Handover is a process for a UE to transfer its sessions from the current network to another one while it is moving towards a neighbor cell. So, the handover procedure ends up with a new connection between the UE and the new eNB. The intra E-UTRAN handover indicates the case where the SGW and/or MME is not relocated whereas the inter E-UTRAN handover is the case where the SGW and/or MME shall be relocated. In this post, the intra E-UTRAN will be described.
It is a serving eNB that determines whether to initiate the handover procedure or not based on measurement reports received from the UE periodically. When handover is to happen, the serving eNB also chooses the target eNB from the list of neighbor eNBs and the type of handover, i.e., X2 handover or S1 handover. If there is an established X2 connection with the target eNB and it is available at the moment, the source eNB performs X2 handover. Otherwise the eNB will perform the S1 handover.
The X2 handover procedure involves signaling transactions among two eNBs and the MME. The following diagram shows the conceptual flow of X2 handover procedure.
(1) UE periodically sends measurement reports to the source eNB.
(2) The source eNB determines X2 handover and requests X2 handover to the target eNB. The target eNB establishes uplink S1 bearer with the same SGW with which the source eNB has been connected. The source eNB establishes a direct tunnel with the target eNB.
(3) UE handover is successfully performed. Hencefortjh, the buffered media is transferred to the UE from the target eNB.
(4) The target eNB informs the SGW of the fact that the handover has been completed successfully. The SGW establishes downlink S1 bearer with the target eNB.
(6) The SGW switches the media path from the source eNB to the target eNB and releases the old S1 bearer.
[1] The UE periodically sends a measurement report to the serving eNB. This reporting mechanism is intended for the UE to find out the best cell to communicate with the network. The measurement report may contain the list of neighbor cells, their signal strength and its current condition.
[2] Based on the received report, the serving eNB determines whether the handover is required and if it is required, the serving eNB selects a target eNB among the list of neighbor eNBs with which X2 connection is established. The source eNB requests the target eNB to prepare for handover by sending Handover Request. The message contains the target Cell ID and the UE Context. The following shows some of parameters included in the UE Context.
[3] The target eNB informs the source eNB about the prepared resources by sending Handover Request Acknowledge.
[4] The source eNB requests the UE to reconfigure the RRC connection by sending RRC Connection Reconfiguration, which also contains Handover Command that was received from the target eNB.
Upon receiving the Handover Command, the UE executes handover from the current eNB to the target eNB.
[5] The source eNB informs the target eNB of the current status of packet transmitter and receiver by sending SN Status Transfer. The message includes the uplink/downlink PDCP SN and HFN.
[6] After the UE has successfully synchronized to the target cell, it sends a target eNB a Handover Confirm informing that the handover has been completed. The buffered data at the target eNB is forwarded to the UE through the DRB. The uplink data from the UE can also be sent hereafter.
[7] The target eNB creates the S1 eNB GTP TEID and sends the MME the Path Switch Request to inform that the UE has changed the cell.
[8] Upon receiving the Path Switch Request, the MME requests the SGW to modify EPS bearers by sending Modify Bearer Request per PDN connection. The Modify Bearer Request contains the list of EPS bearers to be modified.
The PGW may need to inform the PCRF of the fact that the UE's location has been updated based on the request from the PCRF when the Gx session was established. Refer to "Bearer level event and VoLTE call setup failure" for basic understanding as to how the bearer level event reporting mechanism is realized within the PCC architecture.
[9] The SGW establishes the downlink S1 bearer with the target eNB and responds with the Modify Bearer Response, which includes the list of successfully modified EPS bearers.
[10] The SGW acknowledges the target eNB by sending Path Switch by sending Path Switch Acknowledge.
As a result of X2 handover, the UE context that was maintained by the source eNB is moved to the target eNB. The UE's location will be updated (e.g., ECGI, TAI) and the UE's C-RNTI will be re-assigned by the target eNB. The target eNB shall also assign a new eNB S1AP UE ID which will be updated at MME. The S1 bearer between the SGW and the source eNB will be replaced by another S1 bearer between the same SGW and the target eNB, which requires updates of eNB S1 GTP-U TEID. The PCRF may need to update UE's location.
