What are the steps of an LTE handover between two base stations in the same network?

The source BS picks the target and pre-arranges everything (steps 1–2), the mobile switches early (step 3) while the source forwards buffered traffic (step 4), and only afterwards is the S-GW tunnel re-pointed (steps 5–7).

* LTE X2 handover: switch early; the S-GW tunnel is re-pointed afterwards. *

The seven steps:

1. The current (source) BS selects the target BS and sends it a Handover Request
2. The target BS pre-allocates radio time slots and responds with HR ACK, including info for the mobile
3. The source BS informs the mobile of the new BS — the mobile can now send via the new BS: to the mobile, the handover looks complete
4. The source BS stops sending datagrams to the mobile and instead forwards them to the new BS (which forwards over its radio channel)
5. The target BS informs the MME that it is the new BS; the MME instructs the S-GW to change its tunnel endpoint to the target BS
6. The target BS ACKs back to the source BS: handover complete — the source can release resources
7. The mobile's datagrams now flow through the new tunnel from target BS to S-GW

Two elegant details worth noticing:

• Data forwarding (step 4) bridges the gap: packets already in flight toward the old BS aren't lost — they chase the mobile through the old→new BS detour until the tunnel switch catches up
• The mobile is done at step 3, long before the core network (steps 5–7) — radio-side speed matters most, core-side bookkeeping can lag behind

Go deeper:

• E-UTRA (Wikipedia) — the X2 and S1 interfaces that carry the handover signaling and data forwarding.
• LTE handover overview (ShareTechNote) — an engineer's state-diagram view mapping onto the source-eNB → target-eNB → path-switch sequence.