Handover procedure

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In Previous Article we learned about Handover Decision, To continue the Same today we will learn Ho Procedure in the Case of X2 and S1 Handover.

What is X2AP (X2 Application Protocol) and it’s functions?

The X2AP protocol (3GPP TS 36.423) is used to handle the UE mobility within E-UTRAN and provides the following functions:

Load Management ,Resetting the X2 Connection between two eNB ,Mobility Management, , Reporting of General Error Situations, , Setting up the X2 Connection, eNB Configuration Update.

What is S1AP (S1 Application Protocol) and it’s functions?

S1AP (3GPP TS 36.423) provides the signaling service between E-UTRAN and the EPC (evolved packet core) and has following functions:

Paging, Initial Context Transfer function, E-RAB management function, Mobility Functions for Active UE, UE Capability Info Indication function, NAS Signaling transport function, S1 interface management functions, UE Context Modification function, S1 UE context Release function, Location Reporting, Status Transfer, Configuration Transfer function.

X2 and S1 Handover Decision Diagram:

Handover procedure:

The UE reports the measurement results to eNB based on the measurement configuration received. Once the eNB decides a handover is initiated. The handover procedure consists of the following three phases:

1] Preparation phase:

During the preparation phase source eNB and target eNB prepare for handover. As discussed earlier in case of X2 handover the two communicate directly with each other through X2 signaling and carry out the handover accordingly without the intervention of MME. Whereas MME gets involved in case of S1 handover through S1 signaling. 

The source eNB sends the users UE context that is security context, QoS context etc. to the target eNB to check if the target eNB is capable of providing satisfying service quality. 

If it is providing the satisfying service quality the target eNB establishes DL packet forwarding bearer i.e. a transport bearer for packet forwarding. After this it allocated CRNTI value that the UE needs to use when  accessing the eNB also forwards the same to the source eNB, thus completing the preparation phase. 

The Downlink packet forwarding bearer is either a direct tunnel connecting the two eNBs(Serving eNB and Target eNB) in case of X2 HO or an indirect tunnel connecting all the three entities i.e. source eNB, SGW and the target eNB in case of S1 HO. 

Figure above shows packet forwarding path : UL/DL bearer traffic delivery path is shown by two way solid line, control message delivery path is shown by dotted line and DL packet forwarding path is shown by one way solid line.

2] Execution phase:

During this phase the UE disconnects the radio link from source eNB and connects to the target eNB thus accessing a new cell. As soon as the resources needed for packet forwarding between two eNBs are allocated and the new resources for the UE are allocated at the target eNB during preparation phase we can say that the two eNBs are ready for handover. 

After this the source eNB orders the UE to perform a handover by sending a handover command message. During execution phase the UE uses the CRNTI that was allocated by target eNB during the preparation phase. Due to this the UE can access the target eNB faster. 

Once the DL packets are arrived at the source eNB they are forwarded to the target eNB through forward bearer and are stored there until the UE is completely accessed to the target eNB. This makes sure that no packet is lost on the way. UL packets that are coming from the UE are not forwarded until the UE is accessed to the target eNB successfully. 

Once the UE completes its radio access to the target eNB the UL packets are then immediately forwarded to the SGW via the target eNB. Following figure shows the DL traffic delivery path during the execution phase and the UL traffic delivery path through the target eNB after the execution phase.

3] Completion phase:

As soon as the UE completes the radio access to the target eNB successfully, the UEs bearer path gets connected to the target eNB instead of source eNB. Once the path is switched the forwarding bearer used in forwarding DL packets during the execution phase is released. In figure below both UL and DL traffic is delivered via new bearer path once the completion phase is ended.

In Upcoming Part we will learn more about X2 Handover and S1 Handover.

Stay tune for more Update…………………

Pinal Dobariya…

1)3GPP TS 36.300
2)3GPP TS 36.331
3)Netmanias Technical Document, “Eleven EMM Cases in an EMM Scenario


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