Please note that all these changes does not affect the existing VoLTE session. All the EPS bearers being used to transfer VoLTE signaling and data moves to the target eNB. In case there is an existing voice media session, the voice data arriving at the source eNB while the UE handover is in progress will be transferred to the target eNB through the direct tunnel between two eNBs and buffered at the target eNB. The buffered data is eventually transferred to the UE when the handover is completed. Assuming that the handover takes less than a few milliseconds, the user won't notice a voice cracking.
REFERENCES
[1] GPP TS23.401, "General Packet Radio Service (GPRS) enhancement for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access", v12.4.0, Mar 2014
[2] 3GPP TS36.423, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP)", v13.1.0, Sep 2015
[3] 3GPP TS25.331, "Radio Resource Control (RRC) Protocol specification", v10.0.0, Jun 2010
[4] 3GPP TS36.331, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio Resource Control (RRC) Protocol specification", v12.7.0, Sep 2015
[5] 3GPP TS25.323, "Packet Data Convergence Protocol (PDCP) specification (release 9)", v9.0.0, Dec 2012
[6] 3GPP TS36.300, "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2", v10.0.0, Jun 2010
[7] EventHelix.com, "LTE X2 handover sequence diagram", 20th Apr 2013
[8] Netmannias, "EMM Procedure 6. Handover over without TAU - Part2. X2 handover", Mar 21th 2014
It is a serving eNB that determines whether to initiate the handover procedure or not based on measurement reports received from the UE periodically. When handover is to happen, the serving eNB also chooses the target eNB from the list of neighbor eNBs and the type of handover, i.e., X2 handover or S1 handover. If there is an established X2 connection with the target eNB and it is available at the moment, the source eNB performs X2 handover. Otherwise the eNB will perform the S1 handover.
I. Overall scenario
The X2 handover procedure involves signaling transactions among two eNBs and the MME. The following diagram shows the conceptual flow of X2 handover procedure.
 |
| Fig 1. overall scenario - X2 handover |
(1) UE periodically sends measurement reports to the source eNB.
(2) The source eNB determines X2 handover and requests X2 handover to the target eNB. The target eNB establishes uplink S1 bearer with the same SGW with which the source eNB has been connected. The source eNB establishes a direct tunnel with the target eNB.
(3) UE handover is successfully performed. Hencefortjh, the buffered media is transferred to the UE from the target eNB.
(4) The target eNB informs the SGW of the fact that the handover has been completed successfully. The SGW establishes downlink S1 bearer with the target eNB.
(6) The SGW switches the media path from the source eNB to the target eNB and releases the old S1 bearer.
II. X2 handover flow
 |
| Fig 2. X2 handover call flow |
[1] The UE periodically sends a measurement report to the serving eNB. This reporting mechanism is intended for the UE to find out the best cell to communicate with the network. The measurement report may contain the list of neighbor cells, their signal strength and its current condition.
[2] Based on the received report, the serving eNB determines whether the handover is required and if it is required, the serving eNB selects a target eNB among the list of neighbor eNBs with which X2 connection is established. The source eNB requests the target eNB to prepare for handover by sending Handover Request. The message contains the target Cell ID and the UE Context. The following shows some of parameters included in the UE Context.
- UE-AMBR indicates aggregated maximum bit rate for all the bearers of the UE.
- E-RABsToBeSetupList indicates a list of radio access bearer. Each E-RAB is defined by E-RAB ID and corresponding QoS parameters like ARP, QCI, GBR, etc.
- UL GTP TEID indicates the SGW endpoint of the S1 bearer for delivery of uplink packets. It is delivered to the target eNB so the target eNB can establish the UL S1 bearer with the same SGW as like the source eNB.
- E-RABs Admitted List contains the list of E-RABs for which the resources have been allocated. It also contains the DL GTP TEID that identifies the X2 transport bearer that shall be used by the source eNB to forward the downlink packets towards the target eNB.
- E-RABs Not Admitted List contains the list of E-RABs for which resources won't be allocated.
- Target eNB to source eNB transparent container is used by the target eNB to deliver the message to the UE through the source eNB transparently. In this case, it contains the Handover Command which is a command to the UE for handover execution.
[4] The source eNB requests the UE to reconfigure the RRC connection by sending RRC Connection Reconfiguration, which also contains Handover Command that was received from the target eNB.
- C-RNTI (Cell Radio Network Temporary Identifier) is a temporary UE identifier assigned by the serving eNB. It is persistent while the UE is connected to that eNB and re-assigned whenever the serving eNB changes.
