VoLTE S1 Handover preparation (1/3)

While UE crosses the border towards the neighborhood cell, the source eNB determines whether there needs a handover based on the received measurement report from the UE. When there is no established X2 connection with the target eNB, the source eNB determines to perform S1 handover. In this post, the handover preparation phase will be addressed. The handover preparation procedure includes the determination of handover by the eNB, handover request by the MME, the allocation of required resources and establishing the indirect data forwarding tunnels among NEs.

I. Indirect Data Forwarding Tunnel

S1 handover is different from the X2 handover in that the media is anchored by the SGW under the control of MME rather than by eNB. During the S1 handover, a temporary indirect data forwarding tunnel is established between two eNBs via the common SGW. All the media arriving at the source eNB while S1 handover is in progress, which can be uplink data from UE side or the downlink data from the network side, will be re-directed to the target eNB through this indirect data forwarding tunnel and buffered at the target eNB. When the S1 handover is completed, the buffered media at the target eNB is transferred to the UE. The following diagram shows Indirect Data Forwarding Tunnel established between the source eNB and target eNB anchored by the common SGW during S1 handover procedure.

Figure 1. Indirect Data Forwarding Tunnel in S1 handover

II. S1 handover preparation

As a precondition of this practice, the UE has PDN connections with the internet APN and IMS APN and there are three EPS bearers established in total with EPS bearer ID of ‘5’, ‘6’ and ‘7’ and the relocation of SGW/PGW does not occur. The S1 handover procedure is triggered by the source eNB. The MME controls establishing the Indirect Data Forwarding Tunnel by intermediating required data between two eNBs like Bearer Context. The Bearer Context contains EPS bearer ID and their GTP-U TEIDs corresponding to each EPS bearers. The SGW anchors the media transfer between source eNB and the target eNB. As the SGW and two eNBs are involved in the media control, they allocate the required media resources, generate their own GTP-U TEIDs and establishes S1 bearer and indirect data forwarding tunnels based on the GTP-U TEID of the peer node delivered via MME.

Figure 2. S1 handover procedure - preparation

[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, signal strength, current condition, etc.

[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. If there is no X2 connection with the target eNB, the source eNB performs S1 handover and sends Handover Required to the serving MME requesting to prepare for resources at the target. 

•  MME UE S1AP ID: Unique identifier of the UE association over the S1 within the MME
• eNB UE S1AP ID: Unique identifier of the UE association over the S1 within the eNB
• Handover Type: The type of triggered handover in the source side (i.e., intraLTE, LTEtoUTRAN, LTEtoGERAN, UTRANtoLTE, GERANtoLTE)
• Cause: A reason for a particular event for the S1AP. In this practice, the value “Handover Desirable for Radio Reasons” indicates that the cause is related with radio. Refer to section 9.2.1.3 Cause of [TS36.413] for the detail.
• Target ID: The target eNB for the handover. It is composed of “Global eNB ID” and “selected TAI”. The Global eNB ID is composed of PLMN ID (MCC+MNC) and macro eNB-ID identifying the specific eNB of a certain operator in a certain country. The selected TAI uniquely identifies the target Tracking Area i.e., PLMN ID + TAC, identifying a specific tracking area that is served by that eNB.
• Source to Target Transparent Container: Information elements created by the source eNB and transmitted to the target eNB, which contains the RRC related information, E-RAB information, Target Cell-ID and UE History Information as shown in the following bullets.
• RRC Container: RRC Information used by the target eNB during handover preparation including UE capability information. Refer to “HandoverPreparationInformation” in section 10.2.2 “Message definitions” of TS36.331 for the detail.
• e-RABInformationList: The list of eRAB to be handed over. The E-RAB ID identifies a radio access bearer for a particular UE, which makes the E-RAB ID unique over one S1 connection. The “dL-Forwarding-proposed” indicates that the source eNB proposes the list of E-RABs for forwarding of data. In this practice, there are three E-RABs (i.e., E-RAB ID=’5’,’6’ and ‘7’) proposed and they actually represent existing E-RABs of the source eNB. If the target eNB accepts it in the Handover Request Acknowledge, the downlink data received by the source eNB during the handover will be forwarded to the target eNB through the indirect data forwarding tunnel.
• Target Cell ID: Globally unique cell identifier. It is composed of PLMN ID and Cell ID.
• UE-HistoryInformation: The list of cells that a UE has been served by in the active state prior to the target cell.

Figure 3. Handover Required

[3] the MME initiates the procedure by sending the Handover Request to the target eNB requesting to prepare for resources for handover.

•  MME UE S1AP ID: refer to step#2.
• Handover Type: refer to step#2.
• Cause: refer to step#2.
• UE-AMBR: Aggregated maximum bit rates per UE being applicable for Non-GBR bearers.
• E-RAB to be SetupList: A list of E-RAB to be setup where each item contains E-RAB ID, Transport Layer Address, GTP-TEID and E-RAB level QoS parameters as addressed in the following bullets.
• E-RAB ID: Refer to step#2
• The Transport Layer Address : IP address of the source eNB.
• The GTP-TEID : The GTP Tunnel Endpoint Identifier of the SGW towards the source eNB. This has been known to the MME during the initial attach of the UE and used by the source eNB to maintain the S1 bearer with the SGW. Now, the MME intends to take this to the target eNB to replace the existing S1 bearer in the end.
• E-RAB Level QoS Parameters indicates the QoS to be applied to the corresponding E-RAB. It is composed of QCI and ARP. Refer to the section 9.2.1.60 “Allocation of Retention Priority” in TS36.413 for the detail.
• Source to Target Transparent Container: Information elements created by the source eNB and transmitted to the target eNB. Refer to step#2.
• UESecurityCapabilities: Supported algorithms for encryption and integrity protection in the UE.
• HandoverRestrictionList: Roaming or access restrictions for subsequent mobility action for which the eNB provides information about the target of the mobility action towards the UE.
• SecurityContext: Security related parameters to the eNB which are used to derive security keys for user plane traffic and RRC signaling messages and to generate security parameters for the current S1 handover. 

