After the UE confirms the completion of the S1 handover in a new
cell, the target eNB initiates the completion procedure by sending towards the
network. The completion procedure involves the SGW and PGW replacing the
downlink S1 bearer from the source eNB to the source eNB releasing the UE
associated resources and lastly, the release of Indirect Forwarding Tunnels by
the SGW.
Figure
1. S1 handover procedure - complete
[12] The target eNB sends the Handover
Notify to the MME notifying that the UE has been identified in the target
cell and S1 handover has been completed.
- E-UTRAN CGI: globally identifies a cell and composed of PLMN identity and Cell identity.
- TAI: uniquely identify a Tracking Area and composed of PLMN identity and TAC (Tracking Area Code).
Figure
2. Handover Notify
[13] The MME sends the Modify
Bearer Request to the SGW requesting to switch the media path from the source
eNB to the target eNB. The switch of media path from the SGW is done by
updating the S1 bearer tunneling end point.
- Indication: includes various application flags. Refer to section 7.2.7 “Modify Bearer Request” in [TS 29.274].
- Bearer Context: identifies the bearer to be modified. It is composed of EPS Bearer ID and S1 eNB F-TEID sub elements.
In this practice, there are three bearers, i.e., bearer id =
5,6,7. The bearer 5 and 6 belongs to the internet APN, meanwhile the bearer 7
belongs to the IMS APN. The MME sends multiple Modify Bearer Request for each
APN.
Note that in the following message, the value of F-TEID is set to
0x018088ed. This value is the one that was sent by the target eNB in step [4] Handover Request Acknowledge where it
represents the tunneling end point of the target eNB for the downlink S1 bearer.
Now, the MME deliver this value to the S-GW so the S-GW can use this value as a
new target end point for the media path. As such, the downlink media path on S1
bearer is switched from the source eNB to the target eNB.
Figure
3. Modify Bearer Request for bearer 7
The following message shows the Modify Bearer Request for the bearer 5 and 6. In the same as described
in the above, the value of F-TEID is set to 0x018008ed and 0x018048ed for
bearer 5 and 6, respective. They are the same values as were sent by the target
eNB in step [4] Handover Request Acknowledge
where they represent tunneling end points of the target eNB for the downlink S1
bearer.
Figure
4. Modify Bearer Request for bearer 5 and 6
Note that the Modify Bearer
Request is also sent by the SGW to the PGW which is not depicted here. The
SGW and PGW are logically separated but in many cases, they are deployed on the
same platform.
[14] The Modify Bearer
Response sent from the PGW to the SGW and from the SGW to the MME.
- Cause: indicates the result of modifying the requested bearer.
- Bearer Context: identifies the bearer that has been modified. It is composed of EPS Bearer ID, Cause and S1 eNB F-TEID sub elements.
Note that in the following message, the value of F-TEID is set to
0x005b8422. This value is the one that was sent by the MME to the target eNB in
step [3] Handover Request where it
represents the tunneling end point of the SGW for the uplink S1 bearer. The
tunneling end point values of SGW is created when the UE attaches the network
at the very beginning. Upon S1 handover, the MME sends this value to the target
eNB and the target eNB establishes uplink S1 bearer to the SGW using this
value. At this moment, the source eNB also has uplink S1 bearer connection with
the SGW with the same tunneling end points. As both source eNB and target eNB
can send traffic to the SGW, there is no change that the data sent by the UE
gets lost during handover procedure.
Now, the SGW sends this TEID to the MME for the downlink S1 bearer
informing the tunneling end point for the downlink S1 bearer.
Figure
5. Modify Bearer Response for bearer 7
The same principal is applied to the following message which is
for bearer 5 and 6.
Figure
6. Modify Bearer Response for bearer 5 and 6
[15] The MME requests the release of UE-associated S1 logical
connections over S1 interface by sending UE
Context Release Command. The source eNB releases its resources related to
the given UE.
- UE S1AP ID Pair: contains the UE S1AP identifiers. It is composed of MME UE S1AP ID and eNB UE S1AP ID
- Cause: indicates the reason for this command. Refer to section 9.2.1.3 “Cause” in TS36.413 for the detail.
Figure 7. UE Context Release Command
[16] The source eNB confirms the release of UE-associated S1
logical connections over S1 interface by sending UE Context Release Complete.
- MME UE S1AP ID: uniquely identifies the UE association with the S1 interface within the MME.
- eNB UE S1AP ID: uniquely identifies the UE association with the S1 interface within the eNB.
Figure
8. UE Context Release Complete
[17] The MME requests the SGW to delete the Indirect Forwarding
Tunnels by sending Delete Indirect Data
Forwarding Tunnel Request.
Figure
9. Delete Indirect Data Forwarding Tunnel Request
[18] The SGW responds with Delete
Indirect Data Forwarding Tunnel Response to the MME.
Figure 10. Delete Indirect Data Forwarding Tunnel Response
Red Mouse
REFERENCES
[1] 3GPP TS23.401, “General Packet Radio Service (GPRS)
enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN)
access”, v12.4.0, Mar 2014
[2] 3GPP TS29.274, “Evolved General Packet Radion Service (GPRS)
Tunneling Protocol for Control Plane (GTPv2-C); Stage3”, v13.2.0, Jun 2015
[3] 3GPP TS36.411, "Evolved Universal Terrestrial Radio
Access (E-UTRA); S1 Application Protocol (S1AP)", v13.0.0, Jun 2015