- DRB-ID (Data Radio Bearer Identifier) is an identifier of the data bearer between UE and the eNB to be established with the target eNB.
[5] The source eNB informs the target eNB of the current status of packet transmitter and receiver by sending SN Status Transfer. The message includes the uplink/downlink PDCP SN and HFN.
- PDCP(Packet Data Convergence Protocol) SN indicates the sequence number assigned for each packet data unit.
- HFN (Hyper Frame Number) is used to limit the actual number of sequence number bits that's needed to be sent over the radio. When the PDCP SN reaches the maximum value, the PDCP SN is restarted from zero and HFN is incremented by one. This value shall be synchronized between the UE and the eNB.
[6] After the UE has successfully synchronized to the target cell, it sends a target eNB a Handover Confirm informing that the handover has been completed. The buffered data at the target eNB is forwarded to the UE through the DRB. The uplink data from the UE can also be sent hereafter.
[7] The target eNB creates the S1 eNB GTP TEID and sends the MME the Path Switch Request to inform that the UE has changed the cell.
- ECGI (E-UTRAN Cell Global Identifier) is a globally unique cell identifier to which the UE is camping on.
- TAI (Tracking Area Identity) is a globally unique tracking area identifier.
- E-RAB to be switched indicates the list of EPS bearers to be switched.
- S1 eNB GTP TEID indicates the end point of the GTP Tunnel that will be used by the SGW to identify the target eNB.
[8] Upon receiving the Path Switch Request, the MME requests the SGW to modify EPS bearers by sending Modify Bearer Request per PDN connection. The Modify Bearer Request contains the list of EPS bearers to be modified.
The PGW may need to inform the PCRF of the fact that the UE's location has been updated based on the request from the PCRF when the Gx session was established. Refer to "Bearer level event and VoLTE call setup failure" for basic understanding as to how the bearer level event reporting mechanism is realized within the PCC architecture.
[9] The SGW establishes the downlink S1 bearer with the target eNB and responds with the Modify Bearer Response, which includes the list of successfully modified EPS bearers.
[10] The SGW acknowledges the target eNB by sending Path Switch by sending Path Switch Acknowledge.
[11] The target eNB informs the source eNB that the handover has been completed successfully by sending UE Context Release. Upon receiving the UE Context Release, the source eNB releases all the resources associated with the received UE context.
***
As a result of X2 handover, the UE context that was maintained by the source eNB is moved to the target eNB. The UE's location will be updated (e.g., ECGI, TAI) and the UE's C-RNTI will be re-assigned by the target eNB. The target eNB shall also assign a new eNB S1AP UE ID which will be updated at MME. The S1 bearer between the SGW and the source eNB will be replaced by another S1 bearer between the same SGW and the target eNB, which requires updates of eNB S1 GTP-U TEID. The PCRF may need to update UE's location.
Please note that all these changes does not affect the existing VoLTE session. All the EPS bearers being used to transfer VoLTE signaling and data moves to the target eNB. In case there is an existing voice media session, the voice data arriving at the source eNB while the UE handover is in progress will be transferred to the target eNB through the direct tunnel between two eNBs and buffered at the target eNB. The buffered data is eventually transferred to the UE when the handover is completed. Assuming that the handover takes less than a few milliseconds, the user won't notice a voice cracking.
Red Mouse
REFERENCES
[1] GPP TS23.401, "General Packet Radio Service (GPRS) enhancement for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access", v12.4.0, Mar 2014
[2] 3GPP TS36.423, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 application protocol (X2AP)", v13.1.0, Sep 2015
[3] 3GPP TS25.331, "Radio Resource Control (RRC) Protocol specification", v10.0.0, Jun 2010
[4] 3GPP TS36.331, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Radio Resource Control (RRC) Protocol specification", v12.7.0, Sep 2015
[5] 3GPP TS25.323, "Packet Data Convergence Protocol (PDCP) specification (release 9)", v9.0.0, Dec 2012
[6] 3GPP TS36.300, "Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2", v10.0.0, Jun 2010
[7] EventHelix.com, "LTE X2 handover sequence diagram", 20th Apr 2013
[8] Netmannias, "EMM Procedure 6. Handover over without TAU - Part2. X2 handover", Mar 21th 2014
