X2 Handover Case The X2 handover is used when direct connectivity exits between the source and target enbs with X2 interface

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1 LTE X2 Handover X2 Handover Case The X2 handover is used when direct connectivity exits between the source and target enbs with X2 interface Handover Preparation The X2 handover procedure is performed without Evolved Packet Core (EPC) involvement, i.e. preparation messages are directly exchanged between Source enb and Target enb Handover Execution There is direct tunnel formed between source and target enbs for downlink data forwarding in handover execution time 1. Before Handover 2. Handover Preparation 3. Handover Execution 4. Handover Completion 5. After Handover MME SAE-GW MME SAE-GW MME SAE-GW MME SAE-GW MME SAE-GW Source Target Source Target Source 3 Target Source Target Source Target enb enb enb enb enb enb enb enb enb enb UE moving UE moving UE moving UE moving UE Source enb makes the HO decision Direct Tunnel Setup (X2 Transport Bearer between Source enb and Target enb) Detach from Source enb Direct Packet Forwarding (DL traffic from Source enb to Target enb) Path Switch procedure between Target enb and MME Release S1 Bearer of Source enb Release X2 Transport Bearer for direct Packet Forwarding DRB (Data Radio Beaer) Signaling in Radio S1 Bearer (GTP Tunnel) S1 Signaling X2 Bearer (GTP Tunnel) X2 Signaling S11 (GTP-C) Signaling UL Traffic DL Traffic About NMC Consulting Group NMC Consulting Group was founded on year 2002 and is advanced, professional network consulting company which is specialized for IP Network area like FTTH, Metro Ethernet and IP/MPLS, Service area like IPTV and IMS lastly, Wireless network area like Mobile WIMAX and LTE. Copyright NMC Consulting Group. All rights reserved. LTE X2 Handover 1

2 LTE X2 Handover Source Target UE enb enb MME S-GW P-GW HSS PCRF EMM-Registered ECM-Connected RRC-Connected RRC-Connected EMM-Registered ECM-Connected DRB (UL/DL) S1 Bearer (UL/DL) S5 Bearer (UL/DL) Internet Measurement Report Serving Cell (Radio Signal Strength), Target Cell (PCI, Radio Signal Strength) Triggering the UE to send a Measurement Report based on triggering event (e.g., when a neighboring cell becomes better than the serving cell in terms of radio signal strength) Handover Preparation Handover Execution UE can not send & receive packets Receive Target enb TEID (DL) Handover Decision by Source enb Derive KeNB* Handover Request ECGI of the Target Cell (of Target enb), UE-AMBR, UE Security Capability, KeNB*, E-RAB to be setup (E-RAB ID, QCI, ARP, S1 S-GW TEID) enb generates AS keys from KeNB (KeNB = KeNB*) Receive S1 TEID (UL) S1 TEID (UL) for Source enb Admission control with E-RAB QoS (QCI, ARP) Target enb reserves RRC resource and allocates a new C-RNTI (e.g., Target C-RNTI) Handover Request Ack X2 Transport Bearer s {E-RAB ID, Target enb TEID}, Handover Command (Target C-RNTI, Target DRB ID (UL/DL), Target enb AS Security Algorithm) Create Target enb TEID (DL) X2 Transport Bearer (DL) Established DL: From Source enb to Target enb Create DRB ID (UL/DL) Handover Command (RRC Connection Reconfiguration) Target C-RNTI, Target DRB ID (UL/DL), Target enb AS Security Algorithm) Receive DRB ID (UL/DL) Detach from the source enb UE generates KeNB (KeNB = KeNB*) & AS keys SN (Sequence Number) Status Transfer DL Count (DL PDCP SN), UL Count (UL PDCP SN) Synchronization UL Allocation X2 Transport Bearer (DL) S1 Bearer (UL/DL) Buffering DL Packets Handover Confirm (RRC Connection Reconfiguration Complete) Target C-RNTI RRC-Idle S1 Bearer Established (UL) Start to send the Buffered DL Packets to the UE S5 Bearer (UL/DL) Internet DRB Established (UL/DL) RRC-Connected AS Security setup S1 Bearer (UL/DL) S5 Bearer (UL/DL) Internet X2 Transport Bearer (DL) DRB (UL/DL) DRB (UL/DL) S1 Bearer (UL) S5 Bearer (UL/DL) Internet Handover Completion Create S1 TEID (DL) X2 Transport Bearer EM(DL) Path Switch Request ECGI, TAI, Received UE Security Capability, E-RAB to be switched in DL (E-RAB ID, S1 Target enb TEID) Modify Bearer Request EPS Bearer in DL (EPS Bearer ID, S1 Target enb TEID), ECGI, TAI Receive S1 TEID (DL) S1 Bearer Established (DL) Modify Bearer Request EPS Bearer ID, ECGI, TAI Indication of IP-CAN Session Modification (CCR) IMSI, UE IP, ECGI, TAI Acknowledge of IP-CAN Session Modification (CCA) Modify Bearer Response EPS Bearer ID Modify Bearer Response EPS Bearer in DL (EPS Bearer ID) Switch to DL path S1 Bearer (UL/DL) EM Target enb sends packets to UE from old path (X2) until receiving End-Marker, after then it sends packets from new path (S1) Send End Marker Packet For re-ordering in the Target enb, the S-GW sends End-Marker packet(s) on the old path immediately after switch the path DRB (UL/DL) S1 Bearer (UL/DL) S5 Bearer (UL/DL) Internet EMM-Registered ECM-Connected RRC-Connected UE Conext Release Source enb release UE context X2 Transport Bearer (DL) Released RRC-Connected Path Switch Request Ack E-RAB to be switched in UL (E-RAB ID) S1 Bearer Released (UL/DL) EMM-Registered ECM-Connected UE Target enb EM 5 EM End Marker EM Source enb S-GW LTE X2 Handover 2