Saturday, June 20, 2009

LTE X2 Handover without SGW change

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.


Manish said...

I have one doubt

Will S-GW send End -marker message to old eNB immidiately after U-Plane change


S-GW will send End Marker to old eNB when it receives the DL data?

Any answer?

Regards and Keep Smiling

Santosh said...

End marker is sent immediately after switching the path, i.e immediately after user plane change.

Marcin Mikuszewski said...

Hi Santosh,

I am trying to understand this simple handover.
When I read 23.401 in chapter 5 I can find X2 based handover which is as you described. But when I check 36.300 I can see a small difference between some messages i.e. Modify Bearer Request and User Plane Update Request that is being sent from MME to SGW. I have checked some previous version of 23.401 and I found out that in 801 version it was actually User Plane Update Request instead of Modify Bearer Request.
Could you please explain me why and when this one message turns into another within this X2 based handover ?

Santosh said... was a mistake in spec. When target eNB sends PSReq MME needs to inform SGW about the t.eNB. In the call flow it was shown as User Plane Update request because MME should update the SGW with new S1-U interface. However there was no GTPv2 message called User Plane Update, hence they changed it to Modify Bearer Request.

In a nut was a mistake in spec.

Marcin Mikuszewski said...


Thanks for the answer. You are right. There is no such message described in GTP-C spec.
You wrote it was a spec mistake I just would say it is still. I have checked latest version of 36.300 and in chapter there is a picture showing Intra MME/SGW handover which I think is X2 based handover in fact. There is still User Plane Update Request message in this picture. So when someone would like to combine information from those two specs then he would come up with a question that I have.
Once again thanks.

Anonymous said...

Hello Santosh.

I have a doubt, when SGW send the end market message to the source eNB,the source eNB stops reciving packets and forwarding or just stops reciving and send all packets in buffer?


Santosh said...

SGW sends end marker message to eNB informing it that there will no more data for it for the particular UE. After receiving the end marker, the source eNB will not receive the data any more. what ever data source eNB has received till then will be buffered and sent to target eNB.

Anonymous said...


I have read that the intra-technology Handover in LTE is break-before-make, but i also read in a book that it's possible iniciate forwarding before UEs disconnect. In the TS 36.300 this is not clear if forwarding can made after or before UE break link.

I would like know your opinion.


Santosh said...

Intra LTE handover is smooth without any data loss. Forwarding is enabled before handover is complete.

Refer to 3GPP TS 23.401 for call flows.

Anonymous said...


I have a doub in the forwarding process, when this process is iniciated how the packets are routed through X2 interface, is created the same bearer that the UE had?

Relatively to the packet header, is it sent in the same form like for UE?


Anonymous said...

Hi, from the UE's perspective, what's the difference between a intra-eNB handover and a inter-eNB handover? would a UE see any differences? could there maybe some difference in how security keys are handled/generated? (e.g. KeNB?)