What is S1 and X2 handover in LTE?

What is S1 and X2 handover in LTE?

Let me explain to you the difference between S1 and X2 handover in LTE. If you’ve followed earlier discussions about handover concepts in LTE, you already know that handovers are essential for maintaining seamless connectivity when a user moves from one cell to another. But there are different types of handovers based on how the network is structured and how the signal routing takes place — and that’s where S1 and X2 come in.

In LTE, handovers can happen over two main interfaces:

• X2 Interface: Direct connection between two eNodeBs.
• S1 Interface: Involves the core network (MME and Serving Gateway) in the handover process.

Let me break them down for you more clearly.

X2 Handover

When your device moves from one LTE cell to another, and both cells (eNodeBs) are directly connected via the X2 interface, the handover is called an X2 handover. This is a faster and more efficient handover because it avoids involving the core network directly. Here’s what happens:

• The source eNodeB detects that the signal is getting weak and prepares the target eNodeB for the handover.
• It sends a handover request via the X2 interface.
• The target eNodeB allocates resources and sends back a handover response.
• Your UE is then instructed to switch to the new cell.

This process is quick and reduces latency, which is why X2 handover is preferred when both eNodeBs are part of the same LTE network and are connected directly.

S1 Handover

Now, in cases where there is no direct X2 connection between the source and target eNodeB — for example, if they belong to different network operators or regions — the handover has to go through the S1 interface, involving the core network.

Here’s what typically happens during an S1 handover:

• The source eNodeB sends a handover request to the MME.
• The MME then contacts the target eNodeB to prepare for the handover.
• Once the target cell is ready, the MME facilitates the handover by managing the data path switch in the Serving Gateway (SGW).
• Your UE is told to connect to the new eNodeB.

S1 handover takes slightly more time compared to X2, but it’s necessary when X2 isn’t possible.

Comparison Table

So, to sum it up — if your LTE device is handed over between two neighboring cells under the same management, you’ll most likely experience an X2 handover. But if you move across areas where eNodeBs aren’t directly connected, then an S1 handover steps in to keep your session active.

As we explored in previous discussions on mobility and handover types, understanding these interfaces gives you better insight into how LTE networks manage seamless data sessions and voice calls while you’re on the move.