S1 Handover in LTE: Call Flow & Entities?

S1 handover is used when the UE moves between two EnodeB that do not have an X2 interface. This type of handover involves the MME (Mobility Management Entity) and the S-GW (Serving Gateway) for control and data forwarding. S1 handover is slower than X2 handover due to additional signaling via the EPC (Evolved Packet Core).

S1 Handover Call Flow with Entities:

Step 1: Handover Preparation
1️. UE → Source eNodeB → Sends a Measurement Report (signal strength of neighboring cells).
2️. Source eNodeB → MME → Sends Handover Required message (requests handover).
3️.MME → Target eNodeB → Sends Handover Request (includes UE context & QoS details).
4️.Target eNodeB → MME → Responds with Handover Request Acknowledge after resource allocation.

Step 2: Handover Execution
5️. MME → Source eNodeB → Sends Handover Command for UE to move.
6️. Source eNodeB → UE → Sends RRC Connection Reconfiguration with new target eNodeB parameters.
7️. UE → Target eNodeB → Synchronizes and sends RRC Reconfiguration Complete.
8️. Target eNodeB → MME → Sends Handover Notify (UE successfully attached).

Step 3: Handover Completion
9️. MME → S-GW → Sends Modify Bearer Request to update the user plane path.
10. S-GW → MME → Target eNodeB → Path switch is confirmed.
11. MME → Source eNodeB → Sends Release Resource Message, freeing old resources.