X2-based handovers

Posted: March 19, 2010 in technical
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As I was saying in a previous post, one of the handover types that can take place over LTE/SAE is the X2-based handover. This type of handover takes place when the UE moves from a cell controlled by one eNodeB (let’s call it source eNB) to a cell controlled by another eNB – target eNB. When this happens, the MME cannot be changed (basic condition of an X2-based handover). Still, the MME can decide whether or not to change the SGW. This means that there are 2 types of X2-based handover: with no SGW relocation and with SGW relocation. In both of them, the existence of the X2 interface between the source and target eNB is assumed.

As per TS 23.401, the behavior in case of X2 handover is the one below:[quoted from TS 23.401 – 5.5.1 Intra-E-UTRAN handover]

5.5.1.1 X2-based handover
5.5.1.1.1 General
These procedures are used to hand over a UE from a source eNodeB to a target eNodeB using the X2 reference point. In these procedures the MME is unchanged. Two procedures are defined depending on whether the Serving GW is unchanged or is relocated. In addition to the X2 reference point between the source and target eNodeB, the procedures rely on the presence of S1-MME reference point between the MME and the source eNodeB as well as between the MME and the target eNodeB.
The handover preparation and execution phases are performed as specified in TS 36.300 [5].
When the UE receives the handover command it will remove any EPS bearers for which it did not receive the corresponding EPS radio bearers in the target cell. As part of handover execution, downlink packets are forwarded from the source eNodeB to the target eNodeB. When the UE has arrived to the target eNodeB, downlink data forwarded from the source eNodeB can be sent to it. Uplink data from the UE can be delivered via the (source) Serving GW to the PDN GW. Only the handover completion phase is affected by a potential change of the Serving GW, the handover preparation and execution phases are identical.
If the MME receives a rejection to a NAS procedure (e.g. dedicated bearer establishment/modification/release; location reporting control; NAS message transfer; etc.) from the eNodeB with an indication that an X2 handover is in progress (see TS 36.300 [5]), the MME shall reattempt the same NAS procedure either when the handover is complete or the handover is deemed to have failed. The failure is known by expiry of the timer guarding the NAS procedure.
5.5.1.1.2 X2-based handover without Serving GW relocation
This procedure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and decides that the Serving GW is also unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as well as between the Serving GW and the target eNodeB is assumed.
1
NOTE 1: For a PMIP-based S5/S8, procedure steps (A) and (B) are defined in TS 23.402 [2].
1. The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the TAI+ECGI of the target cell and the list of rejected EPS bearers. The MME determines that the Serving GW can continue to serve the UE
2. The MME sends a Modify Bearer Request (eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers) message to the Serving GW. If the PDN GW requested UE’s location info, the MME also includes the User Location Information IE in this message.
The MME releases the non-accepted bearers by triggering the bearer release procedure as specified in clause 5.4.4.2. If the Serving GW receives a DL packet for a non-accepted bearer, the Serving GW drops the DL packet and does not send a Downlink Data Notification to the MME.
3. If the Serving GW has received the User Location Information IE from the MME in step 2 the Serving GW informs the PDN GW(s) about this information that e.g. can be used for charging, by sending the message Modify Bearer Request (Serving GW Address and TEID, User Location Information IE) to the PDN GW(s) concerned. A Modify Bearer Response message is sent back to the Serving GW.
4. The Serving GW starts sending downlink packets to the target eNodeB using the newly received address and TEIDs. A Modify Bearer Response message is sent back to the MME.
5. In order to assist the reordering function in the target eNB, the Serving GW shall send one or more “end marker” packets on the old path immediately after switching the path as defined in TS 36.300 [5], clause 10.1.2.2.
6. The MME confirms the Path Switch Request message with the Path Switch Request Ack message. If the UE AMBR is changed, e.g. all the EPS bearers which are associated to the same APN are rejected in the target eNodeB, the MME shall provide the updated value of UE AMBR to the target eNodeB in the Path Switch Request Ack message.
If some EPS bearers have not been switched successfully in the core network, the MME shall indicate in the Path Switch Request Ack message which bearers failed to be established (see more detail in TS 36.413 [36]) and inititate the bearer release procedure as specified in clause 5.4.4.2 to release the core network resources of the failed EPS bearers. The target eNodeB shall delete the corresponding bearer contexts when it is informed that bearers have not been established in the core network.
7. By sending Release Resource the target eNodeB informs success of the handover to source eNodeB and triggers the release of resources. This step is specified in TS 36.300 [5].
8. The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause “Triggers for tracking area update” applies.
NOTE 2: It is only a subset of the TA update procedure that is performed by the MME, since the UE is in ECM CONNECTED state and the MME is not changed.
5.5.1.1.3 X2-based handover with Serving GW relocation
This procedure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and the MME decides that the Serving GW is to be relocated. The presence of IP connectivity between the source Serving GW and the source eNodeB, between the source Serving GW and the target eNodeB, and between the target Serving GW and target eNodeB is assumed. (If there is no IP connectivity between target eNodeB and source Serving GW, it is assumed that the S1-based handover procedure in clause 5.5.1.2 shall be used instead.)
2
Figure 5.5.1.1.3-1: X2-based handover with Serving GW relocation
NOTE 1: For a PMIP-based S5/S8, procedure steps (A) and (B) are defined in TS 23.402 [2].
1. The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the ECGI of the target cell and the list of rejected EPS bearers. The MME determines that the Serving GW is relocated and selects a new Serving GW according to clause 4.3.8.2 on “Serving GW Selection Function”.
NOTE 2: The MME knows the S GW Service Area with a TA granularity.
2. The MME sends a Create Session Request (bearer context(s) with PDN GW addresses and TEIDs (for GTP-based S5/S8) or GRE keys (for PMIP-based S5/S8) at the PDN GW(s) for uplink traffic, eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers, the Protocol Type over S5/S8) message to the target Serving GW. The target Serving GW allocates the S GW addresses and TEIDs for the uplink traffic on S1_U reference point (one TEID per bearer). The Protocol Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface. If the PDN GW requested UE’s location info, the MME also includes the User Location Information IE in this message.
The MME releases the non-accepted bearers by triggering the bearer release procedure as specified in clause 5.4.4.2. If the Serving GW receives a DL packet for a non-accepted bearer, the Serving GW drops the DL packet and does not send a Downlink Data Notification to the MME.
3. The target Serving GW assigns addresses and TEIDs (one per bearer) for downlink traffic from the PDN GW. The Serving GW allocates DL TEIDs on S5/S8 even for non-accepted bearers. It sends a Modify Bearer Request (Serving GW addresses for user plane and TEID(s)) message to the PDN GW(s). The S GW also includes User Location Information IE if it is present in step 2. The PDN GW updates its context field and returns a Modify Bearer Response (PDN GW address and TEID, Charging Id, MSISDN, etc.) message to the Serving GW. The MSISDN is included if the PDN GW has it stored in its UE context. The PDN GW starts sending downlink packets to the target GW using the newly received address and TEIDs. These downlink packets will use the new downlink path via the target Serving GW to the target eNodeB.
4. The target Serving GW sends a Create Session Response (Serving GW addresses and uplink TEID(s) for user plane) message back to the target MME. The MME starts a timer, to be used in step 7.
5. The MME confirms the Path Switch Request message with the Path Switch Request Ack (Serving GW addresses and uplink TEID(s) for user plane) message. If the UE AMBR is changed, e.g. all the EPS bearers which are associated to the same APN are rejected in the target eNodeB, the MME shall provide the updated value of UE AMBR to the target eNodeB in the Path Switch Request Ack message. The target eNodeB starts using the new Serving GW address(es) and TEID(s) for forwarding subsequent uplink packets.
If some EPS bearers have not been switched successfully in the core network, the MME shall indicate in the Path Switch Request Ack message which bearers failed to be established (see more detail in TS 36.413 [36]) and inititate the bearer release procedure as specified in clause 5.4.4.2 to release the core network resources of the failed EPS bearers. The target eNodeB shall delete the corresponding bearer contexts when it is informed that bearers have not been established in the core network.
6. By sending Release Resource the target eNodeB informs success of the handover to source eNodeB and triggers the release of resources. This step is specified in TS 36.300 [5].
7. When the timer has expired after step 4, the source MME releases the bearer(s) in the source Serving GW by sending a Delete Session Request message, which is acknowledged by the Serving GW.
8. The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause “Triggers for tracking area update” applies.
NOTE 3: It is only a subset of the TA update procedure that is performed by the MME, since the UE is in ECM CONNECTED state. During the TA update procedure the MME deactivates ISR because of the S GW change.
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Comments
  1. Prathvi says:

