What's the difference between X2 handover and S1 handover

In LTE (Long Term Evolution) cellular networks, handover refers to the process of transferring an ongoing call or data session from one cell to another without disrupting the connection. There are different types of handovers, including hard handover, soft handover, and fast handover.

X2 handover and S1 handover are two types of handovers in LTE networks.

• X2 Handover: It is a type of handover that occurs between two base stations (eNodeBs) connected via an X2 interface. This interface is used to exchange signaling and user data between eNodeBs. During an x2 handover, the source eNodeB sends a handover request to the target eNodeB, and after confirmation, the target eNodeB starts serving the user. X2 handover is a fast handover, and it is typically used for intra-LTE handovers within the same operator's network.

• S1 Handover: It is a type of handover that occurs between eNodeB and the core network (Evolved Packet Core, EPC) over the S1 interface. During an S1 handover, the serving eNodeB sends a handover request to the target eNodeB via the EPC. The target eNodeB then confirms its readiness to serve the user, and the handover is completed. S1 handover is typically used for inter-LTE handovers between different operators or networks, or between LTE and other radio access technologies. It also happens when X2 interface is not available.

In summary, the main difference between x2 and s1 handover in LTE is that x2 handover occurs between two eNodeBs connected via an X2 interface, while s1 handover occurs between eNodeB and the EPC over the S1 interface.

X2 Handover:

• The X2 handover procedure is a handover method that takes place between two eNodeBs (base stations).

• They are connected via an X2 interface.

• This type of handover is utilised in situations in which the target cell is connected to an eNodeB that is distinct from the eNodeB that the source cell is connected to.

• During the X2 handover operation, the source eNodeB and the destination eNodeB will exchange signalling messages in order to transmit the UE (user equipment) context as well as any data that has to be sent to the target cell.

• The X2 handover process is commonly utilised for intra-LTE handovers because it is a lot quicker than the S1 handover procedure (i.e., handovers within the LTE network).

S1 Handover:

• The S1 handover is a handover between an eNodeB and the core network (i.e., the mobility management entity or MME).

• It occurs when the target cell is connected to the same eNodeB as the source cell, but is controlled by a different MME. Transferring the UE context and data from the source MME to the target MME is part of the S1 handover protocol.

• S1 is often used for inter-RAT (i.e. inter-technology) handovers

• It is slower than X2 (i.e., handovers between LTE and other wireless technologies such as 3G or 2G).