X2 HO occurs when source and target eNBs are served within the same MME pool.The procedures relies on the presence of X2 interface between S and T eNB AND S1-MME interface between the MME and the source eNodeB as well as between the MME and the target eNodeB. Also as part of handover execution, downlink and optionally also uplink packets are forwarded from the source eNodeB to the target eNodeB
Whereas, S1 handover is when the X2 procedure fails(due to unreachability/Error response etc).
X2 procedure
TeNB ->MME : Path Switch Req (GTPu IP of TeNB and FTEID)
Informs MME that the UE has changed cell, including the TAI+ECGI of the target cell and the list of EPS bearers to be switched, MME may determine SGW change is required or not
MME->SGW: Modify Bearer Req (GTPu IP of TeNB and FTEID)
Message is sent per PDN connection. Includes eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers, ISR Activated. MME determines whether any dedicated bearers havent been accepted by the TeNB and releases those dedicated bearers using bearer release proc. If the default bearer of a PDN connection has not been accepted by the target eNodeB and there are multiple PDN connections active, the MME shall consider all bearers of that PDN connection as failed and release that PDN connection by triggering the MME requested PDN disconnection procedure
SGW->PGW: Modify Bearer Req
PGW->SGW: Modify Bearer Resp
Optional, only if SGW has received User Location Info(ULI) and/or Timezone IE from MME
SGW->MME: Modify Bearer Resp (GTPu IP of SGW and SGW-FTEID)
SGW starts sending DL packets to TeNB
SGW->SeNB: Endmarker req
SeNB->TeNB: Endmarker req
In order to assist the reordering function in the target eNodeB, the Serving GW shall send one or more "end marker" packets on the old path immediately after switching the path
MME->TeNB: Path switch Ack
Confirms path switch. If UE-AMBR is changed,it is indicated to the TeNB
MME->SeNB: Resource release
Informs handover success
TAU procedure follows depending on the triggers
S1 procedure
SeNB->MME: Handover Required
When SeNB decides that UE needs to handover to TeNB(and when X2 is not possible). Msg contains Direct Forwarding Path Availability, Source to Target transparent container, target eNodeB Identity, CSG ID, CSG access mode, target TAI, S1AP Cause, Bearers subject to data fwding. The target TAI is sent to MME to facilitate the selection of a suitable target MME.
S-MME->T-MME: Forward Relocation Request
Message contains MME UE context, Source to Target transparent container, RAN Cause, target eNodeB Identity, CSG ID, CSG Membership Indication, target TAI, MS Info Change Reporting Action, CSG Information Reporting Action,UE Time Zone, Direct Forwarding Flag. The target TAI helps T-MME to determine if SGW change is reqd.The MME UE context includes IMSI, ME Identity, UE security context, UE Network Capability, AMBR, Selected CN operator ID, APN restriction, Serving GW address and TEID for control signalling, and EPS Bearer context(s).
An EPS Bearer context includes the 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, APN, Serving GW addresses and TEIDs for uplink traffic, TI,RAN Cause indicates the S1AP Cause as received from source eNodeB.Direct Forwarding Flag indicates if direct forwarding is applied, or if indirect forwarding is going to be set up by the source side.
T-MME->T-SGW: Create Session Req (Only,if SGW relocation is reqd)
T-SGW->T-MME: Create Session Resp
*Msg consists 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, Serving Network, UE Time Zone) message per PDN connection to the target Serving GW.
T-SGW allocates the SGW addresses and TEIDs for the uplink traffic on S1_U reference point*
T-MME->TeNB: Handover Request
Consists of EPS Bearers to Setup, AMBR, S1AP Cause, Source to Target transparent container, CSG ID, CSG Membership Indication, Handover Restriction List.
For each EPS Bearer, the Bearers to Setup includes Serving GW address and uplink TEID for user plane, and EPS Bearer QoS.
TeNB->T-MME: Handover Request Ack
Msg consists of EPS Bearer Setup list, EPS Bearers failed to setup list Target to Source transparent container. The EPS Bearer Setup list includes a list of addresses and TEIDs allocated at the TeNB for DL traffic on S1 U (one TEID per bearer) and addresses and TEIDs for receiving forwarded data if necessary,UE AMBR (if changed)
T-MME->T-SGW: Create Indirect Data Forwarding Tunnel Request
Consists T-eNB addr and TEIDs for forwarding
T-SGW->T-MME:Create Indirect Data Forwarding Tunnel Resp
Consists T-SGW addresses and TEIDs for forwarding
T-MME->S-MME:Fwd Relocation response
Contains Cause, Target to Source transparent container, Serving GW change indication, EPS Bearer Setup List, Addresses and TEIDs.For indirect forwarding, this msg includes SGW Address and TEIDs for indirect forwarding (source or target)
SMME->S-SGW: Create Indirect Data Forwarding Tunnel Request
addresses and TEIDs for forwarding
S-SGW->S-MME:Create Indirect Data Forwarding Tunnel Resp
SGW addresses and TEIDs for forwarding
SMME->S-eNB: Handover Command
Consists Target to Source transparent container, Bearers subject to forwarding, Bearers to Release.The Bearers subject to forwarding includes list of addresses and TEIDs allocated for forwarding. The Bearers to Release includes the list of bearers to be released.
SeNB->T-eNB:eNB Status Transfer
To convey the PDCP and HFN status of the E-RABs for which PDCP status preservation applies
UE->T-eNB: Handover Confirm
Now, Direct(S-eNB to T-eNB) or Indirect(S-eNB to T-SGW to T-eNB) data for DL data starts flowing, until UE completely detaches from the old-cell and resynchs with the new cell. It then sends Handover confirm
T-eNB->T-MME:Handover Notify
Consists of TAI+ECGI.
T-MME->S-MME:Forward relocation complete notification
S-MME->T-MME:Forward relocation complete notification Ack
A timer in SMME is started to supervise when resources in S-eNB and if the SGW is relocated, also resources in S-SGW shall be released.
Upon receipt of the Forward Relocation Complete Acknowledge message the target MME starts a timer if the target MME allocated S GW resources for indirect forwarding.
T-MME->T-SGW->PGW: Mod Bearer Req
PGW->T-SGW->T-MME: Mod Bearer Resp
Now DL data plane is completely switched from PGW->TSGW->TeNB->UE
***Tracking Area Update procedure ensues, followed by Cleaning up Delete Session Req between S-MME<->S-SGW, Delete Indirect tunnel Req between SMME->S-SGW and TMME<->TSGW***
Ref: http://www.lteandbeyond.com/2012/03/s1-interface-based-handover.html#more
