VoLTE S1 handover execution (2/3)

S1 handover preparation procedure includes the decision of S1 handover by the source eNB, allocation of network resources to establish Indirect Data Forwarding Tunnel among two eNBs and common S-GW and establishment of uplink S1 bearer from the target eNB to the S-GW. Once the S1 handover preparation procedure is completed, the MME initiates the S1 handover by sending Handover Command to the UE via the source eNB. The UE executes the handover by detaching from the source eNB and attaching to the target eNB. Meanwhile, the downlink data is forwarded to the target eNB and buffered while the UE handover is in progress. Lastly, the UE informs the target eNB of the fact that the handover has been successfully completed and thereafter, the buffered data and downlink data is forwarded the UE through the target eNB. 

Figure 1. S1 handover procedure - execution

[8] The Handover Command received from the MME is wrapped by the source eNB within the RRC Connection Reconfiguration and sent to the UE. The RRC Connection Reconfiguration is the message to perform logical, transport and physical channel configurations. In this case, it is used to send NAS signaling to the UE to reduce the latency. Upon receiving the Handover Command, the UE detaches from the source eNB and performs handover to the target eNB.

[9] The source eNB stops assigning PDCP-SNs to downlink packets and sends the eNB Status Transfer to the target eNB via MME that contains uplink and downlink PDCP-SN and HFN (Hyper Frame Number) for each respective E-RAB. This procedure is initiated by the source eNB at the moment when it considers the transmitter/receiver status to be frozen. The use of PDCP-SN and HFN is part of overflow control mechanism for radio. The PDCP-SN is the serial number of PDCP packets increasing up to MAX-PDCN-SN. If the number reaches the MAX-PDCP-SN, the HFN is incremented by one. 

• Subject to Transfer Items: contains uplink/downlink PDCP-SN and HFN for each respective E-RAB.
• E-RAB ID: Identifies a radio access bearer for a particular UE. This value remains the same after S1-handover.
• uL-/dL-Count value: contains the PDCP-SN and HFN values
• Received Status of UL PDCP SDUs: indicates the missing and the received uplink SDUs (Service Data Units) for each bearer for which the source eNB has accepted the request from the target eNB for uplink forwarding.

  

Figure 2. eNB Status Transfer

[10] The MME forwards the received PDCP-SN and HFN information to the target eNB by sending MME Status Transfer. Upon receiving the MME Status Transfer, the target eNB does not deliver any uplink packet whose PDCP-SN is lower than the value received in the PDCP-SN in the uL-COUNT value. The target eNB uses the received PDCP-SN in the dL-COUNT value for the first downlink packet for which no PDCP-SN is assigned yet. The downlink traffic received by the source eNB is routed to the target eNB following the Indirect Data Forwarding Tunnel.

[11] After the UE successfully synchronized with the target cell, it sends a Handover Confirm to the target eNB. The Handover Confirm is contained in the RRC Connection Reconfiguration Complete message on RRC. Please note that, at this moment, there is no direct S1 bearer established yet between S-GW and the target eNB. Therefore, the downlink data is sent to the source eNB and forwarded to the target eNB following the Indirect Data Forwarding Tunnel. The uplink data from the UE will be forwarded to the S-GW following the direct S1 interface.

Red Mouse

REFERENCES

[1] 3GPP TS25.331, "Radio Resource Network (RRC); Protocol specification", v12.3.0, Sep 2014

[2] 3GPP TS23.401, “General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access”, v12.4.0, Mar 2014

[3] 3GPP TS36.331, "Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification", v12.3.0, Sep 2009

[4] Blog, “How LTE Stuff Works?: RRC Connection Reconfiguration”, Oct 2011