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