    In x2-Handover, how a source eNodeB selects the target eNodeB?

    • hi, Prathvi
      the source eNB does not select the target eNB, the UE simply goes under the coverage of another eNB (like you walk towards an antenna or another antenna, it’s the user’s decision, where he/she goes)…

      maybe I don’t get your question right?

  2. Prathvi says:

    Thank you Cristina….

    In X2 based Handover (simple scenario-only eNB is changed) Source eNB sends Handover Request message to Target eNodeB. For instance i have three eNBs other than Source eNB. Say Node1, Node2 & Node3. Now to which Node Source eNB sends HO Request Message? How it selects that particular eNB as Target NodeB.?

    I hope now I am clear about my question. I have searched spec (36.423-X2AP) also. But I could not get this info..

  3. ah, Prathvi…I am not familiar with the X2 AP unfortunately 😦
    still, as far as I know is that the target eNB is chosen based on the measurements received by the source eNB from the UE (as the UE is the actual entity moving _towards_ that eNB)

  4. Prathvi says:

    Cristina you are correct.

    Interface between UE and eNB (i think Uu interface) is defined by RRC. Though UE is connected to the Source eNB, periodically it sends measurement report to the Source eNB regarding the signalling strength of the other eNBs(Spec 3GPP TS 36.331 – RRC). Using this information source eNB selects the target eNB.

    If someone is interested they can refer this pdf “http://go.radisys.com/rs/radisys/images/paper-lte-interoperable.pdf”

  5. waw! Very nice! thank you for the references! I am not familiar with the details of the access side; many thanks!

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