Ok, here it is. First handover scenario in LTE. Inter eNB handover without SGW change which spec calls as X2 handover. X2 ah! sounds like X-Men. :) This handover is pretty simple to understand. This handover is explained in 23.401 section 5. What we have below is a UE is moving from source eNB to target eNB. Since the control plane is already established there will be just a change in the user plane.
One thing. We are taking of scenario where source eNB and target eNB are both connected to same MME and SGW. If the physical connection changes then we will be looking at different handovers, which I will cover soon.
When UE moves from one eNB to other eNB, the eNB's communicate about the UE information using X2 protocol which unfortunately is out of my scope at this moment. :( The target eNB sends path switch request, which I believe is a S1message, to MME indicating that UE has changed its location. Here the bearers come into picture. Say UE has default and dedicated bearers established before the handover. Simply these bearers are to be updated for the new user plane. So MME sends out Modify bearer request which includes all the bearers information and new eNB information to SGW. That is Modify bearer request will contain EBI's (of default and dedicated bearers) and target eNB FTEID. All the info is sent in a single message which SGW may accept and return the response.
After that SGW sends out an end marker message to source eNB. End marker is a GTP-U message which indicates that there will not be any further user plane traffic on that TEID. Here we are sending this message to source eNB to tell that there will not be any user plane traffic sent to it from now on, offcourse for that UE. Source eNB send the same to target eNB. Rest two messages complete the handover.
What about QoS? I dont believe that there will be any qos change in the above scenario. How ever the qos information will have to be sent if the SGW changes. Then there may be new Qos values. This is for further study.
QoS is turning out to be quite complex then i thought. Research is one and will post more.
Thats pretty much it. I hope it was useful and comments are welcome.
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