eUTRAN to UTRAN (4G to 3G) Iu mode handover

Few last articles were about Handover in LTE network. From old eNB to new eNB, with or without using new MME/SGW, with or without using X2 interface, and so on.
Today we will be talking about handovers from eUTRAN to UTRAN, from 4G to 3G.

Sorry guys, today I don't have time to create abstract picture which we could use to start from.
But it should be easy for you to imagine.
UE is moving from area where is 4g coverage to area where is only 3g coverage.
eNBs change to RNCs, MME to SGSN, only the mobility anchor (PGW) stays the same.

More about this you can find in 3GPP TS 23.401.

Inter RAT handover - general information

During Inter RAT handover indirect forwarding may apply for the dowlink data forwarding performed as part of the handover. From its configuration data the MME knows whether indirect forwarding applies and it allocates a dowlink data forwarding path on a Serving GWs for indirect forwarding. From its configuration data the S4 SGSN knows whether indirect forwarding applies and it allocates dowlink data forwarding paths on Serving GWs for indirect forwarding. It is configured on MME and S4 SGSN whether indirect dowlink data forwarding does not apply, applies always or applies only for inter PLMN inter RAT handovers.

eUTRAN to UTRAN Iu mode Inter RAT handover

General information

Pre-conditions:

• The UE is in ECM-CONNECTED state (E-UTRAN mode).

If emergency bearer services are ongoing for an UE, handover to the target RNC is performed independent of the Handover Restriction List. The SGSN checks, as part of the Routing Area Update in the execution phase, if the handover is to a restricted area and if so SGSN deactivate the non-emergency PDP context.

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.

X2-based handover with SGW relocation

Last time article was about handover based on X2 interface but without SGW change, that is why it's time to deal with X2 based handover with SGW relocation.

All of this information you can find by reading 3GPP TS 23.401.
Today as usually we will start with a high level of abstract picture. Please see below.

As you can see Fig. 1. is almost the same as it was in X2 based handover, where the green arrow was demonstrating the case we talked about last time (if interested you can read about it here) where was no need to change SGW. Today we are interested in case illustrated by blue (azure? blueish?) arrow. Where handover is done bye X2 interface with SGW relocation.
Lines that are green, between UE and eNB, showing changed path on the end of handover operation without SGW change. Blue lines between UE, eNB, MME and new SGW are showing new path after changing old SGW to new.

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

Before we will jump into detailed description of handover few general information about X2 interface based handover.

X2-based handover without SGW relocation

Last article was about IMSI, TMSI and GUTI, and how they are created, it was also mentioned there that GUTI is used in handover (HO) procedure. I thought that today is a good time to say few words about handover.
There are two general types of handovers in LTE.

• X2 based, and
• S1 based

In this article I'm going to describe only X2 based HO and to be perfectly honest with you I have to admit, that I will publish information about X2 based handover with case where no SGW is changed.

For others types please look for different article (After publishing articles describing other HOs I will update this line with direct links. [Update-16-03-2012] First one is X2-based handover with SGW relocation).

All of this information you can find by yourself just by reading 3GPP TS 23.401.

As always it's good to start with general picture, so here it is.

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

As you can see, green arrow is demonstrating UE movement between two eNodeBs controlled by the same MME. Such procedure as you should know is called a handover. In this procedure every bearer set between UE and PGW will be moved to new eNodeB(if target eNodeB is able to handle them).
Please see below general description of X2-based handover according to specyfication.