Would there be an advantage of X2 based handover over S1 based handover, when X2 physically spans the same path as S1?

Question

In case of hub-and-spoke topology, X2 messaging can travel all the way back to an aggregation point, which may be co-located with EPC. In such scenario both X2 and S1 traverses same physical path (a.k.a X2 over S1). Please consider both Intra-MME and Inter-MME handover scenarios. Would X2 based handover in such a case yield advantage over S1 based handover. Thanks.

Answer 1

Though there is hub-Spoke topology and eNodeB has to reach at the aggregation point for the messages exchange and for S1 handover also there is same path.

This is the case where according to you If this scenario occurs, what will be the behavior of the Network or else which handover will be advantageous.

As we know if we don't talk about this scenario, In normal scenario X2 based handover is better than S1 based handover because of less signalling, fast and quick. In a same way this scenario seems to me the same situation like the normal has.

X2 based handover will yield the advantage. Just consider the message flow.

Call Flow for X2 based handover:
Source-EnodeB------> Aggregation Point-------->Target EnodeB (Source and target MME is same)
This is how handover will be possible and messages which are to be exchange you know. Only one MME will be there. I mean to say source MME and Target MME is same so It saves the signaling.

Call flow for S1-Based Handover:
SeNB-->aggregation point--->SourceMME---->TargetMME--->Target eNodeB (Source and target MME is diff)

This is the extra signaling. So if both are in same condition and we have to compare between the preferable handover. I think the network should go for the X2. Exceptions are always there. It mainly depends upon the network condition and the implementation by the Operator side. It is the theoretical concept which I have been considered here. If something is slipped, please let me know.

Comments

Well answered. Thanks.

Answer 2

Hi,

as it goes to advantages
Less signaling messages on the S1, less MME involvement (processing power / free cycles) - from top of my head.

It would be much better for you if you would check it comparing the call flow on your own.
Please here you have reference to S1 and X2 handover on my blog

http://www.lteandbeyond.com/2012/03/s1-interface-based-handover.html   -  Figure 2
http://www.lteandbeyond.com/2012/03/x2-based-handover-without-sgw.html   - Figure 2

Mentioned Figures would be the most important for you.
Best

Comments

Thanks for the response, it makes sense.

Answer 3

As far as I know, S1 and X2 both are logical link. It is not necessary that MME should have direct physical connection with eNodeB for S1 interface. Similarly for X2 interface. Underlying IP network may have a number of router or hub etc between any two nodes of LTE network.

In Case of S1 handover - HO Request message is prepared by MME where as in case of X2, source eNodeB itself prepares HO Request. 3GPP specification just covers the logical hops not the underlying IP hops.

Looking for others to share their knowledge/thoughts over this topic.

Comments

Thanks, but my question remain unanswered. I am looking for specific advantages of X2 over S1 based Handover in a typical case when path latency is same in both scenarios.

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The advantage of X2 HO is that there is less chance of data loss in comparision to S1