Difference between X2 and S1 Hand Over

X2 Hand Over:

HO occurs when source and target eNBs are served within the same MME pool. The procedures relies on the presence of X2 interface between Source and Target eNB, which is summarized as follows,

  • Source eNB makes HO decision and setup a direct tunnel i.e X2 transport bearer between Source and target eNB.
  • Detach UE from Source eNB and Forward traffic from source eNB to Target.
  • Path switch procedure between Target eNB and MME
  • Releases S1 bearer of source eNB
  • Release X2 transport bearer for direct packet forwarding.

S1 Hand Over:

S1 handover is when If two eNodeBs are not connected with same MME or the X2 interfaces are not defined between eNB or when X2 procedure fails(due to unreachability/Error response etc). Summary of S1-HO is as follows,

  • Source eNB makes HO decision and setup a indirect tunnel i.e S1 bearer between Source eNB and SGW, and target eNB and SGW.
  • S1 bearer for UL setup between target and source eNB
  • Detach UE from Source eNB and indirect packet forwarding
  • No need for the Path switch procedure between Target eNB and MME, as MME is aware of HO
  • Releases S1 bearer of source eNB
  • Release S1 transport bearer for indirect packet forwarding.

If the two eNodeBs are connected with same MME, it is preferred to perform X2 based handover but there is no restriction in using S1 based handover even in this case. If two eNodeBs are not connected with same MME, you have to perform S1 based handover even in this case.

X2 handover is faster as compared to S1. X2 handover is defined only for intra LTE handover while S1 handover can be used for intra LTE, inter RAT.

 

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