Figure 4. Handover Request

[4] The target eNB allocates all the necessary resources for the admitted E-RABs and establishes uplink S1 bearer with the SGW. The target eNB creates its own GTP TEID for S1 bearers which will be delivered to the SGW later and used by the SGW to establish the downlink S1 bearer (SGWàtarget eNB), thereby replaces that the source eNB. The uplink and downlink GTP-TEID for each admitted E-RABs are also included. These DL/UL GTP-TEIDs are used for indirect media transfer. The target eNB responds with the Handover Request Acknowledge to the MME.

• MME UE S1AP ID: refer to step#2.
• eNB UE S1AP ID: refer to step#2.
• E-RAB Admitted List: The list of E-RAB for which the target eNB admitted to establish. This is composed of E-RAB ID, GTP-TEID, UL/DL GTP-TEIDs and the Transport Layer for each Admitted Item as addressed in the following bullets.
• E-RAB ID : Refer to step#2.
• GTP-TEID indicates GTP Tunneling Endpoint Identifier of the target eNB which will replace the corresponding GTP-TEID of the source eNB in the end (i.e., downlink S1 bearer).
• The DL/UL GTP-TEIDs indicate the Tunneling Endpoint Identifiers of the target eNB for Indirect Data Forwarding Tunnels. The following diagram shows the GTP TEID for indirect data forwarding dedicated for one E-RAB.
•  Target to Source Transparent Container: Information element that is used to transparently pass radio related information from the handover target to the handover source through the MME. The Handover Command is transparently delivered to the source eNB.

  

Figure 5. Handover Request Acknowledge

[5] Upon receiving the Handover Request Acknowledge, the MME sends the Create Indirect Data Forwarding Tunnel Request to the SGW, which contains the Bearer Context for each bearer which was received at step #4 from the target eNB. The Bearer Context represents the attributes of each bearer including bearer identifier and TEIDs as below:

• EPS Bearer ID : Identifier of EPS bearer to be established.
• eNB F-TEID for DL data forwarding: GTP Tunneling Endpoint Identifier created by the target eNB for downlink data forwarding. Refer to step#4 
• eNB F-TEID for UL data forwarding: GTP Tunneling Endpoint Identifier created by the target eNB for uplink data forwarding. Refer to step#4 

  

Figure 6. Create Indirect Data Forwarding Tunnel Request

[6] Upon receiving the Create Indirect Data Forwarding Tunnel Request, the SGW establishes the Indirect Data Forwarding Tunnel towards the target eNB using the received DL/UL eNB F-TEIDs. The SGW allocates its own UL/DL F-TEID for indirect data forwarding tunnel, which is delivered to the source eNB via MME at step #7. The SGW responds to the MME with the Create Indirect Data Forwarding Tunnel Response which contains the Bearer Context of the SGW.

• Cause : Indicates if the Indirect Data Forwarding Tunnel has been created in the SGW. Refer to section 7.2.19 “Create Indirect Data Forwarding Tunnel Response” of TS29.274 for the detail.
• EPS Bearer ID : refer to step #5.
• SGW F-TEID for DL data forwarding: GTP Tunneling Endpoint Identifier created by the SGW for downlink data forwarding.
• SGW F-TEID for UL data forwarding: GTP Tunneling Endpoint Identifier created by the SGW for uplink data forwarding.

  

Figure 7. Create Indirect Data Forwarding Tunnel Response

[7] The MME sends the Handover Command to the source eNB informing that resources for handover has been prepared at the target eNB. Upon receiving the Handover Command, the source eNB establishes Indirect Data Forwarding Tunnel towards the SGW using the received E-RAB Subject to Data Forwarding List, i.e., DL/UL GTP-TEIDs for each E-RAB.

• MME UE S1AP ID: refer to step#2.
• eNB UE S1AP ID: refer to step#2.
• Handover Type: refer to step#2.
• E-RAB Subject to Data Forwarding List : The list of E-RAB items to be used for indirect data forwarding. Each E-RAB Data Forwarding Item identifies the E-RAB context of each E-RAB and it consists of E-RAB ID, DL/UL GTP-TEIDs as addressed below.
• E-RAB ID: Refer to step #2.
• DL/UL GTP-TEIDs : Tunneling End Point Identifier of the SGW created and delivered at step #6 for the Indirect Data Forwarding Tunnel.
• Target To Source Transparent Container: Radio related information transparently delivered to the source eNB through the MME. It was created by the target eNB and sent to the MME at step #4.

Figure 8. Handover Command

Red Mouse

REFERENCES

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

[2] 3GPP TS24.301, "Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage3", v12.4.0, Mar 2014

[3] 3GPP TS24.008, "Mobile radio interface Layer 3 specification; Core network protocols; Stage3", v13.4.0, Dec 2015

[4] 3GPP TS29.274, "3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS); Tunneling Protocol for Control Plane (GTPv2-C); Stage3", v13.0.0, Dec 2014

[5] 3GPP TS36.331, "Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification", v12.3.0, Sep 2009

[6] 3GPP TS36.413, "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP)", v12.3.0, Sep 2014