by eNodeB.

This image should look familiar for those who was reading about X2-based handover. The thing what has change in this scenario is that there is lack of connectivity between two eNBs between which UE moves.
That's why, to do the handover the MMEl has to be involved directly. If you compare previous cases with this one, the first thing that each one of you should notice is that here eNodeB is contacting with MME, and the target eNodeB address is found because of SGW.
Before we will see detailed call flow please keep in mind few general S1 handover information.

Interfaces and their protocol stacks

After familiarising with main network elements, time has come to better know interfaces between those elements.
Interfaces are allowing MME, SGW and PGW cooperating with other network elements( e.g. HSS or PCRF).
Each one of them is built in standardised way described by 3GPP.org. Each interface described here is taken from 23.401 3GPP.org documentation.
Please keep in mind, that the documentation is (sometimes) bigger than we need it to be, so not every aspect of interfaces is described here.

Lets start with big picture of situation in which user is not roaming.

Fig.1. Non-roaming architecture by 3GPP